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Re: URLClassloader and native objects


Andrew Haley wrote:

There's no code to allow a jarfile to be separately compiled to a .so
and loaded automagically, though.  It's quite tricky to figure out how
to do it.

This is one of the things I would like to get to work, once an elegant solution is found. In a way its a simpler issue in that the code is already compiled, we just need a way to tell gcj to grab a hold of it from within a custom classloader.


A variation of the issue I'm having, is when the compiled code already exists but is statically linked into my executable. Everything works great, and I can even use Class.forName() to grab the object. But my CCLs are unable to pull this object out, nor load it from the disk because it is combined into the executable. Do you have any insight on how I may be able get around this?

An ugly hack I can think of would be to have defineClass return some custom data, such that it doesn't define a class with VM bytecode, but returns some string that references a native object (ugly_native_hack://foo.bar.classname). We'de have to bypass any standard bytecode verification in these special cases and pull that statically linked image out of storage instead. (Of course this is just speculation, I don't know enough about GCJ architecture to say it would be possible.) Has this already been accounted for in some design work previously?

I'm trying to think of a way to handle it without blatantly breaking any of the JVM specs... but its difficult as I think it's a unique GCJ situation. A more elegant solution may be to force the application to load the .class data anyway from some file, then introspect and get the CRC/signature of the object. Then check all statically linked images that they match and swap for that image. The issue here is that even though two classes may have the same method signatures, they could contain different code in the methods and still be incompatible.

A third solution might be to have some eternal override. Maybe a directory with SOs or a configuration file that will list objects. Any objects in this list, when referenced via defineClass will 'thunk' down to use a staticly compiled version, or a native .so regardless of what data the application is trying to define the class with. So under a normal JVM the application will function fine in the usual way, but in GCJ if a native object exists, it will get used. The downside here is of course, it contradicts the usage of a custom classloader in the first place, as a global override will always give the same object that matches a certain name, despite what the application wants/sends. It allows a CCL to 'own' a native object though, which fixes the problems I've been having under GCJ since I don't have to delegate to the system classloader for these objects.

Anyway, if you have some ideas here I would like to hear them... as it's the issue I would likely try to address first.

Great.  Well, there's a branch called gcj-abi-2-dev-branch, which you
should be able to build.  -findirect-dispatch enables the new ABI.

I'll start digging through code. GCC is still a bit daunting as I'm new to it, so it may take a little while before I'll be able to contribute... if you know of some reading offhand (online or off) to bring me up to speed that would be great.


- Sal


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