doc/en/java.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/java.xml,v 1.49 2010/07/06 11:51:28 jkt Exp $ -->

<guide>
<title>Gentoo Java Guide</title>

<author title="Author">
 <mail link="nichoj@gentoo.org">Joshua Nichols</mail>
</author>
<author title="Author">
 <mail link="karltk@gentoo.org">Karl Trygve Kalleberg</mail>
</author>
<author title="Editor">
 <mail link="nightmorph@gentoo.org">Joshua Saddler</mail>
</author>

<abstract>
This guide will introduce you to Java and explain how to use Java with Gentoo
Linux.
</abstract>

<!-- The content of this document is licensed under the CC-BY-SA license -->
<!-- See http://creativecommons.org/licenses/by-sa/2.5 -->
<license/>

<version>1</version>
<date>2011-05-09</date>

<chapter>
<title>What is Java?</title>
<section>
<title>Overview</title>
<body>

<p>
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. To make
platform independence possible the Java compiler compiles the Java code to an
intermediate representation called "Java bytecode" that runs on a JRE (Java
Runtime Environment) and not directly on the operating system.
</p>

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

</body>
</section>
</chapter>

<chapter>
<title>Before You Begin</title>
<section>
<title>Existing installs</title>
<body>

<p>
For existing installs, regardless of if you have installed anything Java
before, make sure you have followed the <uri
link="/proj/en/java/java-upgrade.xml">Java Upgrade Guide</uri>.
</p>

</body>
</section>
<section>
<title>New installs</title>
<body>

<p>
New installs should require no further preparation.
</p>

</body>
</section>
</chapter>

<chapter>
<title>Installing a Virtual Machine</title>
<section>
<title>The choices</title>
<body>

<p>
Gentoo provides numerous Runtime Environments (JREs) and Development Kits
(JDKs). Among the current choices, we have:
</p>

<table>
<tr>
  <th>Vendor</th>
  <th>JDK</th>
  <th>JRE</th>
</tr>
<tr>
  <ti>Sun's Java Kit</ti>
  <ti>dev-java/sun-jdk</ti>
  <ti>dev-java/sun-jre-bin</ti>
</tr>
<tr>
  <ti>The IBM Java Kit</ti>
  <ti>dev-java/ibm-jdk-bin</ti>
  <ti>dev-java/ibm-jre-bin</ti>
</tr>
<tr>
  <ti>BEA WebLogic's J2SE Development Kit</ti>
  <ti>dev-java/jrockit-jdk-bin</ti>
</tr>
<tr>
  <ti>The IcedTea Open Java Kit</ti>
  <ti>dev-java/icedtea</ti>
  <ti>dev-java/icedtea6-bin</ti>
</tr>
</table>

<!--
TODO: list free implementations?
note about not drop-in replacements
kaffe/sablevm/gcj/jamvm
-->
</body>
</section>
<section>
<title>Installing a JRE/JDKs</title>
<body>

<p>
To install your profile's default JDK, you can run <c>emerge virtual/jdk</c>.
Or to install your profile's default JRE, you can <c>emerge virtual/jre</c>.
</p>

<p>
Some JDKs and JREs, including the Sun packages, require accepting an End User
License Agreement, or EULA. If its license (such as dlj-1.1) is not listed in
ACCEPT_LICENSE in <path>/etc/make.conf</path>, then you won't be able to install
the JDK/JRE. For more information on how to add acceptable licenses to
<path>make.conf</path>, please read the <uri
link="/doc/en/handbook/handbook-x86.xml?part=2&amp;chap=1#doc_chap4">Portage
Handbook</uri>.
</p>

<p>
To avoid any restrictive license hassle, consider installing <c>icedtea6-bin</c>
or <c>icedtea</c>, which are more open Java implementations from the OpenJDK
project.
</p>

<note>
A JDK also includes a JRE, so if you install a JDK you shouldn't have to also
have to install a JRE.
</note>

</body>
</section>
<section>
<title>Installing fetch-restricted virtual machines</title>
<body>

