java.lang.Thread.contextClassLoader : what if it were a weak reference ?

Tom Hawtin Thomas.Hawtin at Sun.COM
Mon Mar 23 11:11:39 UTC 2009

Sylvain Laurent wrote:
> I'm tracking classloader leaks more or less as a hobby and found out 
> that one of the (many) source of leaks come from threads that still have 
> a reference to a classloader through their contextClassLoader instance 
> variable.
> So, I'm wondering : what if instead of 
> "private ClassLoader contextClassLoader;" we had "private 
> WeakReference<ClassLoader> contextClassLoaderRef" and the method 
> getContextClassLoader would more or less 
> returned contextClassLoaderRef.get() ?
> Would it break many things ? is it incompatible with the specs ?

How much it would break, I wouldn't like to guess.

Incompatible? Yes. It is legitimate to use a class loader only through 

As for solutions: I guess containers could add a fake context class 
loader with a weak reference to the real one (perhaps parent could 
optionally become a weak reference?). Really there needs to be more care 
taken when creating threads to make sure the context class loader (and 
indeed InheritableThreadLocals) are inherited only when appropriate.

Tom Hawtin

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