1. /usr/bin/下多存放用户的安装软件 /usr/lib/下存放系统安装软件

    ll /usr/bin/java
    lrwxrwxrwx 1 root root 22 Mar 13 22:31 **java** -> /etc/alternatives/**java**

    1. alternatives是linux多版本管理软件，利用它可以实现jdk切换

    查看响应的jdk是否在菜单里，选择序号

    1. [root@izuf65mhum14tjzqrobqrsz alternatives]# update-alternatives --config java
    3. There are 2 programs which provide 'java'.
    5.   Selection    Command
    6. -----------------------------------------------
    7. *+ 1           /usr/java/jdk1.8.0_291-amd64/bin/java
    8.    2           java-11-openjdk.x86_64 (/usr/lib/jvm/java-11-openjdk-11.0.14.1.1-1.el7_9.x86_64/bin/java)

    如果没有可以如下添加：update-alternatives --install /usr/bin/java java /usr/lib/jvm/java/jdk1.6.0_12/bin/java 300

    1. java -version查看版本