Now that Java is working, the next pieces depend on what you are building. If you are running a JVM app on Debian, you probably also need a reverse proxy, a database, and a CI pipeline. Jenkins still rules the JVM CI world, so install Jenkins on Debian is a common next step. For the database, PostgreSQL on Debian is the usual choice for Spring Boot and Micronaut apps. For TLS termination or HTTP routing, Nginx on Debian handles it cleanly.

Original content from computingforgeeks.com - post 49033

For build tooling, Maven is in the Debian repo as maven and pulls in a sane default JDK. For Gradle, use Gradle’s own distribution because Debian’s is always a few versions behind. Apache Ant is still alive for legacy projects.

Stuck deploying a JVM app in production, or need help wiring Java services into CI? We do Claude Code, Java, and DevOps consulting. Reach out at [email protected] for one-hour paid sessions to multi-week engagements.

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Either the install step failed (check apt install output for 404s on packages.adoptium.net) or your shell opened before the alternatives symlink was in place. Fix by opening a new shell or running hash -r. If the problem persists, confirm /usr/bin/java exists with ls -la /usr/bin/java.

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What to install next

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

This shows up when an app reads JAVA_HOME from a service unit that captured the old value before you edited /etc/profile.d/java.sh. systemctl daemon-reload does not re-read profile files. Fix by either setting Environment=JAVA_HOME=/usr/lib/jvm/... inside the service file, or using EnvironmentFile=/etc/profile.d/java.sh with the export keywords removed.

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Debian 13’s main repo does not ship openjdk-17-jdk or openjdk-11-jdk. The only OpenJDK versions in trixie main are 21 and 25. For 17 or 11 on trixie, use the Adoptium Temurin repo from Step 4. Do not try to pull openjdk-17-jdk from the bookworm pool, glibc symbols differ and the package will not run reliably.

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

If your output prints a different version than you expect, re-check update-alternatives --config java and update-alternatives --config javac. The runtime matches whichever /usr/bin/java is currently linked to.

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

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What to install next

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Notice Oracle’s build string says Java(TM) SE and includes the LTS marker, which is how you tell it apart from OpenJDK at a glance. The Oracle postinst also registers the java alternative with a priority of 352403456, which forces it to win auto-mode selection. Step 6 shows how to override that if you do not want Oracle as the default.

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The file is around 160 MB. The latest path always serves the current point release, so the same URL works past 21.0.10. Installation completes quickly and registers the binaries under /usr/lib/jvm/jdk-21.0.10-oracle-x64/.

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Use Oracle JDK only if your employer’s license already covers it or if you specifically need Oracle’s commercial features (Flight Recorder extensions, Mission Control, GraalVM Enterprise). For everything else, OpenJDK or Temurin is the same code without the licensing complexity.

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The $(lsb_release -cs) expansion resolves to trixie on Debian 13 or bookworm on Debian 12, so the same block works for either. Install lsb-release first if it is missing: apt install -y lsb-release.

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Debian 13’s main repo carries only OpenJDK 21 and 25. Anything older (OpenJDK 17, 11, or 8) has to come from a third-party repo. The cleanest option is Eclipse Adoptium, which publishes Temurin builds with TCK-certified quality and a proper trixie suite.

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

The default-jdk meta-package pulls in whatever the Debian release considers the default (OpenJDK 21 on trixie, OpenJDK 17 on bookworm). Adding openjdk-21-jdk explicitly pins the version so you do not get surprised when the meta-package’s preferred version shifts in a future release.

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

OpenJDK 21 is the current long-term support release and the one most readers want. It ships in Debian 13 main with no extra repo:

sudo apt update
sudo apt install -y default-jdk openjdk-21-jdk

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

If the output is empty and you see no java yet, you have a clean slate. If something is already installed (common on images that ship Jenkins, Elasticsearch, or Tomcat), decide whether to keep it or purge it with apt purge 'openjdk-*' before continuing.

