更新jdk

1.从官网下载jdk-8u191-linux-x64.tar.gz，然后放在ubuntu里的Downloads文件夹里。然后解压到/usr/lib/jvm文件夹中。

 sudo tar -zxvf Downloads/jdk-8u191-linux-x64.tar.gz -C /usr/lib/jvm

2.修改环境变量

sudo vim ~/.bashrc

修改jdk路径

export JAVA_HOME=/usr/lib/jvm/jdk1.8.0_191

3.设置系统默认jdk 版本

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk1.8.0_191/bin/java 300  
sudo update-alternatives --install /usr/bin/javac javac /usr/lib/jvm/jdk1.8.0_191/bin/javac 300  
sudo update-alternatives --install /usr/bin/jar jar /usr/lib/jvm/jdk1.8.0_191/bin/jar 300   
sudo update-alternatives --install /usr/bin/javah javah /usr/lib/jvm/jdk1.8.0_191/bin/javah 300   
sudo update-alternatives --install /usr/bin/javap javap /usr/lib/jvm/jdk1.8.0_191/bin/javap 300  

 执行

sudo update-alternatives --config java

成功：