一、下载JDK
到官网下载：Oracle JDK 官网
选择 *.tar.gz 对应自己系统的版本
我下载的是：jdk-8u144-linux-x64.tar.gz
jdk-7u80-linux-x64.tar.gz (老版本的JDK在Oracle 官网下载需要登录，在Windows 下用迅雷下载对应的JDK可以不用登录，不过下载后文件名字可能需要改，包括后缀名)
二、解压到指定目录
创建目录：
sudo mkdir /usr/lib/jvm解压到刚才创建的目录：
sudo tar -zxvf jdk-8u144-linux-x64.tar.gz -C /usr/lib/jvm/三、使用update-alternatives进行配置
sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.8.0_144/bin/java 400
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.8.0_144/bin/javac 400
sudo update-alternatives --install /usr/bin/jar jar /usr/lib/jvm/jdk1.8.0_144/bin/jar 400
sudo update-alternatives --install /usr/bin/javah javah /usr/lib/jvm/jdk1.8.0_144/bin/javah 400
sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/jdk1.8.0_144/bin/javap 400初次安装JDK：
$sudo update-alternatives --config java
There is only one alternative in link group java (providing /usr/bin/java): /usr/lib/jvm/jdk1.8.0_144/bin/java
Nothing to configure.验证:
$ java -version
java version "1.8.0_144"
Java(TM) SE Runtime Environment (build 1.8.0_144-b01)
Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)JDK 1.7 那个版本的安装类似。
update-alternatives 设置不同于1.8的优先级。
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0_80/bin/java 300
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.7.0_80/bin/javac 300
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --install /usr/bin/jar jar /usr/lib/jvm/jdk1.7.0_80/bin/jar 300
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --install /usr/bin/javah javah /usr/lib/jvm/jdk1.7.0_80/bin/javah 300
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/jdk1.7.0_80/bin/javap 300
经过上步配置，终端输入
toby@toby-ThinkPad:~/Downloads$ java -version
java version "1.8.0_144"
Java(TM) SE Runtime Environment (build 1.8.0_144-b01)
Java HotSpot(TM) 64-Bit Server VM (build 25.144-b01, mixed mode)使用 sudo update-alternatives –config name 在不同的Java版本之间切换：
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --config java
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/lib/jvm/jdk1.8.0_144/bin/java 400 auto mode
1 /usr/lib/jvm/jdk1.7.0_80/bin/java 300 manual mode
2 /usr/lib/jvm/jdk1.8.0_144/bin/java 400 manual mode
Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/lib/jvm/jdk1.7.0_80/bin/java to provide /usr/bin/java (java) in manual mode
切换后 /usr/bin/java java 就变成手动模式了。其他的不受影响。
此时，终端输入：
toby@toby-ThinkPad:~/Downloads$ java -version
java version "1.7.0_80"
Java(TM) SE Runtime Environment (build 1.7.0_80-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.80-b11, mixed mode)
toby@toby-ThinkPad:~/Downloads$ javac -version
javac 1.8.0_144
toby@toby-ThinkPad:~/Downloads$ ll /etc/alternatives/java*
lrwxrwxrwx 1 root root 33 9月 9 19:21 /etc/alternatives/java -> /usr/lib/jvm/jdk1.7.0_80/bin/java*
lrwxrwxrwx 1 root root 35 9月 9 18:01 /etc/alternatives/javac -> /usr/lib/jvm/jdk1.8.0_144/bin/javac*
lrwxrwxrwx 1 root root 35 9月 9 18:05 /etc/alternatives/javah -> /usr/lib/jvm/jdk1.8.0_144/bin/javah*
lrwxrwxrwx 1 root root 35 9月 9 18:07 /etc/alternatives/javap -> /usr/lib/jvm/jdk1.8.0_144/bin/javap*
终端输入：
toby@toby-ThinkPad:~/Downloads$ sudo update-alternatives --auto java
update-alternatives: using /usr/lib/jvm/jdk1.8.0_144/bin/java to provide /usr/bin/java (java) in auto mode
把Java那项改为 自动模式。
此时再次查看：
toby@toby-ThinkPad:~/Downloads$ ll /etc/alternatives/java*
lrwxrwxrwx 1 root root 34 9月 9 19:22 /etc/alternatives/java -> /usr/lib/jvm/jdk1.8.0_144/bin/java*
lrwxrwxrwx 1 root root 35 9月 9 18:01 /etc/alternatives/javac -> /usr/lib/jvm/jdk1.8.0_144/bin/javac*
lrwxrwxrwx 1 root root 35 9月 9 18:05 /etc/alternatives/javah -> /usr/lib/jvm/jdk1.8.0_144/bin/javah*
lrwxrwxrwx 1 root root 35 9月 9 18:07 /etc/alternatives/javap -> /usr/lib/jvm/jdk1.8.0_144/bin/javap*
参考：
Ubuntu 安装 JDK 7 / JDK8 的两种方式
update-alternatives 使用参考：
update-alternatives详解
update-alternatives命令
update-alternatives命令
