Introduction

There is also a method using multiple versions of Java by switching Java, such as method using jenv, but here the method using orthodox alternatives I'd like to introduce_______

For example, I think it will be useful in the following scenes, so it may be useful to remember just the command name alternatives.

When you want to install new Java in a production environment where old Java is installed, but want to switch back to old Java if something goes wrong.
When you want to use multiple versions of Java in a test environment.
When you want to quickly switch from Oracle JDK to OpenJDK.

What is the alternatives command?

alternatives are commands that come standard with CentOS.

If you look up alternatives with the man command, it says "maintain symbolic links determining default commands". It's hard to understand in English, but it's an excellent command that says, "You can use symbolic links to switch between software with similar functions and software with different versions."

In other words, it can be applied not only to Java but also to various commands **.

testing environment

OS：CentOS 6.10
Installed Java: Java12 (Oracle JDK)
Installed with RPM.

[root@akagi ~]# java -version
java version "12.0.1" 2019-04-16
Java(TM) SE Runtime Environment (build 12.0.1+12)
Java HotSpot(TM) 64-Bit Server VM (build 12.0.1+12, mixed mode, sharing)

Advance preparation

Installation of new Java (OpenJDK)

Download Java12 tar.gz from OpenJDK Site.
Place and extract tar.gz under an appropriate directory (* here / usr / local) to complete the installation.

Installation is complete, but the path does not pass.

[root@akagi ~]# cd /usr/local/
[root@akagi local]# tar zxvf openjdk-12.0.1_linux-x64_bin.tar.gz 
[root@akagi local]# ls | grep jdk
jdk-12.0.1
openjdk-12.0.1_linux-x64_bin.tar.gz

Switching Java with the alternatives command

Check the current status

ʻAlternatives --config {generic name}` allows you to see the current status in interactive mode.
For the generic name, specify the command you are actually using, such as "java" or "perl".
Since there is only one command corresponding to "java" here, press [Enter] to exit interactive mode while preserving the current settings.

[root@akagi local]# alternatives --config java

There is one program'java'To provide.

Select command
-----------------------------------------------
*+ 1           /usr/java/jdk-12.0.1/bin/java

Press Enter to select the current[+]Or enter the selection number:

Command registration-Switching the JVM to use

ʻAlternatives --install {link} {generic name} {path} {priority} You can register alternative commands by typing `.
If you specify a "generic name" that does not exist, a new group of "generic names" will be created.
If you specify an existing "generic name", it will be registered in the same "generic name" group.
A link is a symbolic link that is called when a command with a generic name is executed, and specifies "/ usr / bin / java" or "/ usr / local / bin / perl".
The path is the command that is actually executed when the command with the generic name is executed.
Type ʻalternatives --config java` again to switch the JVM to be called.
[+] Is currently selected, and Oracle JDK is currently selected.
Therefore, enter 2 (No. 2) in the "Selection number" requested at the end to switch to OpenJDK.

[root@akagi local]# alternatives --install /usr/bin/java java /usr/local/jdk-12.0.1/bin/java 2
[root@akagi local]# alternatives --config java

There are 2 programs'java'To provide.

Select command
-----------------------------------------------
*+ 1           /usr/java/jdk-12.0.1/bin/java
   2           /usr/local/jdk-12.0.1/bin/java

Press Enter to select the current[+]Or enter the selection number:2

Check the status after setting

After completing the configuration, enter ʻalternatives --config java` to see the currently selected JVM.
At this point, No. 2 (OpenJDK) is selected.

[root@akagi local]# alternatives --config java

There are 2 programs'java'To provide.

Select command
-----------------------------------------------
*  1           /usr/java/jdk-12.0.1/bin/java
 + 2           /usr/local/jdk-12.0.1/bin/java

Press Enter to select the current[+]Or enter the selection number:

Finally, display the Java version to make sure that OpenJDK is actually working.

[root@akagi local]# java -version
openjdk version "12.0.1" 2019-04-16
OpenJDK Runtime Environment (build 12.0.1+12)
OpenJDK 64-Bit Server VM (build 12.0.1+12, mixed mode, sharing)

Other

According to the following article, it seems that you can switch the Java version with the alternatives command even in CentOS 7 series.
Switch with CentOS7 series + JDK-9.0.1 + JDK1.8.0 + alternatives

Reference URL

Switch Java version using "alternatives" command on CentOS

[LINUX] [Java] How to switch between multiple versions of Java