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From "Mark Thomas (JIRA)" <j...@apache.org>
Subject [jira] Commented: (POOL-161) ContextClassLoader problems for the Evictor thread
Date Tue, 01 Jun 2010 19:40:40 GMT

    [ https://issues.apache.org/jira/browse/POOL-161?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12874193#action_12874193
] 

Mark Thomas commented on POOL-161:
----------------------------------

My main concern is what other as yet undiscovered memory leaks may be lurking in the code.
Having each web app (in this use case) use it's own copy of pool will avoid any such issues
entirely. Fixing this issue sets us off down the potentially hugely complicated path of supporting
the use of a single instance of commons pool across multiple class loaders. That is the complexity
that concerns me.

> ContextClassLoader problems for the Evictor thread
> --------------------------------------------------
>
>                 Key: POOL-161
>                 URL: https://issues.apache.org/jira/browse/POOL-161
>             Project: Commons Pool
>          Issue Type: Bug
>    Affects Versions: 1.5.4
>            Reporter: Sylvain Laurent
>             Fix For: 2.0
>
>         Attachments: patch_Evictor_CCL.txt, TestGenericObjectPoolClassLoader.patch.txt
>
>
> Since a single Timer is used for several GenericObjectPool instances, this may create
classloader issues and a memory leak of one classloader :
> Let's imagine the following scenario :
> - commons-pool.jar is in the classpath of a webapp container (e.g. tomcat).
> - 2 webapps A and B are deployed, each creating an instance of GenericObjectPool for
its own usage.
> - each webapp makes use of the idle object evictor and sets a positive number for minIdle
> - first, webapp A instantiates its GenericObjectPool. Since this is the first TimerTask
to be created, the Timer instance is created, thus creating a Thread whose ContextClassLoader
is the current one, that is webapp A's ContextClassLoader.
> The TimerTask properly creates instances of idle objects in the pool, making use of the
ObjectFactory provided by A.
> - then B instantiates its GenericObjectPool. A new TimerTask is created, and it tries
to invoke the ObjectFactory provided by B. But when it needs a class that only exists in B
webapp, it cannot find it because the ContextClassLoader of the Timer Thread is A's classloader.
> Other side effect : if webapp A is undeployed, but B is still running, then A's webappClassLoader
cannot be GCed because the Timer Thread keeps a strong reference to A's classloader (as its
context classloader).

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