将Linux系统默认的java修改过来

sudo update-alternatives --install /usr/bin/java java /opt/java/jdk1.8.0_40/bin/java 300
 sudo update-alternatives --install /usr/bin/javac javac /opt/java/jdk1.8.0_40/bin/javac 300
 sudo update-alternatives --config java 
 sudo update-alternatives --config javac

来源：http://blog.chinaunix.net/uid-20680966-id-4916603.html