<p>
Some of the JDKs and JREs require you to jump through a few hoops before
installing. Simply emerge the packages as you normally would.  The ebuilds will
then instruct you where to go and what to download.
</p>

<p>
You should download the indicated file(s) into
<path>/usr/portage/distfiles</path>. Once there, you can rerun the emerge
command, at which point the JRE/JDK will be begin to install.
</p>

</body>
</section>
</chapter>

<chapter>
<title>Configuring your virtual machine</title>
<section>
<title>Overview</title>
<body>

<p>
Gentoo has the ability to have multiple JDKs and JREs installed without causing
conflicts.
</p>

<p>
Using the <c>java-config</c> tool, you can set the system-wide default
(provided you have root access). Users can also use <c>java-config</c> to set
up their own personal default.
</p>

<note>
You can also use <e>eselect</e> to change the system and user vm.  See
<c>eselect java-vm help</c>.
</note>

</body>
</section>
<section>
<title>Setting a default virtual machine</title>
<body>

<p>
Running the command <c>java-config --list-available-vms</c> will give you a
list of all JREs and JDKs installed on your system. Here is an example of
output:
</p>

<pre caption="Listing available VMs">
# <i>java-config --list-available-vms</i>
The following VMs are available for generation-2:
1)      IcedTea6-bin 1.4.1 [icedtea6-bin]
2)      Sun JDK 1.5.0.20 [sun-jdk-1.5] <comment>(Build Only)</comment>
*)      Sun JDK 1.6.0.16 [sun-jdk-1.6]
</pre>

<note>
VMs marked as Build Only may contain security vulnerabilities and/or be EOL.
Gentoo recommends not setting these VMs as either your system or user VM.
Please see <uri link="java.xml#build-only">Build Only VM</uri> for more
information.
</note>

<p>
The <e>*</e> indicates this is the current active vm (system-vm or user-vm when
set). The name in the brackets (<e>[]</e>) is the handle or ID for that
particular VM. You use the handle or the number to <c>java-config
--set-system-vm</c>. Here is an example of how to set the system VM.
</p>

<pre caption="Setting the System VM">
<comment>(By handle (preferred))</comment>
# <i>java-config --set-system-vm sun-jdk-1.6</i>
Now using sun-jdk-1.6 as your generation-2 system JVM
<comment>(By number)</comment>
# <i>java-config --set-system-vm 3</i>
Now using sun-jdk-1.6 as your generation-2 system JVM
</pre>

<p>
As a regular user, you can use <c>java-config --set-user-vm</c>.
</p>

<note>
You no longer have to <c>source</c> the profile for updates to the user/system
VM take place.
</note>

</body>
</section>
<section id="build-only">
<title>Build Only VM</title>
<body>

<p>
Some virtual machines are flagged as build-only due to being EOL and/or
containing security vulnerabilities. These virtual machines will not
automatically be used by Gentoo for the running of applications using Gentoo
launchers but will still be available for use by Gentoo's build environment as
some packages may require them for building. The setting of these virtual
machines as either your system or user VM is strongly discouraged as these VMs
will then be used when running the <path>/usr/bin/{java,javac,..}</path>
executables and will also be used by any packages not using Gentoo's launcher
scripts.
</p>

</body>
</section>
<section id="preferred-vm">
<title>Preferred Build VM</title>
<body>

<p>
While merging Java packages, the VM used for building can sometimes be different
from the one currently set as the system VM.
</p>

<p>
This merge time VM switching is needed when, for example, your system-vm is
set to a 1.6 VM and the package you are merging requires a 1.5 VM. While merging
it will select and use a 1.5 VM, leaving your system-vm choice intact.
</p>

<p>
To define which VM is selected when a switch is needed, we have created a list of <e>default/supported
VMs</e> per arch. You can find them in
<path>/usr/share/java-config-2/config/jdk-defaults.conf</path>. 
</p>

<p>
You can override these defaults (and even your selected system VM) in
<path>/etc/java-config-2/build/jdk.conf</path> and have complete control over
which VM will get used for merging. Some examples:
</p>

