As you may see from the title, for some reason I need to make running .class files on Minix possible (a compiler is not necessary). So could somebody point me in any direction, suggest some literature or give some advice? Generally, how would you do it?
Until now I found OpenJDK (but it's not exactly what I am looking for). I have also read Tanenbaum's "operating systems design and implementation". It gave me a lot of insight of minix internals.
If you just want to run .class files without much concern for performance, you could create a bytecode interpreter, which might be simpler than porting / creating a full compiler. You can find the format of these class files detailed here, and the behavior of the VM specified here.
You'll also need to pick a runtime -- OpenJDK and GNU Classpath are probably the best bets -- and port it to MINIX by implementing its native methods in C. native methods are usually concerned with platform-specific stuff, like calls to file I/O, and therefore cannot be implemented in the platform-independent Java language.
There are a number of other links and resources that you might find useful on this wiki page.