Gentoo Java Guide

1. What is Java?

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.

2. Installing a JRE/JDK

The choices

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.

Installing the Sun/IBM JRE/JDKs

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.

Note: ibm-jdk-bin is currently masked, you may have to unmask it to use it.

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.

3. Configuring your JRE/JDK

Overview

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.

Setting a default JRE/JDK

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:

Code Listing 3.1: Listing available VMs

# 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.

Code Listing 3.2: Setting 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)

Note: You will have to be root to run --set-system-vm.

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:

Code Listing 3.3: Regenerating /etc/profile.env

# 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).

4. USE flags for use with Java

Setting USE flags

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

The flags

The java flag adds support for Java in a variety of programs.
The nsplugin flag adds a symlink to the javaplugin for Mozilla-like browsers (including Firefox) if you are using, for example, Blackdown's Java kit, IBM Java Kit or Sun's Java Kit. You will need this for viewing Java applets in your Mozilla-like browser.

5. Additional resources

Off-line resources