第一步：下载 jdk-7u51-linux-x64.tar.gz，并解压
sudo tar -zxvf jdk-7u51-linux-x64.tar.gz
将解压的文件夹移到/usr/lib/jvm，并更名为 java-7-sun
第二步：修改环境变量
export JAVA_HOME=/usr/lib/jvm/java-7-sun
export JRE_HOME=${JAVA_HOME}/jre
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib
export PATH=${JAVA_HOME}/bin:$PATH
export JRE_HOME=${JAVA_HOME}/jre
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib
export PATH=${JAVA_HOME}/bin:$PATH
第三步：配置当前安装的JDK为系统默认
sudo update-alternatives --install
/usr/bin/java java /usr/lib/jvm/java-7-sun/bin/java 300
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/java-7-sun/bin/javac 300
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/java-7-sun/bin/javac 300
sudo update-alternatives --config java
这时，执行：
java -version
可以看到：
java version
"1.7.0_51"
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)
Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)