Centos java版本变更

先下载jdk源码，再解压:

[root@master ~]tar -zxvf jdk-7u55-linux-x64.tar.gz -C /usr/java/

开始配置前的java 版本:

[root@master ~]#java -version
java version "1.7.0_21"
Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode)

更改java版本

[root@master ~]# /usr/sbin/alternatives --install /usr/bin/java  java  /usr/java/jdk1.7.0_55/bin/java  17055
[root@master ~]# /usr/sbin/alternatives --install /usr/bin/javac javac /usr/java/jdk1.7.0_55/bin/javac 17055

上面两行命令的中最后面的数字表示优先级，建议根据版本号修改，如我的JDK版本为1.7.0_55，则为17055

查看可用的版本信息

[root@master ~]# /usr/sbin/alternatives --display java
java - status is auto.
 link currently points to /usr/java/jdk1.7.0_55/bin/java
/usr/lib/jvm/jre-1.5.0-gcj/bin/java - priority 1500
 slave keytool: /usr/lib/jvm/jre-1.5.0-gcj/bin/keytool
 slave rmiregistry: /usr/lib/jvm/jre-1.5.0-gcj/bin/rmiregistry
 slave jre_exports: /usr/lib/jvm-exports/jre-1.5.0-gcj
 slave jre: /usr/lib/jvm/jre-1.5.0-gcj
/usr/java/jdk1.7.0_55/bin/java - priority 17055
 slave keytool: (null)
 slave rmiregistry: (null)
 slave jre_exports: (null)
 slave jre: (null)
Current `best' version is /usr/java/jdk1.7.0_55/bin/java.

更改版本信息

[root@master ~]# /usr/sbin/alternatives --config java


There are 2 programs which provide 'java'.


  Selection    Command
-----------------------------------------------
   1           /usr/lib/jvm/jre-1.5.0-gcj/bin/java
*+ 2           /usr/java/jdk1.7.0_55/bin/java


Enter to keep the current selection[+], or type selection number: 2


[root@master ~]# java -version
java version "1.7.0_21"
Java(TM) SE Runtime Environment (build 1.7.0_21-b11)
Java HotSpot(TM) 64-Bit Server VM (build 23.21-b01, mixed mode)
[root@master ~]# /usr/sbin/alternatives --config javac


There is 1 program that provides 'javac'.


  Selection    Command
-----------------------------------------------
*+ 1           /usr/java/jdk1.7.0_55/bin/javac


Enter to keep the current selection[+], or type selection number: 1

配置环境变量
    vi /etc/profile
在文件末尾加上
export JAVA_HOME=/usr/java/jdk1.7.0_55
export PATH=$PATH:$JAVA_HOME/bin
    

最后使环境变量生效

[root@master ~]# source /etc/profile
[root@master ~]# java -version
java version "1.7.0_55"
Java(TM) SE Runtime Environment (build 1.7.0_55-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.55-b03, mixed mode)

参考：http://www.gokhanatil.com/2011/07/how-to-installupdate-java-jdk-on-oracle.html