The canonical JVM implementation from Sun applies some pretty sophisticated optimization to bytecode to obtain near-native execution speeds after the code has been run a few times.
The question is, why isn't this compiled code cached to disk for use during subsequent uses of the same function/class?
As it stands, every time a program is executed, the JIT compiler kicks in afresh, rather than using a pre-compiled version of the code. Wouldn't adding this feature add a significant boost to the initial run time of the program, when the bytecode is essentially being interpreted?
Without resorting to cut'n'paste of the link that @MYYN posted, I suspect this is because the optimisations that the JVM performs are not static, but rather dynamic, based on the data patterns as well as code patterns. It's likely that these data patterns will change during the application's lifetime, rendering the cached optimisations less than optimal.
So you'd need a mechanism to establish whether than saved optimisations were still optimal, at which point you might as well just re-optimise on the fly.
Oracle's JVM is indeed documented to do so -- quoting Oracle,
the compiler can take advantage of
Oracle JVM's class resolution model to
optionally persist compiled Java
methods across database calls,
sessions, or instances. Such
persistence avoids the overhead of
unnecessary recompilations across
sessions or instances, when it is
known that semantically the Java code
has not changed.
I don't know why all sophisticated VM implementations don't offer similar options.
An updated to the existing answers - Java 8 has a JEP dedicated to solving this:
=> JEP 145: Cache Compiled Code. New link.
At a very high level, its stated goal is:
Save and reuse compiled native code from previous runs in order to
improve the startup time of large Java applications.
Hope this helps.
Excelsior JET has a caching JIT compiler since version 2.0, released back in 2001. Moreover, its AOT compiler may recompile the cache into a single DLL/shared object using all optimizations.
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.
