[RFC]: Linkage Error fix.

[Denis Lila]

Hi.

Andrew Su pointed out this bug to me a few days ago.
In the page
http://www-archive.mozilla.org/quality/browser/front-end/testcases/oji/objtest2.html
one of the applets does not load once in a while (it happens
very often on my machine (>80% of the time)).

A LinkageError is thrown.

The attached patch fixes it.

The problem was that "JNLPClassloader.loadClass" was being called
explicitly by the applet instance creation process. That page loads
the same applet many times, so there was a race condition where
findLoadedClassAll(name) returned null (meaning the class has not
been loaded yet), so then we moved on to actually loading it. However
by the time we got to the loading code another applet had finished
loading the class. Trying to define a class twice in the same
classloader is an error.

The patch fixes it by not allowing concurrent loading of classes
with the same name. Another fix would be to just synchronize
the method body on a static lock, but that might be too slow.
The only thing I'm worried about with the attached patch is that
the map might get a bit big after a while. Is this a realistic concern?
If yes, it could be fixed by changing the hash map to <Integer, Object>,
and instead of putting (name, new Object()) in it, I could
put(name.hashCode()%83, new Object()). That way key,value pairs would
never take up more than about 2Kb, and with 83 locks, I don't think
we would have to worry about reduced concurrency.

ChangeLog:
2011-04-15  Denis Lila  <dlila at redhat.com>

	* netx/net/sourceforge/jnlp/Launcher.java
	Remove unused import.
	* netx/net/sourceforge/jnlp/runtime/JNLPClassLoader.java
	(loadedClassNames): New map. Records all the arguments to loadClass.
	(loadClass): Does not atomically load classes with identical names.


Regards,
Denis.
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