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【翻译完成】20151116 Linux FAQs with Answers--How to set JAVA_HOME environment variable automatically on Linux.md
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Linux FAQs with Answers--How to set JAVA_HOME environment variable automatically on Linux
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> **Question**: I need to compile a Java program on my Linux box. For that I already installed JDK (Java Development Kit), and now I'm trying to set JAVA_HOME environment variable to point to the installed JDK. What is the recommended way to set JAVA_HOME environment variable on Linux?
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Many Java programs or Java-based IDE environments require JAVA_HOME environment variable being set. This environment variable is supposed to point to the top directory where the Java development kit (JDK) or Java runtime environment (JRE) is installed. The JDK contains everything the JRE offers, but also provides additional binaries and libraries needed to compile Java programs (e.g., compilers, debugger, JavaDoc). While the JDK is needed to build Java applications, the JRE alone is sufficient to simply run already built Java programs.
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When you are trying to set JAVA_HOME environment variable, the complication is that JAVA_HOME variable will change depending on (1) whether you installed JDK or JRE, (2) which version of JDK/JRE you installed, and (3) whether you installed Oracle JDK or Open JDK.
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So whenever your build or run-time environment changes (e.g., upgrade to a newer JDK), you need to adjust JAVA_HOME accordingly, which is cumbersome.
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The following export commands will allow you to set JAVA_HOME environment variable automatically regardless of these factors.
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If you installed JRE:
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export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which java))))
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If you installed JDK:
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export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which javac))))
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Place either of the above commands in ~/.bashrc (or /etc/profile) depending on you installed JDK or JRE, and it will set JAVA_HOME permanently.
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Note that "readlink -f" command is used to get the canonical path since java or javac can be set up with multiple symlinks.
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For example, if you installed Oracle JRE 7, the first export command will automatically set JAVA_HOME to:
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/usr/lib/jvm/java-7-oracle/jre
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If you installed Open JDK version 8, the second export command will set JAVA_HOME to:
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/usr/lib/jvm/java-8-openjdk-amd64
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In short, these export commands will automatically update JAVA_HOME variable as you re-install/upgrade your JDK/JRE packages or [change default Java version][1]. No need to adjust JAVA_HOME manually.
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--------------------------------------------------------------------------------
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via: http://ask.xmodulo.com/set-java_home-environment-variable-linux.html
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作者：[Dan Nanni][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:http://ask.xmodulo.com/author/nanni
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[1]:http://ask.xmodulo.com/change-default-java-version-linux.html
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Linux 有问必答 - 如何在 Linux 上自动设置 JAVA_HOME 环境变量
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> **问题**：我需要在我的 Linux 机器上编译 Java 程序。为此我已经安装了 JDK (Java Development Kit)，而现在我正试图设置 JAVA\_HOME 环境变量使其指向安装好的 JDK 。关于在 Linux 上设置 JAVA\_HOME 环境变量，最受推崇的办法是什么？
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许多 Java 程序或基于 Java 的*集成开发环境* (IDE)都需要设置好 JAVA_HOME 环境变量。该变量应指向 *Java 开发工具包* (JDK)或 *Java 运行时环境* (JRE)的安装目录。JDK 不仅包含了 JRE 提供的一切，还带有额外的二进制代码和库文件（例如编译器，调试器及 JavaDoc 文档生成器）用于编译 Java 程序。JDK 是用来构建 Java 程序的，如果只是运行已经构建好的 Java 程序，单独一份 JRE 就足够了。
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当您正试图设置 JAVA\_HOME 环境变量时，麻烦的事情在于 JAVA\_HOME 变量需要根据以下几点而改变：(1) 您是否安装了 JDK 或 JRE；(2) 您安装了哪个版本；(3) 您安装的是 Oracle JDK 还是 Open JDK。
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因此每当您的开发环境或运行时环境发生改变（例如为 JDK 更新版本）时，您需要根据实际情况调整 JAVA\_HOME 变量，而这种做法是繁重且缺乏效率的。
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以下 export 命令能为您自动设置 JAVA\_HOME 环境变量，而无须顾及上述的因素。
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若您安装的是 JRE：
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export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which java))))
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若您安装的是 JDK：
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export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which javac))))
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根据您的情况，将上述命令中的一条写入 ~/.bashrc（或 /etc/profile）文件中，它就会永久地设置好 JAVA\_HOME 变量。
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注意，由于 java 或 javac 可以建立起多个层次的符号链接，为此"readlink -f"命令是用来获取它们真正的执行路径的。
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举个例子，假如您安装的是 Oracle JRE 7，那么上述的第一条 export 命令将自动设置 JAVA\_HOME 为：
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/usr/lib/jvm/java-7-oracle/jre
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若您安装的是 Open JDK 第8版，那么第二条 export 命令将设置 JAVA\_HOME 为：
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/usr/lib/jvm/java-8-openjdk-amd64
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简而言之，这些 export 命令会在您重装/升级您的JDK/JRE，或[更换默认 Java 版本][1]时自动更新 JAVA\_HOME 变量。您不再需要手动调整它。
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--------------------------------------------------------------------------------
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via: http://ask.xmodulo.com/set-java_home-environment-variable-linux.html
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作者：[Dan Nanni][a]
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译者：[Ricky-Gong](https://github.com/Ricky-Gong)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:http://ask.xmodulo.com/author/nanni
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[1]:http://ask.xmodulo.com/change-default-java-version-linux.html
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