Step 2: Install OpenJDK 21 LTS from the Debian repo

OpenJDK 21 is the current long-term support release and the one most readers want. It ships in Debian 13 main with no extra repo:

sudo apt update
sudo apt install -y default-jdk openjdk-21-jdk

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Next, check whether any JDK is already on the system. A fresh Debian netinst has none:

apt list --installed 2>/dev/null | grep -iE 'jdk|jre|java'
which java || echo "no java yet"

Step 2: Install OpenJDK 21 LTS from the Debian repo

OpenJDK 21 is the current long-term support release and the one most readers want. It ships in Debian 13 main with no extra repo:

sudo apt update
sudo apt install -y default-jdk openjdk-21-jdk

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

On the test box this reports Debian 13 trixie:

PRETTY_NAME="Debian GNU/Linux 13 (trixie)"
NAME="Debian GNU/Linux"
VERSION_ID="13"
6.12.57+deb13-amd64

Next, check whether any JDK is already on the system. A fresh Debian netinst has none:

apt list --installed 2>/dev/null | grep -iE 'jdk|jre|java'
which java || echo "no java yet"

Step 2: Install OpenJDK 21 LTS from the Debian repo

OpenJDK 21 is the current long-term support release and the one most readers want. It ships in Debian 13 main with no extra repo:

sudo apt update
sudo apt install -y default-jdk openjdk-21-jdk

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

Confirm the Debian release first so you know which package pool you are pulling from:

cat /etc/os-release | head -3
uname -r

On the test box this reports Debian 13 trixie:

PRETTY_NAME="Debian GNU/Linux 13 (trixie)"
NAME="Debian GNU/Linux"
VERSION_ID="13"
6.12.57+deb13-amd64

Next, check whether any JDK is already on the system. A fresh Debian netinst has none:

apt list --installed 2>/dev/null | grep -iE 'jdk|jre|java'
which java || echo "no java yet"

Step 2: Install OpenJDK 21 LTS from the Debian repo

OpenJDK 21 is the current long-term support release and the one most readers want. It ships in Debian 13 main with no extra repo:

sudo apt update
sudo apt install -y default-jdk openjdk-21-jdk

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

You need Java on your Debian server because something you want to run, a Tomcat app, a Jenkins agent, a Minecraft server, an Elasticsearch node, refuses to start without it. This guide gets you from zero to a working java --version on Debian 13 (trixie) and Debian 12 (bookworm), with OpenJDK 21 LTS as the default, plus clear paths for OpenJDK 17, OpenJDK 11, OpenJDK 25, and Oracle JDK when you need them.

Every command here was tested on a real Debian 13 box. The output blocks are copied from the terminal, not made up. You will see how to install JDK packages from Debian’s own repo, add Eclipse Temurin for versions Debian does not ship, switch the system default with update-alternatives, set JAVA_HOME cleanly, and compile a HelloWorld as a final sanity check.

Tested April 2026 on Debian 13 (trixie, kernel 6.12) with OpenJDK 21.0.10 LTS, OpenJDK 25.0.2, Eclipse Temurin 17.0.18 and 11.0.30, Oracle JDK 21.0.10 LTS. Debian 12 (bookworm) notes where packaging differs.

Prerequisites

Debian 13 (trixie) or Debian 12 (bookworm), fresh or existing install
A user with sudo, or root (examples use root for brevity)
Outbound HTTPS for deb.debian.org, packages.adoptium.net, and download.oracle.com
~500 MB free disk for a single JDK, ~2 GB if you install all four covered here

Step 1: Check what you already have

Confirm the Debian release first so you know which package pool you are pulling from:

cat /etc/os-release | head -3
uname -r

On the test box this reports Debian 13 trixie:

PRETTY_NAME="Debian GNU/Linux 13 (trixie)"
NAME="Debian GNU/Linux"
VERSION_ID="13"
6.12.57+deb13-amd64

Next, check whether any JDK is already on the system. A fresh Debian netinst has none:

apt list --installed 2>/dev/null | grep -iE 'jdk|jre|java'
which java || echo "no java yet"

Step 2: Install OpenJDK 21 LTS from the Debian repo

OpenJDK 21 is the current long-term support release and the one most readers want. It ships in Debian 13 main with no extra repo:

sudo apt update
sudo apt install -y default-jdk openjdk-21-jdk

Verify:

java --version
javac --version
readlink -f $(which java)

Real output from the test VM:

openjdk 21.0.10 2026-01-20
OpenJDK Runtime Environment (build 21.0.10+7-Debian-1deb13u1)
OpenJDK 64-Bit Server VM (build 21.0.10+7-Debian-1deb13u1, mixed mode, sharing)
javac 21.0.10
/usr/lib/jvm/java-21-openjdk-amd64/bin/java

The binary lives at /usr/lib/jvm/java-21-openjdk-amd64/. That path matters in a moment when you set JAVA_HOME.

