执行命令：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-8-openjdk-amd64/jre/bin/java 1081 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 1081 manual mode

选择 1

然后服务器的全局变量java就是变成

java -version
java version "1.7.0_121"
OpenJDK Runtime Environment (IcedTea 2.6.8) (7u121-2.6.8-1ubuntu0.14.04.3)
OpenJDK 64-Bit Server VM (build 24.121-b00, mixed mode)

如果只是要更换个别用户的java环境，则进行以下步骤：

cd ~
vim .profile (加入以下java的路径)
export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64                        主要要看java的根目录在哪，然后在用户profile里面加进去就好了
export PATH=$JAVA_HOME/bin:$PATH 
export CLASSPATH=.:$JAVA_HOME/lib/dt.jar:$JAVA_HOME/lib/tools.jar 

wq保存退出

souce .profile