yum -y install java-1.8.0-openjdk java-1.8.0-openjdk-devel

    dirname $(readlink $(readlink $(which java)))

【/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.181-3.b13.el7_5.x86_64/jre/bin】

vim /etc/profile

   export JAVA_HOME=上面dirname命令獲取到的路徑，不要jre/bin最后這段
   export PATH=$PATH:$JAVA_HOME/bin
   export CLASSPATH=.:$JAVA_HOME/jre/lib:$JAVA_HOME/lib:$JAVA_HOME/lib/tools.jar

保存退出文件，然后執行一下使設置立即生效

   source /etc/profile

   [root@iZm5e46125tswtbtri94d1Z ~]# java -version
   openjdk version "1.8.0_181"
   OpenJDK Runtime Environment (build 1.8.0_181-b13)
   OpenJDK 64-Bit Server VM (build 25.181-b13, mixed mode)