Debian 12 difference: On bookworm, default-jdk resolves to openjdk-17-jdk. OpenJDK 21 is available in bookworm-backports:

# Debian 12 only, enable backports first
echo "deb http://deb.debian.org/debian bookworm-backports main" \
  | sudo tee /etc/apt/sources.list.d/backports.list
sudo apt update
sudo apt install -y -t bookworm-backports openjdk-21-jdk

Step 3: Install OpenJDK 25 for the latest LTS

OpenJDK 25 is the newer LTS line and Debian 13 security-backports it alongside 21. Useful if your workload targets the newest LTS or you are evaluating the switch:

sudo apt install -y openjdk-25-jdk

After install, both live side by side under /usr/lib/jvm/. The package does not change which java is the default. That is handled by the alternatives system in Step 6.

Confirm the 25 binary runs:

/usr/lib/jvm/java-25-openjdk-amd64/bin/java --version

The output shows the 25.0.2 build with trixie’s security patch suffix:

openjdk 25.0.2 2026-01-20
OpenJDK Runtime Environment (build 25.0.2+10-Debian-1deb13u2)
OpenJDK 64-Bit Server VM (build 25.0.2+10-Debian-1deb13u2, mixed mode, sharing)

Step 4: Install OpenJDK 17 or 11 from Eclipse Adoptium (Temurin)

Add the repo:

sudo apt install -y wget gpg apt-transport-https
sudo mkdir -p /etc/apt/keyrings
wget -qO- https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | sudo gpg --dearmor -o /etc/apt/keyrings/adoptium.gpg
echo "deb [signed-by=/etc/apt/keyrings/adoptium.gpg] https://packages.adoptium.net/artifactory/deb $(lsb_release -cs) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list
sudo apt update

Now install the Temurin versions you need:

sudo apt install -y temurin-17-jdk temurin-11-jdk

Temurin 8, 22, 23, 24, 25, and 26 are also in the repo under the same temurin-N-jdk naming. List them with apt-cache search temurin | sort if you are curious.

Verify both binaries:

/usr/lib/jvm/temurin-17-jdk-amd64/bin/java --version
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java --version

The Temurin 17 output is first, and the build string names the upstream Adoptium project so you can tell it apart from Debian’s OpenJDK packaging:

openjdk 17.0.18 2026-01-20
OpenJDK Runtime Environment Temurin-17.0.18+8 (build 17.0.18+8)
OpenJDK 64-Bit Server VM Temurin-17.0.18+8 (build 17.0.18+8, mixed mode, sharing)

Step 5: Install Oracle JDK 21 (opt-in, LTS)

Oracle publishes a .deb package. Download and install:

cd /tmp
wget -q https://download.oracle.com/java/21/latest/jdk-21_linux-x64_bin.deb
sudo dpkg -i jdk-21_linux-x64_bin.deb

Verify:

/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java --version

Oracle’s build string makes the commercial provenance obvious:

java 21.0.10 2026-01-20 LTS
Java(TM) SE Runtime Environment (build 21.0.10+8-LTS-217)
Java HotSpot(TM) 64-Bit Server VM (build 21.0.10+8-LTS-217, mixed mode, sharing)

Step 6: Switch between installed JDKs

After following the steps above, five JDKs are registered with update-alternatives. List them:

sudo update-alternatives --list java

All five paths show up:

/usr/lib/jvm/java-21-openjdk-amd64/bin/java
/usr/lib/jvm/java-25-openjdk-amd64/bin/java
/usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java
/usr/lib/jvm/temurin-11-jdk-amd64/bin/java
/usr/lib/jvm/temurin-17-jdk-amd64/bin/java

Now switch the default with the interactive menu. This updates /usr/bin/java symlink for every user on the box:

sudo update-alternatives --config java

The menu shows every alternative with its priority. Pick the number next to the JDK you want:

There are 5 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                          Priority   Status
------------------------------------------------------------
  0            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 auto mode
* 1            /usr/lib/jvm/java-21-openjdk-amd64/bin/java    2111      manual mode
  2            /usr/lib/jvm/java-25-openjdk-amd64/bin/java    2511      manual mode
  3            /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/java   352403456 manual mode
  4            /usr/lib/jvm/temurin-11-jdk-amd64/bin/java     1111      manual mode
  5            /usr/lib/jvm/temurin-17-jdk-amd64/bin/java     1711      manual mode

Press <enter> to keep the current choice[*], or type selection number:

Do the same for the compiler, because java and javac are separate alternatives:

sudo update-alternatives --config javac

Running both is critical. If you switch java to 17 but forget javac, your runtime and compiler versions drift, and builds will fail with cryptic class-file errors.

