執行命令：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
選擇 1
然后服務器的全局變量java就是變成
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)
如果只是要更換個別用戶的java環境，則進行以下步驟：
cd ~
vim .profile (加入以下java的路徑)
export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64 主要要看java的根目錄在哪，然后在用戶profile里面加進去就好了
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