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From Kevan Miller <kevan.mil...@gmail.com>
Subject Re: OutOfMemoryException while redeploying WAR
Date Wed, 25 Apr 2007 15:11:41 GMT

On Apr 25, 2007, at 10:37 AM, DRobert@bfm.bm wrote:

>
> It's related to the way Sun's JVM handles class data and  
> internalized string. They make the assumption that you'd never want  
> that information flushed from the VM, so they put that chunk of  
> memory out of the reach of the garbage collector (i.e. "permanent  
> generation"). Unfortunately, when you redeploy an application,  
> you're really attempting to remove a bunch of classes and replace  
> them with new ones. That simply doesn't work with Sun's VM; you  
> only end up with both the old and the new classes in PermGen, even  
> if the old class data is no longer useful or even accessible.
>
> There's no way around this one if you're using Sun's VM. Raising  
> the amount of memory available for the permanent generation is  
> really only a stop gap measure. If you feel you're going to  
> redeploy your app very often, you should look into using another  
> VM. I personally use JRockit for development (which is when I do  
> most of my redeployment), even if I only run Sun's VM for staging  
> and production servers (in order to take advantage of the deeper  
> testing done on that VM).

That's not accurate. PermGen memory *is* GC'ed. However, a Class can  
only be GC'ed when its ClassLoader has been GC'ed. In Geronimo, when  
you deploy an application, one (or more) ClassLoaders are created to  
load your application classes. When you undeploy your application, we  
free up references to that ClassLoader. Once the ClassLoader has been  
GC'ed, the application classes will be GC'ed from the PermGen space.

Denis describes the behavior of classes loaded by the System  
ClassLoader. Since the system ClassLoader will never be GC'ed,  
classes loaded by the System ClassLoader will never be GC'ed. That is  
not a problem for Geronimo, since application classes will never be  
loaded using the system ClassLoader...

--kevan



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