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Java是一种编程语言，最初由Sun Microsystems开发，它使用一个运行时允许在各种平台上运行Java开发的应用程序。对于想要创建跨平台应用程序的开发人员来说，这是一个受欢迎的选择。

什么是 Java?

概观

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.

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.

安装虚拟机

选择

Gentoo提供了许多Java运行环境（JRE）和Java开发工具包（JDK）。目前的选择包括：

Vendor	JDK	JRE
The IcedTea Open Java SE (formerly icedtea6-bin)	dev-java/icedtea and dev-java/icedtea-bin
Oracle's Java	dev-java/oracle-jdk-bin	dev-java/oracle-jre-bin
The GCJ IcedTea GNU GCC Java (currently IcedTea-7)	dev-java/icedtea sys-devel/gcc USE="gcj"

安装 JRE/JDK

安装profile的默认JDK ，请运行：

root #emerge --ask virtual/jdk

安装profile的默认JRE ，请运行：

root #emerge --ask virtual/jre

Some JDKs and JREs, including the Oracle packages, require accepting an End User License Agreement, or EULA. If its license (such as dlj-1.1) is not listed in the ACCEPT_LICENSE variable (found in /etc/portage/make.conf), then the JDK/JRE will be unable to be installed. For more information on how to add acceptable licenses to make.conf 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.

Note
请注意，每个“JDK”将包含一个“JRE”;如果已经emerge JDK，则不需要安装JRE。

安装 IcedTea GCJ 虚拟机

Important
gcj is deprecated and is removed in GCC versions 7.0 and greater. It is unavailable from GCC 6.0 on Gentoo. Consider this section deprecated.

The IcedTea virtual machine is provided in source code and requires compilation by a Java bytecode compiler. This bootstrapping can be executed by the GNU Compiler for Java (GCJ). This compiler is available when GCC is built with the gcj USE flag.

When GCC is rebuilt with this USE flag set, the dev-java/icedtea package can be installed. Because of a Portage bug, users need to install dev-java/gcj-jdk and dev-java/ant-core explicitly first if they are not already present.

root #emerge --ask --oneshot dev-java/gcj-jdk dev-java/ant-core

root #emerge --ask dev-java/icedtea:7

安装fetch-restricted虚拟机

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 /usr/portage/distfiles 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 (sometimes also referred to as headless) JRE, a few USE flags need to be changed for the selected JRE flavor.

FILE /etc/portage/package.useRequired USE flag changes

dev-java/icedtea headless-awt -alsa -cups
dev-java/icedtea-bin headless-awt -alsa -cups
dev-java/oracle-jre-bin headless-awt -alsa -cups

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:

user $emerge --pretend --verbose virtual/jre

配置java虚拟机

概观

Gentoo has the ability to have multiple JDKs and JREs installed without causing conflicts.

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.

Note
eselect can also be used to change the system and user VM. See eselect java-vm help.

设置默认虚拟机

Running the java-config command with the --list-available-vms option will output a list of all JREs and JDKs installed on the system. Here is an example of the output:

root #java-config --list-available-vms

The following VMs are available for generation-2:
*      IcedTea JDK 7.2.6.3 [icedtea-7]
*      IcedTea JDK 3.0.1 [icedtea-8]

Note
VMs marked as 'Build Only' may contain security vulnerabilities and/or be end of life (EOL). Gentoo recommends not setting these VMs as the system's or the user's VM. Please see Build Only VM for more information.

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.

Setting the system VM by handle (preferred):

root #java-config --set-system-vm icedtea-7

Now using icedtea-7 as your generation-2 system JVM

Alternate method: select VM by number handle number:

root #java-config --set-system-vm 3

Now using icedtea-7 as your generation-2 system JVM

As a regular user, use java-config --set-user-vm.

Note
source-ing the profile for existing shell sessions is not usually needed when changing the user or system VM. The only exception is that variables such as JAVA_HOME will still point to the old location after setting a user VM for the first time or revert back to the system VM.

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 /usr/bin/{java,javac,..} executables, as well as used by any packages not using Gentoo's launcher scripts.

设置默认 CLASSPATH 变量

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 CLASSPATH variable. Here is an example of output:

root #java-config --list-available-packages

[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)
...

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

root #java-config --set-system-classpath log4j,xerces-2

Note
The current directory (.) will not be part of the system classpath, as that should be added to the system's login profile.

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

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

CODE Sourcing user specific classpath

if [[ -f "${HOME}/.gentoo/java-env-classpath" ]]; then
       source ${HOME}/.gentoo/java-env-classpath
fi

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

root #export CLASSPATH="${CLASSPATH}:$(java-config --classpath log4j,xerces-2)"

Java浏览器插件

The Java browser plugin used to be managed via eselect java-nsplugin. This has now been removed in favor of a simpler out-of-the-box experience. For basic usage, simply emerge the chosen JVM with the nsplugin USE flag enabled. Note that Oracle's plugin is only available for amd64 and x86.

Also note that Chromium-based browsers no longer support NPAPI-based plugins since the code was removed in September 2015. The list of supporting browsers is shrinking but still includes Firefox as of version 46.

For more information, including JVM selection, Web Start, and multilib, see the README installed with icedtea-web.

USE flags for use with Java

设置 USE 标签

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

USE 标记

• The java flag adds support for Java in a variety of programs;
• The nsplugin flag adds support for Mozilla-like browsers (including Firefox). This is needed for viewing Java applets in a Mozilla-like browser;
• The jce flag adds support for the Java Cryptography Engine;

Following USE flags go in JAVA_PKG_IUSE.

• The source 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;
• For Java packages, the doc flag will build API documentation using javadoc.

另请参阅

• Java Developer Guide
• Project:Java/Why_build_from_source
• Project:Java/Getting_Involved

外部资源

• The gentoo-java, gentoo-user, and gentoo-dev mailing list archives
• #gentoo and #gentoo-java on IRC
• Java 7 and bootstrapping icedtea on the Gentoo forums

更多信息可以离线查找：

• man java-config
• java-config --help

有关本文档的建议或问题，请发送电子邮件至Gentoo Java团队：java@gentoo.org

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This page is based on a document formerly found on our main website gentoo.org.
The following people contributed to the original document: Joshua Nichols, Karl Trygve Kalleberg, Joshua Saddler (nightmorph)
They are listed here because wiki history does not allow for any external attribution. If you edit the wiki article, please do not add yourself here; your contributions are recorded on each article's associated history page.