How to use the command update-alternatives –config java

The update-alternatives --config java command is used in Linux to manage and switch between different installed versions of Java on your system. This command is part of the update-alternatives system, which maintains symbolic links to set default versions of commands, libraries, and more.

Steps to Use update-alternatives --config java:

1. Open a Terminal: Launch your terminal application.
2. Run the Command: Use the following command:

   sudo update-alternatives --config java
   

3. View Installed Java Versions: When you run the above command, it lists all the installed Java versions on your system along with their priority and path. It will look something like this:

   There are 3 choices for the alternative java (providing /usr/bin/java).
   
     Selection    Path                                            Priority   Status
   ------------------------------------------------------------
   * 1            /usr/lib/jvm/java-11-openjdk-amd64/bin/java      1100      auto mode
     2            /usr/lib/jvm/java-8-openjdk-amd64/bin/java       1080      manual mode
     3            /usr/lib/jvm/java-17-openjdk-amd64/bin/java      1700      manual mode
   
   Press <enter> to keep the current choice[*], or type selection number:
   

4. Select the Desired Version:
   • Type the number corresponding to the version you want to use.
   • Press Enter.

   For example, to select Java 17, type 3 and press Enter.

5. Verify the Default Version: After switching, you can confirm the active Java version using:

   java -version
   

Explanation of Output Fields:

• Selection: The number you enter to select a version.
• Path: The location of the Java binary.
• Priority: Determines the default version if in auto mode.
• Status:
  • auto mode: The system automatically selects the highest-priority version.
  • manual mode: A version was manually selected.

Example:

Current Setup:

sudo update-alternatives --config java

Output:

There are 2 choices for the alternative java (providing /usr/bin/java).

  Selection    Path                                            Priority   Status
------------------------------------------------------------
* 1            /usr/lib/jvm/java-8-openjdk-amd64/bin/java       1080      auto mode
  2            /usr/lib/jvm/java-11-openjdk-amd64/bin/java      1100      manual mode

Press <enter> to keep the current choice[*], or type selection number:

• If you want to switch to Java 11, type 2 and press Enter.

Notes:

• Run as Root: You need sudo privileges to use update-alternatives.
• Custom Versions: If you’ve manually installed a Java version not listed, you can add it to the alternatives system using:

  sudo update-alternatives --install /usr/bin/java java /path/to/java 1000
  

• This command can also manage other Java-related commands like javac and jar.