<pre caption="Example /etc/java-config-2/build/jdk.conf">
<comment>(I always want it to use a sun-jdk, ie sun-jdk-1.4 for 1.4, sun-jdk-1.5 for 1.5, etc)</comment>
*=sun-jdk
</pre>

<pre caption="Example /etc/java-config-2/build/jdk.conf">
<comment>(Always use sun-jdk-1.5 wherever possible, except for when a 1.4 or 1.3 VM is explicitly required)</comment>
*=sun-jdk-1.5
</pre>

<pre caption="Example /etc/java-config-2/build/jdk.conf">
<comment># For 1.3 I prefer sun-jdk 1.4 but when it is not available, use ibm-jdk-bin,
# For 1.5, use sun-jdk </comment>
1.3=sun-jdk-1.4 ibm-jdk-bin
1.5=sun-jdk
</pre>

<warn>
You do not <e>have</e> to edit this file. If you change these options to use a
unsupported VM, things could possibly break. Because of the wide variety of
available VMs, we do not have the resources to test and verify every package
works on all of them.  Bugs reported with a unsupported VM won't be prioritized
as much as bugs present within supported VMs.
</warn>

</body>
</section>
</chapter>

<chapter>
<title>Compilers</title>
<section>
<body>

<p>
The standard Java compiler used for building is <c>javac</c>, which comes with
each JDK. In addition to configuring the VM used at build time, it is also
possible configure which compiler is used. Essentially, you define a list your
preference for which compiler to use in
<path>/etc/java-config-2/build/compilers.conf</path>.
</p>

<pre caption="/etc/java-config-2/build/compilers.conf">
# If the ebuild supports it
# it will check the COMPILERS var front to back and
# use the first compiler that is installed

COMPILERS="ecj-X.Y jikes javac"
</pre>

<p>
Some compilers don't support all possible -target and -source arguments.
Therefore, each compiler in the list is checked to see if it can support the
desired -source/-target. javac will work in all cases, so if no other suitable
compiler is found, it will be used instead.
</p>

<p>
More details about each compiler are provided below:
</p>

<table>
<tr>
  <th>Name</th>
  <th>Handle</th>
  <th>Package</th>
  <th>Description</th>
</tr>
<tr>
  <ti>javac</ti>
  <ti>javac</ti>
  <ti>N/A</ti>
  <ti>
    This is the default compiler that will be used, and comes with each JDK.
  </ti>
</tr>
<tr>
  <ti>jikes</ti>
  <ti>jikes</ti>
  <ti>dev-java/jikes</ti>
  <ti>
    Jikes was originally developed by IBM. Anecdotally, it is generally quicker
    than javac. Note however, that it is more pedantic, and will fail under a
    few circumstances where javac has no issue. It also does not support Java
    1.5 syntax yet.
  </ti>
</tr>
<tr>
  <ti>Eclipse Compiler for Java</ti>
  <ti>ecj</ti>
  <ti>dev-java/eclipse-ecj</ti>
  <ti>
    ECJ is the compiler used by the Eclipse software development kit. It is
    very full featured, and is pretty fast. It does support Java 1.5 syntax.
  </ti>
</tr>
</table>

</body>
</section>
</chapter>

<chapter>
<title>Setting a default CLASSPATH</title>
<section>
<body>

<warn>
The options explained in this section should be considered deprecated and will
most likely be removed in the future. We strongly recommend against using
these, because your Java projects or application should ideally manage their
own classpaths. If you choose to specify a default CLASSPATH, some applications
may behave unexpectedly, because classes they weren't expecting would be on the
classpath.
</warn>

<p>
<c>java-config</c> can also be used to set a system-wide default CLASSPATH, as
well a user-specific default CLASSPATH.
</p>

<p>
First, you will want to list available Java libraries installed on your system
that might want to be put in your CLASSPATH. Here is an example of output:
</p>

