1. 查看系统所安装的JDK版本

root@ubuntu:~# java -version
java version "1.7.0_79"
OpenJDK Runtime Environment (IcedTea 2.5.5) (7u79-2.5.5-0ubuntu1)
OpenJDK 64-Bit Server VM (build 24.79-b02, mixed mode)

2.将JDK上传到/opt目录中

root@ubuntu:~# cd /opt
root@ubuntu:/opt# ls
jdk-6u45-linux-x64.bin

3.修改JDK的权限

root@ubuntu:/opt# chmod +x jdk-6u45-linux-x64.bin

4.执行JDK文件

root@ubuntu:/opt# ./jdk-6u45-linux-x64.bin

5.查看解压出来的目录

root@ubuntu:/opt# ls
jdk1.6.0_45  jdk-6u45-linux-x64.bin

6.创建软链接

root@ubuntu:/opt# ln -s jdk1.6.0_45/ java
root@ubuntu:/opt# ls
java  jdk1.6.0_45

7.修改环境变量配置文件

root@ubuntu:/opt# vim /etc/profile

#set java environment
export JAVA_HOME=/opt/java
export JRE_HOME=/opt/java
export CLASSPATH==.:$CLASSPATH:$JAVA_HOME/lib:$JRE_HOME/lib
export PATH=$PATH:$JAVA_HOME/bin:$JRE_HOME/bin

8.让配置文件生效

root@ubuntu:/opt# source /etc/profile

9.查看java的版本

root@ubuntu:~# java -version
java version "1.7.0_79"
OpenJDK Runtime Environment (IcedTea 2.5.5) (7u79-2.5.5-0ubuntu1)
OpenJDK 64-Bit Server VM (build 24.79-b02, mixed mode)

10.查看新配置的JAVA版本

root@ubuntu:~# /opt/java/bin/java -version
java version "1.6.0_45"
Java(TM) SE Runtime Environment (build 1.6.0_45-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.45-b01, mixed mode)

11.创建链接

root@ubuntu:~# update-alternatives --install /usr/bin/java java /opt/java/bin/java 300
root@ubuntu:~# update-alternatives --install /usr/bin/javac javac /opt/java/bin/javac 300
update-alternatives: using /opt/java/bin/javac to provide /usr/bin/javac (javac) in auto mode

12.配置系统默认的JAVA环境

root@ubuntu:~# update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
* 0            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      auto mode
  1            /opt/java/bin/java                               300       manual mode
  2            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      manual mode

Press enter to keep the current choice[*], or type selection number: 1

root@ubuntu:~# update-alternatives --config javac
There is only one alternative in link group javac (providing /usr/bin/javac): /opt/java/bin/javac
Nothing to configure.

13.查看是否生效

root@ubuntu:~# java -version
java version "1.6.0_45"
Java(TM) SE Runtime Environment (build 1.6.0_45-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.45-b01, mixed mode)