configuring multiple JDKs
introduction
there are many reasons to install more than 1 jdk on a system.
it can be that a part of your codebase is running on JDK_X while the newer services run on JDK_Y
it can be that you are trying some features of a newer version, but still develop under an LongTermSupport version (LTS)
whatever the reason may be, having more that 1 JDK on the system is not a problem if properly configured for this note, we’re going to install via apt, 2 different jdk versions and manually a third one
installing a jdk via apt
you can check this note -installing_openjdk- or continue reading here…
to install a jdk if you don’t have any or to install a second one if you already have one, just do the following
open a terminal like:
- KDE’s Konsole
- GNOME’s Terminal
# for the java openjdk 14 you need to run
you@host:~$ sudo apt install openjdk-14-jdk
wait for the installation to complete…
# confirm that you have the Openjdk java runtime installed correctly by running the java command
# and checking you get a similar result as below
user@hostname:~$ java -version
openjdk version "14.0.2" 2020-07-14
OpenJDK Runtime Environment (build 14.0.2+12-Ubuntu-120.04)
OpenJDK 64-Bit Server VM (build 14.0.2+12-Ubuntu-120.04, mixed mode, sharing)
# confirm that you have the Openjdk java compiler installed correctly by running the javac command
# and checking you get a similar result as below
user@hostname:~$ javac -version
javac 14.0.2
to install a second one, repeat the above but selecting a different jdk, java 8 for example like so:
# for the java openjdk 8 you need to run
you@host:~$ sudo apt install openjdk-8-jdk
wait for the installation to complete… and confirm like before that java vas installed…
if you ran the java -version again, you will see now the java 8 info being displayed, as it was the last one you installed…
if it is not the case, and you still see java 14 info, continue reading to see how to select the other version
selecting a java version
now that you have more than one jdk installed on your system you can easily see them and switch between them with the update-alternatives command
the open-jdk installs several java commands, for example:
- java
- javac
- javadoc
- jar
- etc…
to switch between the java command installed versions do the following:
# let's first check which java version we are currently running
you@host:~$ java -version
# in my case it's a graalvm v21 running on java 11
openjdk version "11.0.10" 2021-01-19
OpenJDK Runtime Environment GraalVM CE 21.0.0 (build 11.0.10+8-jvmci-21.0-b06)
OpenJDK 64-Bit Server VM GraalVM CE 21.0.0 (build 11.0.10+8-jvmci-21.0-b06, mixed mode, sharing)
# let's now change it to something else, like java 8
you@host:~$ sudo update-alternatives --config java
# the above command will display something similar to the following
There are 4 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/lib/jvm/java-14-openjdk-amd64/bin/java 1411 auto mode
* 1 /home/username/bin/jvm/graalvm/bin/java 1000 manual mode
2 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 manual mode
3 /usr/lib/jvm/java-14-openjdk-amd64/bin/java 1411 manual mode
4 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode
Press <enter> to keep the current choice[*], or type selection number: 4
# as you can see the graalvm version is selected which corresponds with what we found before.
# I will select option 4 which is java 8
update-alternatives: using /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java to provide
/usr/bin/java (java) in manual mode
# if you run again update-alternatives you will see that java 8 is now the selected version
you@host:~$ sudo update-alternatives --config java
# the above command will display something similar to the following
There are 4 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
------------------------------------------------------------
0 /usr/lib/jvm/java-14-openjdk-amd64/bin/java 1411 auto mode
1 /home/username/bin/jvm/graalvm/bin/java 1000 manual mode
2 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 manual mode
3 /usr/lib/jvm/java-14-openjdk-amd64/bin/java 1411 manual mode
* 4 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode
Press <enter> to keep the current choice[*], or type selection number:
# and to confirm...
you@host:~$ java -version
openjdk version "1.8.0_282"
OpenJDK Runtime Environment (build 1.8.0_282-8u282-b08-0ubuntu1~20.04-b08)
OpenJDK 64-Bit Server VM (build 25.282-b08, mixed mode)
the update-alternatives is a very solid and tidy way of keeping many JDKs versions on your system and switch among them without much effort.
manually installing a jdk
if you want to know how to manually install a jdk using update-alternatives, you can check this note -installing_graal_vm- for a detailed step by step explanation on how to manually install GraalVM