Ubuntu14.04安装jdk1.8

//新建文件夹
sudo mkdir /usr/java

//解压jdk
sudo tar zxvf jdk-8u25-linux-x64.tar.gz -C /usr/java

//添加环境变量
sudo gedit ~/.bashrc
//在文档末尾添加如下代码：
    export JAVA_HOME=/usr/java 
    export JRE_HOME=${JAVA_HOME}/jre  
    export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib  
    export PATH=${JAVA_HOME}/bin:$PATH 

//配置jdk版本
sudo update-alternatives --install /usr/bin/java java /usr/java/jdk1.8.0_25/bin/java 300  
sudo update-alternatives --install /usr/bin/javac javac /usr/java/jdk1.8.0_25/bin/javac 300  
sudo update-alternatives --install /usr/bin/jar jar /usr/java/jdk1.8.0_25/bin/jar 300   
sudo update-alternatives --install /usr/bin/javah javah /usr/java/jdk1.8.0_25/bin/javah 300   
sudo update-alternatives --install /usr/bin/javap javap /usr/java/jdk1.8.0_25/bin/javap 300 

sudo update-alternatives --config java
//若是初次安装，则会提示没有选项，否则会有选项可选

//测试
java -version
//会有如下输出：
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)