Gentoo provides numerous JDKs and JREs. The default is the Blackdown JDK/JRE pair, as it is freely (beer) available without any registration fuss.

Both the Sun JDK/JRE and the IBM JDK/JRE are generally faster, but getting them is a bit more work, as you are required to read and accept their license before downloading (IBM additionally requires you to register).

Our ebuilds for the Sun and IBM JDK/JREs will notify you of where to go to download them.

If you run emerge dev-java/sun-jdk-1.3.1 or emerge dev-java/ibm-jdk-1.3.1, you will be notified that you are required to download the actual tarballs yourself. This has to do with license restrictions for the Sun JDK/JRE (online click-wrap license) and registration issues with the IBM JDK/JRE.

You should download the indicated file(s) into /usr/portage/distfiles. Once that is done, you can rerun the emerge command, then the JDK/JRE will be installed properly into /opt.

Gentoo has the ability to have multiple JDKs and JREs installed without them conflicting. There are a few caveats to this, as noted below.

Using the java-config tool, you can set the system-wide default if you have root access. Users can also use java-config to set up their own personal default, that is different from the system-wide default.

Running the command java-config --list-available-vms will give you a list of all availble JREs and JDKs on your system, thus:

[%1 ~] java-config --list-available-vms
[blackdown-jdk-1.3.1] Blackdown JDK 1.3.1 (/etc/env.d/java/20blackdown-jdk-1.3.1)
[blackdown-jre-1.3.1] Blackdown JRE 1.3.1 (/etc/env.d/java/20blackdown-jre-1.3.1)
[ibm-jdk-1.3.0] IBM JDK 1.3.0 (/etc/env.d/java/20ibm-jdk-1.3.0)
[ibm-jdk-1.3.1] IBM JDK 1.3.1 (/etc/env.d/java/20ibm-jdk-1.3.1)
[ibm-jre-1.3.1] IBM JRE 1.3.1 (/etc/env.d/java/20ibm-jre-1.3.1)
[sun-jdk-1.4.0] Sun JDK 1.4.0 (/etc/env.d/java/20sun-jdk-1.4.0)

The name in the brackets "[]" is the handle or ID for that particular VM. You use pass that ID to java-config --set-system-vm, thus:

[#1 ~] java-config --set-system-vm=ibm-jdk-1.3.1
Now using IBM JDK 1.3.1 (/etc/env.d/java/20ibm-jdk-1.3.1)

Once you have issued java-config --set-system-vm with a particular VM ID, you will need to regenerate your /etc/profile.env, thus:

[#1 ~] env-update

After this, you will either want to relogin, or resource /etc/profile into your environment.

As a regular user, you can use java-config --set-user-vm, which will create $HOME/.gentoo/java-env with all required env vars. You would normally source this from your shell's startup script ($HOME/.zshenv in my case).

java-config can also be used to set a system-wide default CLASSPATH, and of course a user-specific default CLASSPATH.

First you want to list available java libraries that might be interesting to put in your CLASSPATH, thus:

[%1 ~] java-config --list-available-packages
[ant] No description (/usr/share/ant/classpath.env)
[java-gnome] No description (/usr/share/java-gnome/classpath.env)
[java-gtk] No description (/usr/share/java-gtk/classpath.env)
[log4j] "" (/usr/share/log4j/package.env)

Again, the name in brackets "[]" are the IDs that you have to pass to java-config --set-system-classpath, thus:

java-config --set-system-classpath=log4j,java-gtk,java-gnome

Again, you will want to run env-update to update your system's environment, and you might also want to relogin or resource the /etc/profile.

For users, java-config --set-user-classpath will create $HOME/.gentoo/java-env-classpath, which is automatically included by $HOME/.gentoo/java-env.

• java-config man page
• java-config --help
• The /usr/bin/java-config script itself

• The gentoo-dev , gentoo-user mailing list archives
• #gentoo on irc.openprojects.net

If you select a JRE as your default VM, you may not have a javac command handy at all times, unless you have manually installed a symlink to take care of that.

Jikes will be used to compile some of the largest packages, if it is available. With time, we hope to be able to compile all java-dependent packages with Jikes, so some dependencies on the JDK can be replaced with a dependency on the JRE.

With some versions of Portage, doing an emerge --world update will install the JDK regardless if whether you have it installed before or not. This is a known bug addressed in later versions of Portage.