<pre caption="Listing classes">
# <i>java-config --list-available-packages</i>
[xerces-2] The next generation of high performance, fully compliant XML parsers in the Apache Xerces family (/usr/share/xerces-2/package.env)
[junit] Simple framework to write repeatable tests (/usr/share/junit/package.env)
[bsh] BeanShell: A small embeddable Java source interpreter (/usr/share/bsh/package.env)
[bcel] The Byte Code Engineering Library: analyze, create, manipulate Java class files (/usr/share/bcel/package.env)
[log4j] A low-overhead robust logging package for Java (/usr/share/log4j/package.env)
...
</pre>

<p>
Again, the names in brackets (<e>[]</e>) are the IDs that you have to pass to
<c>java-config --set-system-classpath</c>. Here is an example:
</p>

<pre caption="Setting classpaths">
# <i>java-config --set-system-classpath log4j,xerces-2</i>
</pre>

<note>
The current directory (<path>.</path>) will not be part of the system
classpath, as that should be added in your system's login profile.
</note>

<p>
You will have to update your environment by logging out, then in again or
sourcing <path>/etc/profile</path>.
</p>

<p>
For users, <c>java-config --set-user-classpath</c> will create
<path>~/.gentoo/java-env-classpath</path>, which you should then source from
your shell's profile.
</p>

<pre caption="Sourcing user specific classpath">
<i>if [[ -f "${HOME}/.gentoo/java-env-classpath" ]]; then
       source ${HOME}/.gentoo/java-env-classpath
fi</i>
</pre>

<p>
If you really want a system wide or user default classpath you can add
something like the following to your shell's profile. But we would advise
against it.
</p>

<pre caption="Setting classpath">
# <i>export CLASSPATH="${CLASSPATH}:$(java-config --classpath log4j,xerces-2)"</i>
</pre>

</body>
</section>
</chapter>

<chapter>
<title>Java Browser Plugins</title>
<section>
<title>Installing a plugin</title>
<body>

<p>
You can install a Java plugin for your web browser by emerging a Java VM with
the <c>nsplugin</c> USE flag set.
</p>

<note>
<c>nsplugin</c> is not available for all architectures. Check for available
plugins on your arch before trying to install a VM by running <c>emerge -pv
&lt;java-vm&gt;</c>.
</note>

<p>
Portage will allow you to install multiple versions of Java plugins, though
only one will be used by your browser. You can check the list of available
plugins by running:
</p>

<pre caption="Viewing available plugins">
# <i>eselect java-nsplugin list</i>
   [1]   sun-jre-bin-1.6
   [2]   icedtea6-bin
</pre>

<p>
In this example, <c>sun-jre-bin</c> is selected for the browser plugin.
</p>

<pre caption="Selecting a plugin">
# <i>eselect java-nsplugin set sun-jre-bin-1.6</i>
</pre>

<p>
Verify that the correct plugin was selected:
</p>

<pre caption="Verifying the correct plugin">
# <i>eselect java-nsplugin list</i>
   [1]   sun-jre-bin-1.6  current
   [2]   icedtea6-bin
</pre>

<p>
Java.com also provides a link to <uri
link="http://java.com/en/download/installed.jsp">verify your installed
plugin</uri>. Additionally, if you are using a Mozilla-based browser, you can
verify your Java plugin by typing <c>about:plugins</c> into the address bar.
</p>

</body>
</section>
<section>
<title>Plugins on multilib systems</title>
<body>

<p>
If you are running a mixed 64-bit and 32-bit multilib system (for example, on
AMD64), you can use 64-bit and 32-bit Java plugins. Unless you have a pressing
need to run 32-bit Java applications, we recommend using native 64-bit plugins
on 64-bit web browsers.
</p>

<p>
There are several native 64-bit browser plugins available. The default JDK/JRE
pair, <c>sun-jdk</c> and <c>sun-jre-bin</c>, both include browser plugins. Just
emerge one of them with the <c>nsplugin</c> USE flag enabled.
</p>

<pre caption="Installing a 64-bit plugin">
# <i>echo "dev-java/sun-jre-bin nsplugin" >> /etc/portage/package.use</i>
# <i>emerge sun-jre-bin</i>
</pre>

