Particularly for Minecraft, you might want to use Microsoft's build of OpenJDK; details are at https://learn.microsoft.com/en-us/java/openjdk/install.

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.