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Linux FAQs with Answers--How to change default Java version on Linux
================================================================================
> **Question**: When I am trying to run a Java program on Linux, I am getting the following error. Looks like the Java program is compiled for a different Java version than the default Java program installed on my Linux. How can I switch the default Java version on Linux?
>
> Exception in thread "main" java.lang.UnsupportedClassVersionError: com/xmodulo/hmon/gui/NetConf : Unsupported major.minor version 51.0
When a Java program is compiled, the build environment sets a "target" which is the oldest JRE version the program can support. If you run the Java program on a Linux system which does not meet the lowest JRE version requirement, you will encounter the following error while starting the program.
Exception in thread "main" java.lang.UnsupportedClassVersionError: com/xmodulo/hmon/gui/NetConf : Unsupported major.minor version 51.0
For example, in this case the program is compiled for Java JRE 1.7 but the system only has Java JRE 1.6.
To solve this problem, you need to change the default Java version you are using to Java JRE 1.7 or higher (assuming that such JRE is already installed).
First, **check available Java versions** on your Linux system by using update-alternatives command:
$ sudo update-alternatives --display java
![](https://c2.staticflickr.com/6/5663/22661333316_81fe1ab7da_c.jpg)
In this example, there are four different Java versions that are installed: OpenJDK JRE 1.6, Oracle Java JRE 1.6, OpenJDK JRE 1.7 and Oracle Java JRE 1.7. The default Java version is currently set to OpenJDK JRE 1.6.
If the necessary Java JRE is not installed, you can always install it using [these instructions][1].
Now that there are suitable candidates to change to, you can **switch the default Java version** among available Java JREs by running the following command:
$ sudo update-alternatives --config java
When prompted, select the Java version you would like to use. In this example, we choose Oracle Java JRE 1.7.
![](https://c2.staticflickr.com/6/5651/22066181083_b9c4c5b676_c.jpg)
Now you can verify the default Java version as follows.
$ java -version
![](https://c1.staticflickr.com/1/634/22499411280_1d702a4101_c.jpg)
Finally, if you defined JAVA_HOME environment variable somewhere, update the variable according to the newly set default Java version.
--------------------------------------------------------------------------------
via: http://ask.xmodulo.com/change-default-java-version-linux.html
作者：[Dan Nanni][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:http://ask.xmodulo.com/author/nanni
[1]:http://ask.xmodulo.com/install-java-runtime-linux.html

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Linux又问必答-- 如何在Linux中改变默认的Java版本
================================================================================
> **提问**：当我尝试在Linux中运行一个Java程序时，我遇到了一个错误。看上去像程序编译所使用的Javab版本与我本地的不同。我该如何在Linux上切换默认的Java版本？
>
> Exception in thread "main" java.lang.UnsupportedClassVersionError: com/xmodulo/hmon/gui/NetConf : Unsupported major.minor version 51.0
当Java程序编译时，编译环境会设置一个“target”变量来设置程序可以运行的最低Java版本。如果你Linux系统上运行的程序不满足最低的JRE版本要求，那么你会在运行的时候遇到下面的错误。
Exception in thread "main" java.lang.UnsupportedClassVersionError: com/xmodulo/hmon/gui/NetConf : Unsupported major.minor version 51.0
比如，这种情况下程序在Java JRE 1.7下编译，但是系统只有Java JRE 1.6。
要解决这个问题，你需要改变默认的Java版本到Java JRE 1.7或者更高（假设JRE已经安装了）。
首先，试用下面的update-alternatives命令**检查你系统上可用的Java版本**：
$ sudo update-alternatives --display java
![](https://c2.staticflickr.com/6/5663/22661333316_81fe1ab7da_c.jpg)
本例中，总共安装了4个不同的Java版本：OpenJDK JRE 1.6、Oracle Java JRE 1.6、OpenJDK JRE 1.7 和 Oracle Java JRE 1.7。现在默认的Java版本是OpenJDK JRE 1.6。
如果没有安装需要的Java JRE,你可以参考[这些指导][1]来完成安装。
现在有可用的候选版本，你可以用下面的命令在可用的Java JRE之间**切换默认的Java版本**：
$ sudo update-alternatives --config java
看到提示的时候，选择你想试用的Java版本。本例中，我们选择Oracle Java JRE 1.7。
![](https://c2.staticflickr.com/6/5651/22066181083_b9c4c5b676_c.jpg)
现在用下面的命令验证默认的Java版本。
$ java -version
![](https://c1.staticflickr.com/1/634/22499411280_1d702a4101_c.jpg)
最后，如果你定义了JAVA_HOME环境变量，根据你设置的Java版本更新变量。
--------------------------------------------------------------------------------
via: http://ask.xmodulo.com/change-default-java-version-linux.html
作者：[Dan Nanni][a]
译者：[geekpi](https://github.com/geekpi)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:http://ask.xmodulo.com/author/nanni
[1]:http://ask.xmodulo.com/install-java-runtime-linux.html