系統是 Debian8, Jessie, 預設java版本是：OpenJDK 1.7

1. 檢查當前系統的java版本

[email protected]:~$ java -version
java version "1.7.0_65"
OpenJDK Runtime Environment (IcedTea 2.5.2) (7u65-2.5.2-4)
OpenJDK 64-Bit Server VM (build 24.65-b04, mixed mode)

2. 配置安裝java：

[email protected]:~/Downloads$ tar zxf jdk-7u67-linux-x64.tar.gz

3. 編輯/etc/profile，新增環境變數：

[email protected]:~/Downloads$ sudo vi /etc/profile
export JAVA_HOME=/usr/lib/jvm/jdk1.7.0_67
export JRE_HOME=${JAVA_HOME}/jre
export CLASSPATH=.:${JAVA_HOME}/lib:${JRE_HOME}/lib
export PATH=${JAVA_HOME}/bin:$PATH

4. 使用環境變數生效，不必重新登入shell：

[email protected]:~/Downloads$ source /etc/profile

5. 配置JDK1.7的java，javac工具：
[email protected]:~$ sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.7.0_67/bin/java 300
[email protected]:~$ sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.7.0_67/bin/javac 300
update-alternatives: using /usr/lib/jvm/jdk1.7.0_67/bin/javac to provide /usr/bin/javac (javac) in auto mode

6. 設定系統預設的java版本，這裡選擇2：
[email protected]:~$ 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/java-7-openjdk-amd64/jre/bin/java   1071      auto mode
  1            /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java   1071      manual mode
  2            /usr/lib/jvm/jdk1.7.0_67/bin/java                300       manual mode

Press enter to keep the current choice[*], or type selection number: 2          
update-alternatives: using /usr/lib/jvm/jdk1.7.0_67/bin/java to provide /usr/bin/java (java) in manual mode

[email protected]:~$ sudo update-alternatives --config javac
There is only one alternative in link group javac (providing /usr/bin/javac): /usr/lib/jvm/jdk1.7.0_67/bin/javac
Nothing to configure.

7. 確認設定的java版本：

[email protected]:~$ java -version
java version "1.7.0_67"
Java(TM) SE Runtime Environment (build 1.7.0_67-b01)
Java HotSpot(TM) 64-Bit Server VM (build 24.65-b04, mixed mode)