到 Sun 的官网下载
http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html
选择 accept license ，然后选择适合自己机型的JDK下载。
解压文件，修改文件名
$ sudo mkdir /usr/lib/jvm
$ sudo tar zxvf jdk-7u21-linux-i586.tar.gz -C /usr/lib/jvm
$ cd /usr/lib/jvm
$ sudo mv jdk1.7.0_21 java
添加环境变量
$ sudo vim ~/.bashrc
加入如下内容
export JAVA_HOME=/usr/lib/jvm/java
export JRE_HOME=${JAVA_HOME}/jre
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib
export PATH=${JAVA_HOME}/bin:$PATH
配置默认JDK版本
sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/java/bin/java 300
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/java/bin/javac 300
sudo update-alternatives --install /usr/bin/jar jar /usr/lib/jvm/java/bin/jar 300
sudo update-alternatives --install /usr/bin/javah javah /usr/lib/jvm/java/bin/javah 300
sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/java/bin/javap 300
然后执行 sudo update-alternatives --config java
若是初次安装 JDK， 将提示There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/java/bin/java无需配置。若是非初次安装，将有不同版本的 JDK 选项。
测试
$ java -version
java version "1.7.0_21"
Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
Java HotSpot(TM) Server VM (build 23.21-b01, mixed mode)