For an older piece on this exact topic, see how to set the default Java version on Ubuntu and Debian. The principles carry over.

Step 7: Set JAVA_HOME system-wide

Most Java-based apps read JAVA_HOME instead of java from PATH. Jenkins, Tomcat, Maven, Gradle, and every IDE do. Set it in /etc/profile.d/ so every login shell picks it up:

sudo nano /etc/profile.d/java.sh

Paste this, adjusting the path if you picked a different JDK:

export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

Make it executable and load it in the current session:

sudo chmod +x /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Confirm:

echo $JAVA_HOME
echo "java binary: $(which java)"

Both lines should report the 21 path:

JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64
java binary: /usr/lib/jvm/java-21-openjdk-amd64/bin/java

For a portable alternative that follows whatever the alternatives symlink points to, use /usr/lib/jvm/default-java:

export JAVA_HOME=/usr/lib/jvm/default-java

That way, switching with update-alternatives also shifts JAVA_HOME without editing the profile file. Handy on multi-tenant boxes.

Step 8: Smoke test with Hello.java

A HelloWorld proves the compiler and runtime agree on the same JDK:

mkdir -p /root/hello && cd /root/hello
nano Hello.java

Drop in this source. It prints the JDK’s own version string so you see which runtime actually ran the class:

public class Hello {
    public static void main(String[] args) {
        String version = Runtime.version().toString();
        System.out.println("Hello from Debian 13 on JDK " + version);
    }
}

Compile, then run:

javac Hello.java
java Hello

On the test VM with OpenJDK 21 active, this prints:

Hello from Debian 13 on JDK 21.0.10+7-Debian-1deb13u1

Here is the full terminal session as one screenshot:

Troubleshooting

Error: “E: Unable to locate package openjdk-17-jdk” on Debian 13

Error: “The java_home file does not exist”

Error: “Oracle JDK installed but update-alternatives –display javac does not list it”

Oracle’s .deb registers most binaries with update-alternatives but skips javac as a master alternative in some versions. Register it manually:

sudo update-alternatives --install /usr/bin/javac javac \
  /usr/lib/jvm/jdk-21.0.10-oracle-x64/bin/javac 3000

Then re-run sudo update-alternatives --config javac and pick the Oracle entry.

Error: “java: command not found” after install

Secure the server you just set up

What to install next

5 thoughts on “How To Install Java OpenJDK and Oracle JDK on Debian 13 / 12”

Anony

September 17, 2021 at 6:42 am

Note: The first method installs early access build which is not the one you would generally use.
Sukasa

October 8, 2022 at 6:25 pm

This doesn’t work for me. The files are there but java -version says it’s v16, and update-alternatives –config java doesn’t list any options for jdk 17

Debian 10
- Josphat Mutai
  
  October 13, 2022 at 12:54 am
  
  I guess you have multiple versions of Java. Set 17 as default. Alternatively uninstall any other version before you install Java 17.
Souli'

July 28, 2023 at 4:20 pm

After using the first and the seccond method, Im still having :

X@X:/X/X# sudo tee /etc/profile.d/jdk.sh < export JAVA_HOME=/opt/jdk17
> export PATH=\$PATH:\$JAVA_HOME/bin
> EOF
export JAVA_HOME=/opt/jdk17
export PATH=$PATH:$JAVA_HOME/bin
X@X:/X/X# source /etc/pro
profile profile.d/ protocols
X@X:/X/X# source /etc/pro
profile profile.d/ protocols
X@X:/X/X# source /etc/profile.d/jdk.sh
X@X:/X/X# echo $JAVA_HOME
/opt/jdk17
X@X:/X/X# java -version
openjdk version “1.8.0_292”
OpenJDK Runtime Environment (AdoptOpenJDK)(build 1.8.0_292-b10)
OpenJDK 64-Bit Server VM (AdoptOpenJDK)(build 25.292-b10, mixed mode)
efu

March 12, 2024 at 3:19 am

nice, It has run perfectly