ubuntu16.04下，JDK的安装，卸载和切换

一 已有jdk卸载

1.whereis java

2.which java
3.sudo rm -rf /usr/local/jdk1.8.0_131
4.sudo gedit /etc/profile
删除代码:  
#set java environment  
export JAVA_HOME=/opt/jdk1.6.0_20
export JRE_HOME=$JAVA_HOME/jre
export CLASSPATH=$CLASSPATH:$JAVA_HOME/lib:$JRE_HOME/lib
export PATH=$PATH:$JAVA_HOME/bin:$JRE_HOME/bin
保存退出。

二 安装和切换JDK

1.官网下载JDK
2. 解压缩,放到指定目录
sudo mkdir /usr/lib/jvm
sudo tar -zxvf ~/下载/jdk-7u79-linux-x64.tar.gz -C /usr/lib/jvm
3.修改环境变量:　
sudo vim ~/.bashrc
文件的末尾追加下面内容:
#set oracle jdk environment
export JAVA_HOME=/usr/lib/jvm/jdk1.7.0_60  ## 这里要注意目录要换成自己解压的jdk 目录
export JRE_HOME=${JAVA_HOME}/jre  
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib  
export PATH=${JAVA_HOME}/bin:$PATH  

使环境变量马上生效
source ~/.bashrc

4.设置系统默认jdk 版本

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0_79/bin/java 300  
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.7.0_79/bin/javac 300  
sudo update-alternatives --install /usr/bin/jar jar /usr/lib/jvm/jdk1.7.0_79/bin/jar 300   
sudo update-alternatives --install /usr/bin/javah javah /usr/lib/jvm/jdk1.7.0_79/bin/javah 300   
sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/jdk1.7.0_79/bin/javap 300   

然后执行:
sudo update-alternatives --config java
若是初次安装jdk,会有下面的提示     
   There is only one alternative in link group java (providing /usr/bin/java):
    /usr/lib/jvm/jdk1.7.0_60/bin/java
否者,选择合适的jdk
5.测试jdk
java -version