关于Linux系统下jdk版本切换问题（alternatives命令的使用）

[root@danny lib]# cd /usr/lib/jvm
[root@danny jvm]#
[root@danny jvm]# ls
java                                                java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64        jre-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64
java-1.7.                                          java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug  jre-1.8.
java-1.7.-openjdk                                  java-openjdk                                           jre-1.8.-openjdk
java-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64  jre                                                    jre-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64
java-1.8.                                          jre-1.7.                                              jre-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug
java-1.8.-openjdk                                  jre-1.7.-openjdk                                      jre-openjdk
[root@danny jvm]# ll
total
lrwxrwxrwx   root root    Nov   : java -> /etc/alternatives/java_sdk
lrwxrwxrwx   root root    Nov   : java-1.7. -> /etc/alternatives/java_sdk_1.7.0
lrwxrwxrwx   root root    Nov   : java-1.7.-openjdk -> /etc/alternatives/java_sdk_1..0_openjdk
drwxr-xr-x  root root  Nov   : java-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64
lrwxrwxrwx   root root    Sep   : java-1.8. -> /etc/alternatives/java_sdk_1.8.0
lrwxrwxrwx   root root    Sep   : java-1.8.-openjdk -> /etc/alternatives/java_sdk_1..0_openjdk
drwxr-xr-x   root root  Nov   : java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64
drwxr-xr-x   root root  Sep   : java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug
lrwxrwxrwx   root root    Nov   : java-openjdk -> /etc/alternatives/java_sdk_openjdk
lrwxrwxrwx   root root    Nov   : jre -> /etc/alternatives/jre
lrwxrwxrwx   root root    Nov   : jre-1.7. -> /etc/alternatives/jre_1.7.0
lrwxrwxrwx   root root    Nov   : jre-1.7.-openjdk -> /etc/alternatives/jre_1..0_openjdk
lrwxrwxrwx   root root    Nov   : jre-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64 -> java-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64/jre
lrwxrwxrwx   root root    Sep   : jre-1.8. -> /etc/alternatives/jre_1.8.0
lrwxrwxrwx   root root    Sep   : jre-1.8.-openjdk -> /etc/alternatives/jre_1..0_openjdk
lrwxrwxrwx   root root    Sep   : jre-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64 -> java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64/jre
lrwxrwxrwx   root root    Sep   : jre-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug -> java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug/jre
lrwxrwxrwx   root root    Nov   : jre-openjdk -> /etc/alternatives/jre_openjdk
[root@danny jvm]# ls /etc/alternatives/java_sdk
bin  demo  include  jre  lib  sample  src.zip  tapset
[root@danny jvm]# alternatives --config java (版本切换)

There are  programs which provide 'java'.

  Selection    Command
-----------------------------------------------
              java-1.8.-openjdk.x86_64 (/usr/lib/jvm/java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug/jre/bin/java)
*+            java-1.8.-openjdk.x86_64 (/usr/lib/jvm/java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64/jre/bin/java)    （当前版本）
              java-1.7.-openjdk.x86_64 (/usr/lib/jvm/java-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64/jre/bin/java)

Enter to keep the current selection[+], or type selection number: 3     （切换为1.7版本）
[root@danny jvm]# alternatives --config java   （以下可以看到+号在1.7版本处，不变还是输入3退出）

There are  programs which provide 'java'.

  Selection    Command
-----------------------------------------------
              java-1.8.-openjdk.x86_64 (/usr/lib/jvm/java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64-debug/jre/bin/java)
*             java-1.8.-openjdk.x86_64 (/usr/lib/jvm/java-1.8.-openjdk-1.8.0.181-.b13.el7_5.x86_64/jre/bin/java)
 +            java-1.7.-openjdk.x86_64 (/usr/lib/jvm/java-1.7.-openjdk-1.7.0.191-2.6.15.4.el7_5.x86_64/jre/bin/java)

Enter to keep the current selection[+], or type selection number:
[root@danny jvm]# java -version    （可以看到已经变为1.7版本了）
java version "1.7.0_191"
OpenJDK Runtime Environment (rhel-2.6.15.4.el7_5-x86_64 u191-b01)
OpenJDK -Bit Server VM (build 24.191-b01, mixed mode)

注释：在探究linux中jdk版本切换时发现了如下一行

lrwxrwxrwx   root root    Nov   : java -> /etc/alternatives/java_sdk

网上搜了以下alternatives这个文件是什么东东时，才发现这是一个可以进行jdk版本切换的命令，命令格式如下：

alternatives --config java