<p>
To use a 32-bit plugin on a 32-bit browser, you will need to emerge
<c>emul-linux-x86-java</c> with the <c>nsplugin</c> USE flag enabled.
</p>

<pre caption="Installing a 32-bit plugin">
# <i>echo "app-emulation/emul-linux-x86-java nsplugin" >> /etc/portage/package.use</i>
# <i>emerge emul-linux-x86-java</i>
</pre>

<p>
Next, check which plugins are available:
</p>

<pre caption="Viewing available plugins">
# <i>eselect java-nsplugin list</i>
Available 32-bit Java browser plugins
  [1]   emul-linux-x86-java-1.5
  [2]   emul-linux-x86-java-1.6
Available 64-bit Java browser plugins
  [1]   icedtea6-bin
  [2]   sun-jre-bin-1.6
</pre>

<p>
Now select the right plugin for your browsers:
</p>

<pre caption="Selecting plugins">
<comment>(Choose the plugins for 32-bit and 64-bit browsers)</comment>
# <i>eselect java-nsplugin set 32bit emul-linux-x86-java-1.6</i>
# <i>eselect java-nsplugin set 64bit sun-jre-bin-1.6</i>
</pre>

<p>
Verify the correct plugin was selected:
</p>

<pre caption="Verifying the correct plugin">
# <i>eselect java-nsplugin list</i>
Available 32-bit Java browser plugins
  [1]   emul-linux-x86-java-1.5
  [2]   emul-linux-x86-java-1.6  current
Available 64-bit Java browser plugins
  [1]   icedtea6-bin
  [2]   sun-jre-bin-1.6  current
</pre>

</body>
</section>
</chapter>

<chapter>
<title>USE flags for use with Java</title>
<section>
<title>Setting USE flags</title>
<body>

<p>
For more information regarding USE flags, refer to the <uri
link="/doc/en/handbook/handbook-x86.xml?part=2&amp;chap=2">USE flags</uri>
chapter from the Gentoo Handbook.
</p>

</body>
</section>
<section>
<title>The flags</title>
<body>

<ul>
  <li>The <b>java</b> flag adds support for Java in a variety of programs</li>
  <li>
    The <b>nsplugin</b> flag adds support for Mozilla-like browsers (including
    Firefox). You will need this for viewing Java applets in your Mozilla-like
    browser.
  </li>
  <li>
    The <b>source</b> flag installs a zip of the source code of a package.
    This is traditionally used for IDEs to 'attach' source to the libraries you
    are using.
  </li>
  <li>The <b>jce</b> flag adds support for the Java Cryptography Engine</li>
  <li>
    For Java packages, the <b>doc</b> flag will build API documentation using
    javadoc.
  </li>
</ul>

</body>
</section>
</chapter>

<chapter>
<title>Additional resources</title>
<section>
<title>Off-line resources</title>
<body>

<ul>
  <li>java-config man page</li>
  <li><c>java-config --help</c></li>
</ul>

</body>
</section>
<section>
<title>Online resources</title>
<body>

<ul>
  <li>
    The <uri link="http://www.gentoo.org/proj/en/java/">Java Project
    Page</uri>
  </li>
  <li>
    The <uri
    link="http://news.gmane.org/gmane.linux.gentoo.java">gentoo-java</uri>,
    <uri
    link="http://news.gmane.org/gmane.linux.gentoo.user">gentoo-user</uri>, and
    <uri
    link="http://news.gmane.org/gmane.linux.gentoo.devel">gentoo-dev</uri>
    mailing list archives
  </li>
  <li>
    <uri link="irc://irc.gentoo.org/gentoo">#gentoo</uri> and <uri
    link="irc://irc.gentoo.org/gentoo-java">#gentoo-java</uri> on IRC
  </li>
  <li>
    <uri
    link="http://en.wikipedia.org/wiki/Java_programming_language">Wikipedia's
    entry for Java</uri>
  </li>
  <li>
    If you have suggestions or questions regarding this document, please email
    the Gentoo Java team: <mail>java@gentoo.org</mail>
  </li>
</ul>

</body>
</section>
</chapter>
</guide>