I do not know the actual reasons, not being in any way involved in the JVM implementation, but I can think of some plausible ones:
- The idea of Java is to be a write-once-run-anywhere language, and putting precompiled stuff into the class file is kind of violating that (only "kind of" because of course the actual byte code would still be there)
- It would increase the class file sizes because you would have the same code there multiple times, especially if you happen to run the same program under multiple different JVMs (which is not really uncommon, when you consider different versions to be different JVMs, which you really have to do)
- The class files themselves might not be writable (though it would be pretty easy to check for that)
- The JVM optimizations are partially based on run-time information and on other runs they might not be as applicable (though they should still provide some benefit)
But I really am guessing, and as you can see, I don't really think any of my reasons are actual show-stoppers. I figure Sun just don't consider this support as a priority, and maybe my first reason is close to the truth, as doing this habitually might also lead people into thinking that Java class files really need a separate version for each VM instead of being cross-platform.
My preferred way would actually be to have a separate bytecode-to-native translator that you could use to do something like this explicitly beforehand, creating class files that are explicitly built for a specific VM, with possibly the original bytecode in them so that you can run with different VMs too. But that probably comes from my experience: I've been mostly doing Java ME, where it really hurts that the Java compiler isn't smarter about compilation.