查看已安装JDK：
$ update-java-alternatives -l
java-1.7.0-openjdk-amd64 1071 /usr/lib/jvm/java-1.7.0-openjdk-amd64
java-1.8.0-openjdk-amd64 1069 /usr/lib/jvm/java-1.8.0-openjdk-amd64
切换版本：
$ sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1071 auto mode
1 /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java 1071 manual mode
* 2 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1069 manual mode
Press enter to keep the current choice[*], or type selection number:
//输入数字
切换JDK工具：
$ sudo update-alternatives --config javac $ sudo update-alternatives --config javap $ sudo update-alternatives --config javadoc