Is installing openjdk-21-jdk on Debian 12 from jdk.java.net .deb safe, if its openjdk-21 package looks not working on Debian tracker?

Question

Because Java 25 LTS is coming soon (September 2025) and I've not even yet installed Java JDK 21 LTS (September 2023), I've attempted to do so with a sudo apt install openjdk-21-jdk on my Debian 12. But it failed with a package not found message.

Its package isn't available yet on Debian 12: openjdk-21 (except on sid).

I'm considering downloading it from https://jdk.java.net/archive/ as a tar.gz of 21.0.2 (build 21.0.2+13) version (at the cost of forgetting apt commands to handle Java, and writing on my agenda to check manually for updates there)

But I'm fearing this:

Won't the .deb included in this .tar.gz break my system if I attempt to install it,
if the dedicated openjdk-21-jdk package for Debian 12 cannot be created for years?

• Stéphane Chazelas: AFAICT, openjdk-21 is available and the default-jre on Debian trixie (currently in testing, the future stable), not just sid
• Stéphane Chazelas: My first question would be: do you need 21? Is there any feature that you're missing from the versions that are maintained by your operating system?
• Marc Le Bihan: @StéphaneChazelas I need to install it because I'm a Java developer. Staying one version below the latest LTS is acceptable, but being two below is difficult to stand. Java 17 has been published 3.5 years ago. It is a bit old now, and many projects have started with Java 21 now.
• Stéphane Chazelas: And would the software you publish expected to only work on bleeding age operating systems? In which case, shouldn't you use such a bleeding edge operating system as your development environment as well?
• Stéphane Chazelas: If not, then it seems to me the question shouldn't be a about installing a newer openjdk system-wide in your stable OS, but how your software should bundle the specific JRE it needs in a way that doesn't interfere with the system's JRE(s).

Answer 1

Installing an archive from java.jdk.net isn’t safe, but not because of packaging issues. The Java 21 archives there don’t receive updates, and 21.0.2 is affected by a number of issues which have been fixed in later releases.

Since Debian doesn’t provide a package for Debian 12 (and probably won’t, ever — you’ll get it in 13 but I doubt it will be backported), you’ll need to download it elsewhere. A good option is to use one of the verified releases available from Adoptium.

As long as you extract the tarball in a location under /opt, /usr/local, or even your home directory, it won’t break your system. You should resist the temptation to make it available using update-alternatives, and instead rely on setting JAVA_HOME and PATH appropriately where necessary.

Stéphane Chazelas’ comments regarding the target environment are worth bearing in mind — if your goal is to produce applications for use on Debian 12, you might be better off sticking with Java 17. The upgrade to Java 21 will be much easier on Debian 13, released in August 2025. Otherwise you’ll need to manage the runtime environment wherever you need to deploy the applications you build, or use something like Quarkus and build a native application.

• Marc Le Bihan: Oh, if Debian 13 comes this summer, it's worth waiting it, yes. But you gave me the answer I was searching for: it's not recommended to install Java 21 yet on Debian 12 as a natural upgrade for Java, without being cautious.

Answer 2

If the tarball contains a set of files, keep them out of the system standard directories and put them under either /opt or /usr/local. Easier to delete manually when no longer required.

If the tarball contains a deb then provided you know what you're installing, the package manager will handle dependencies and you can remove the package manually (apt remove {package}) when no longer required