Java is a programming language developed by engineers of Sun Microsystems. The language is object-oriented and designed to run on multiple platforms without the need of recompiling code for each platform. Although Java can be compiled as a native program, much of Java's popularity can be attributed to its portability, along with other features such as garbage collection. The ability to compile once and run in various platforms is achieved through the use of just-in-time compilers (JIT), which compile Java bytecodes into native code when a given program is run.

In order to run Java bytecodes, one needs to have a JRE (Java Runtime Environment) installed. A JRE provides core libraries, a platform dependent Java Virtual Machine, plug-ins for browsers, among other things. A JDK (Java Development Kit) adds programming tools, such as a bytecode compiler and a debugger.

Gentoo provides numerous JREs and JDKs. Among the current alternatives, we have:

• blackdown-jre and blackdown-jdk, the Blackdown Java Kit
• sun-jre-bin and sun-jdk, Sun's Java Kit
• ibm-jre-bin and ibm-jdk-bin, the IBM Java Kit
• compaq-jre and compaq-jdk, the Compaq Java Kit for Alpha/Linux/GNU
• jrockit-jdk-bin, BEA WebLogic's J2SE Development Kit

The default is the Blackdown JRE/JDK pair, as it is freely ("free as in beer") available without any registration fuss.

Both the Sun JRE/JDK and the IBM JRE/JDK 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 JRE/JDKs will notify you of where to go to download them.

If you run emerge =sun-jdk-1.4.2.06 or =ibm-jdk-bin-1.4.2, you will be notified that you are required to download the actual tarballs yourself. This has to do with license restrictions for the Sun JRE/JDK (online click-wrap license) and registration issues with the IBM JRE/JDK.

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

Gentoo has the ability to have multiple JDKs and JREs installed without them conflicting.

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 available JREs and JDKs on your system. Here is an example of output:

# 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. Here is an example of how to set the system VM.

# 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. You can do it like this:

# 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 ~/.gentoo/java-env with all required env vars. You would normally source this from your shell's startup script (generally ~/.bashrc).

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. Here is an example of output:

# java-config --list-available-packages
[bsh] BeanShell is a small, free, embeddable, Java
source interpreter with object scripting language
features. (/usr/share/bsh/package.env)
[oro] A set of text-processing Java classes that
provide Perl5 compatible regular expressions, AWK-like
regular expressions, glob expressions, and utility
classes for performing substitutions, splits, filtering
filenames, etc. (/usr/share/oro/package.env)
[bcel] The Byte Code Engineering Library: analyze, create,
manipulate Java class files (/usr/share/bcel/package.env)
[jsch] JSch is a pure Java implementation of SSH2.
(/usr/share/jsch/package.env)
...

Again, the name in brackets "[]" are the IDs that you have to pass to java-config --set-system-classpath. Here is an example:

# 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 ~/.gentoo/java-env-classpath, which is automatically included by ~/.gentoo/java-env.

For more information regarding USE flags, refer to the USE flags chapter from the Gentoo Handbook.

• The java flag adds support for Java in a variety of programs.
• The mozilla flag adds support for Mozilla-like browsers (including Firefox) if you are using blackdown's Java kit. You will need this for viewing Java applets in your Mozilla-like browser.

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

• The gentoo-dev, gentoo-user mailing list archives
• #gentoo and #gentoo-java on irc.freenode.net
• Wikipedia's entry for Java