Java/de

This guide introduces the reader to Java and explains how to use Java with Gentoo Linux.

Übersicht
Java ist eine Programmiersprache die von Sun Microsystems entwickelt wurde. Die Sprache ist objektorientiert. Javaprogramme sind auf verschiedenen Platformen lauffähig, ohne das sie neu kompiliert werden müssen. Es ist zwar möglich Javacode für spezielle Platformen zu kompilieren, aber gerade die Portabilität ist, neben etwa dem Garbage Collector, ein Grund für Javas Popularität. Um die Platformunabhängigkeit zu gewährleisten, erstellt der Javacompiler sogenannten "Java Bytecode". Dies ist eine Zwischenstufe zwischen Quelltext und ausführbarem Binärformat. Dieser Bytecode wird nicht direkt sondern in einer Java Laufzeitumgebung (JRE) ausgeführt.

Um Java Bytecode auszuführen, muss eine Java Laufzeitumgebung (JRE) installiert sein. Sie stellt die nötigen Bibliotheken sowie die platformabhängige Java Virtual Machine (JVM) bereit. Ein Java Development Kit (JDK) enthält zusätzliche Entwicklungswerkzeuge, etwa einen Java Bytecode Compiler und einen Debugger.

Die Auswahl
Gentoo provides numerous Runtime Environments (JREs) and Development Kits (JDKs). The current choices include:

Eine JRE/JDK installieren
To install the profile's default JDK run emerge virtual/jdk. Or run emerge virtual/jre to install the profile's default JRE.

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 the  variable (found in ), then  the JDK/JRE will be unable to be installed. For more information on how to add acceptable licenses to read the Licenses chapter of the Portage Handbook.

To avoid any restrictive license hassle, consider installing icedtea-bin, which is an open Java implementation from the OpenJDK project.

Installation von "fetch-restricted" virtuellen Maschinen
Some of the JDKs and JREs require a few extra steps in their configurations. Emerge the packages as normal. If additional steps are required the ebuilds will provide instruction for the user on where to go and what to download.

Download the indicated file(s) into Once the files are in the right directories, rerun the emerge command, at which point the JRE/JDK will be begin to install.

Setting up a headless JRE
Sometimes there is no need for a full JRE with all the capabilities of java. Using java on a server often does not require any GUI, graphical, sound or even printer related features. To install a simplified (also called headless) JRE 3 USE flags need to be unset.

Depending on the current Gentoo profile, this might already be the case. As usual, the USE flag settings that are applicable to a particular package can be checked by running emerge in pretend mode:

Übersicht
Gentoo bietet die Möglichkeit mehrere JDKs oder JREs parallel zu benutzen.

Using the java-config tool with root privileges, a system-wide default java virtual machine (VM) can be set. Users can also use java-config to custom set their personal VM on a user-by-user basis.

Setzen der Standard virtuellen Maschine
Running the java-config --list-available-vms command will output a list of all JREs and JDKs installed on the system. Here is an example of the output:

The * indicates this is the current active JVM (system-vm or user-vm when set). The name in the brackets ([]) is the handle or ID for that particular VM. The handle or the number to java-config --set-system-vm can be used to set the VM. The following text provides an example of how to set the system VM.

Setzen der System-VM via Handle (bevorzugte Methode):

Alternate method: select VM by number handle number:

As a regular user, use java-config.

Build-Only-VM
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 (run-java-tool script designed for switching VMs), 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 the system or user VM is strongly discouraged as these VMs will then be used when running the executables, as well as used by any packages not using Gentoo's launcher scripts.

Bevorzugte Build-VM
Beim Bauen von Java-Packeten, kann die verwendete VM von der als System-VM gesetzten abweichen.

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

To define which VM is selected when a switch is needed, a list of default/supported VMs per arch has been created. It can be found in.

These defaults can be over written (even the selected system VM) in for complete control over which VM will get used for merging.

For example, to always use a sun-jdk:

Or, to always use sun-jdk-1.5 wherever possible, except for when a 1.4 or 1.3 VM is explicitly required:

Or to use different providers for different versions, such as requiring  if a 1.3 (yes, 1.3) VM is asked, and fall back to ibm-jdk-bin otherwise:

Compiler
The standard Java compiler used for building is javac, 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, define a list with preference for which compiler to use in.

Some compilers do not support all possible  and   arguments. Therefore, each compiler in the list is checked to see if it can support the desired /. javac will work in all cases, so if no other suitable compiler is found, it will be used instead.

Die folgende Tabelle enthält eine kurze Beschreibung einiger Compiler:

Setzen des standard Klassenpfads, CLASSPATH
java-config can also be used to set a system-wide default CLASSPATH, as well a user-specific default CLASSPATH.

First, list available Java libraries installed on the system to possibly put in the  variable. Here is an example of output:

Again, the names in brackets ([]) are the IDs to pass to java-config --set-system-classpath. Here is an example:

Update the environment by logging out, then in again or by typing source.

For users, java-config --set-user-classpath will create, which should then source from the shell's profile.

If desiring a system wide or user default classpath add something like the following to the shell's profile. This is advised against:

Installation eines Plugins
It is possible to install a Java plugins for a web browsers by emerging a Java VM with the  USE flag set.

Portage will allow installations of multiple Java plugins versions, though only one will be used by the web browser. Check the list of available plugins by running:

In this example, sun-jre-bin is selected for the browser plugin.

Wie folgt kann überprüft werden, ob das Setzen erfolgreich war:

Java.com also provides a link to verify the installed plugin. Additionally, if a Mozilla-based browser is being used, verification of the Java plugin can be performed by typing about:plugins into the address bar.

Plugins auf Multilib-Systemen
If running a mixed 64-bit and 32-bit multilib system (for example, on AMD64), use both 64-bit and 32-bit Java plugins. Unless there is a pressing need to run 32-bit Java applications, users have been recommended to use native 64-bit plugins on 64-bit web browsers.

There are several native 64-bit browser plugins available. The default JDK/JRE pair,  and , both include browser plugins. Just emerge one of them with  USE enabled.

To use a 32-bit plugin on a 32-bit browser, the package will need to be emerged with   USE enabled.

Eine Liste mit verfügbaren Plugins erhält man durch:

Now select the right plugin for the browser:

Eine erneute Auflistung zeigt die aktuelle Wahl:

Setzen der USE-Flags
For more information regarding USE flags, refer to the USE flags chapter from the Gentoo Handbook.

The USE flags

 * The  flag adds support for Java in a variety of programs;
 * The  flag adds support for Mozilla-like browsers (including Firefox). This is needed for viewing Java applets in a Mozilla-like browser;
 * The  flag installs a zip of the source code of a package. This is traditionally used for IDEs to 'attach' source to the libraries that are being use;
 * The  flag adds support for the Java Cryptography Engine;
 * For Java packages, the  flag will build API documentation using javadoc.

Offline verfügbares Material

 * man java-config
 * java-config --help

Online Material

 * The Java Project Page
 * The gentoo-java, gentoo-user, and gentoo-dev mailing list archives
 * #gentoo and #gentoo-java on IRC
 * Wikipedia's entry for Java
 * For suggestions or questions regarding this document, please email the Gentoo Java team: