gaocegege/install-java-versions.md

## install-java-versions.md

      
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              install-java-versions.md
            
          
    Install and manage multiple Java versions on Ubuntu

This guide shows per example the installation of the Java SDK versions 8 (LTS, already installed) and 11 (LTS).
1. Check which Java versions are avaialable or already installed

apt --names-only search "openjdk-.*jre$"

> Sorting... Done
  Full Text Search... Done
  openjdk-11-jre/focal-updates,focal-security 11.0.8+10-0ubuntu1~20.04 amd64
    OpenJDK Java runtime, using Hotspot JIT

  openjdk-13-jre/focal-updates 13.0.3+3-1ubuntu2 amd64
    OpenJDK Java runtime, using Hotspot JIT

  openjdk-14-jre/focal-updates 14.0.1+7-1ubuntu1 amd64
    OpenJDK Java runtime, using Hotspot JIT

  openjdk-8-jre/focal-updates,focal-security,now 8u265-b01-0ubuntu2~20.04 amd64 [installed,automatic]
    OpenJDK Java runtime, using Hotspot JIT

Thanks to: https://askubuntu.com/a/1112189/393623

2. Install additional Java SDKs

Since the openjdk-8-jre is already installed we only need to install the openjdk-11-jre package:
sudo apt install openjdk-11-jre
3. Make sure that you are using the correct Java version

Open a terminal and run java -version:
java -version

# Output for OpenJDK 8
> openjdk version "1.8.0_265"
  OpenJDK Runtime Environment (build 1.8.0_265-8u265-b01-0ubuntu2~20.04-b01)
  OpenJDK 64-Bit Server VM (build 25.265-b01, mixed mode)

# Output for OpenJDK 11
> openjdk version "11.0.8" 2020-07-14
  OpenJDK Runtime Environment (build 11.0.8+10-post-Ubuntu-0ubuntu120.04)
  OpenJDK 64-Bit Server VM (build 11.0.8+10-post-Ubuntu-0ubuntu120.04, mixed mode, sharing)
If you see the output for the OpenJDK 8 but you want to use the OpenJDK 11 (e.g. for starting eclipse) you can easily switch between the different versions:
sudo update-alternatives --config java

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

    Selection    Path                                            Priority   Status
  ------------------------------------------------------------
  * 0            /usr/lib/jvm/java-11-openjdk-amd64/bin/java      1111      auto mode
    1            /usr/lib/jvm/java-11-openjdk-amd64/bin/java      1111      manual mode
    2            /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java   1081      manual mode

  Press <enter> to keep the current choice[*], or type selection number:
After selecting the version you can run the java -version command again in your terminal to verify that the selected Java SDK is now being used.