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Date: Fri, 15 Aug 2008 16:02:25 +0200
From: Grzegorz Kossakowski <grek@tuffmail.com>
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To: Cocoon's dev mailing list <dev@cocoon.apache.org>
Subject: RCL plug-in and different classloaders issues
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Hi,

I'm just playing around with Cocoon's RCL plug-in in some more complicated scenario and stumbled 
upon a problem when the same class is loaded by two different classloaders.

In brief, following steps are performed:
1. Class from my block is loaded using ResourceStoreClassLoader. Let's call this class MAIN.
2. This class accesses some static method of another class loaded by ResourceStoreClassLoader 
(because this class comes from my block as well) and sets some value on static field. Let's call 
this class STATIC
3. Then another class (3RDPARTY) is being loaded which is not part of my block, it's from 
third-party jar. It's loaded by parent classloader to ResourceStoreClassLoader. As part of 
configuration of this class I pass a name of class which is some kind of extension point and is 
implemented by class coming from my block; let's call this class EXTENSION.
4. 3RDPARTY creates new instance of EXTENSION class (and this class is again not loaded by 
ResourceStoreClassLoader but by parent), but in EXTENSION class one tries to access field set in STATIC.
5. While EXTENSION tries to access STATIC, STATIC is being loaded again, using parent classloader. 
The value in STATIC is gone, obviously.

So the basic problem is that, this STATIC class is loaded by two different classloaders.

I'm wondering if there is any kind of general solution to such problems. The only one I can think of 
is to enforce loading of 3RDPARTY class by ResourceStoreClassLoader so everything stays within one 
classloader and even reflection tricks are not dangerous. However, that would mean that one needs to 
modify commons-jci project.

Is there any simpler method?

-- 
Grzegorz Kossakowski