ubuntu14 update jdk 1.7 to 1.8

有效解决：https://stackoverflow.com/questions/30177455/moving-from-jdk-1-7-to-jdk-1-8-on-ubuntu
我下载的是：https://www.oracle.com/cn/java/technologies/javase/javase8-archive-downloads.html
将路径中的版本更换即可：

sudo update-alternatives --install /usr/bin/java java /opt/jdk/jdk1.8.0_71/bin/java 1
sudo update-alternatives --install /usr/bin/javac javac /opt/jdk/jdk1.8.0_71/bin/javac 1