Re: [Springframework-developer] Cleanup of context resources on webapp reload

[Guillaume P. <gpo...@gl...>]

I experimented some more about this cleanup issue, and I was able to narrow
down the problem to the caching done by the java.beans.Introspector.  It
stores the BeanInfo instances in a WeakHashMap, but in that Map
implementation, only the keys uses WeakReference, the values are stored with
hard references since BeanInfo has an hard reference on the class it gives
info about (indirectly through BeanDescriptor and others), any time
Introspector.getBeanInfo(Class) is used, that class and it's static members
will not ever be able to be garbage collected.  I kind of remember someone
mentioning something related to this in the mailling list, but I cannot find
the mail.  I wonder if there's other case where the java[x] classes might
have an hard reference on a class or its instances.  A fix for this
particular problem is to have a ServletContextListener call
Introspector.flushCaches() when the context is destroyed.

It seems like a known issue at Sun :

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4291376
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4730581
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4809008

So, unless I'm missing something here, that means fixes like using
synchronization and WeakReference on singleton probably won't help much if
at all.  The only way that I can see for a class not to be gargage collected
when no more active thread use it, is if another ClassLoader has an hard
reference to the class instance, or an instance of that class.  Having the
class itself have an hard reference on a its own singleton has no effect,
it's a circular reference that will not prevent the class or the instance to
be gargabe collected when neither is being refered to by something else.

Guillaume