Ubuntu 安裝 Java JDK

1. 先到 http://www.oracle.com/technetwork/java/javase/downloads/jdk7-downloads-1880260.html
    下載 jdk
2. 解壓縮
    $ tar zxvf jdk-7u51-linux-i586.tar.gz
3. 建立jdk目錄
    $ sudo mkdir /usr/lib/jdk
4. Copy jdk 到 /usr/lib 目錄下
    $ sudo cp -r jdk1.7.0_51 /usr/lib/jdk
5. 設定環境變數
    $ sudo vim /etc/profile
    加上以下內容:
        export JAVA_HOME=/usr/lib/jdk/jdk1.7.0_51
        export JRE_HOME=/usr/lib/jdk/jdk1.7.0_51/jre
        export PATH=$PATH:$JAVA_HOME/bin:$JAVA_HOME/jre/bin
        export CLASSPATH=.:$CLASSPATH:$JAVA_HOME/lib:$JAVA_HOME/jre/lib:$JAVA_HOME/lib/tools.jar
6. 置換系統默認 jdk
    $ sudo update-alternatives --install /usr/bin/java java /usr/lib/jdk/jdk1.7.0_51/bin/java 300
    $ sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jdk/jdk1.7.0_51/bin/javac 300
    $ sudo update-alternatives --config java
    $ sudo update-alternatives --config javac
7. 確認安裝
    $ java -version
     java version "1.7.0_51"
     Java(TM) SE Runtime Environment (build 1.7.0_51-b13)
    Java HotSpot(TM) Client VM (build 24.51-b03, mixed mode)

8. 完成