1. 下载安装
2. 设置安装的默认程序
$ sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0_21/bin/java 300
$ sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.7.0_21/bin/javac 300
sudo update-alternatives --config java
sudo update-alternatives --config javac