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Date: Thu, 16 Aug 2012 17:38:38 +1100 (NCT)
From: "Costin Leau (JIRA)" <jira@apache.org>
To: common-issues@hadoop.apache.org
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Subject: [jira] [Updated] (HADOOP-8632) Configuration leaking class-loaders
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     [ https://issues.apache.org/jira/browse/HADOOP-8632?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Costin Leau updated HADOOP-8632:
--------------------------------

    Attachment: HADOOP-8632-trunk.patch

Attaching the patch against the trunk (the previous patch as mentioned, was against branch-2.1.0-alpha).
Ran
git diff --no-prefix 7a3427de68 518c39814e

The github link is: http://j.mp/Nor44u

Did a cherry pick of the commit against 2.1.0-alpha.
                
> Configuration leaking class-loaders
> -----------------------------------
>
>                 Key: HADOOP-8632
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8632
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: conf
>    Affects Versions: 2.0.0-alpha
>            Reporter: Costin Leau
>         Attachments: 0001-wrapping-classes-with-WeakRefs-in-CLASS_CACHE.patch, HADOOP-8632.patch, HADOOP-8632-trunk.patch
>
>
> The newly introduced CACHE_CLASSES leaks class loaders causing associated classes to not be reclaimed.
> One solution is to remove the cache itself since each class loader implementation caches the classes it loads automatically and preventing an exception from being raised is just a micro-optimization that, as one can tell, causes bugs instead of improving anything.
> In fact, I would argue in a highly-concurrent environment, the weakhashmap synchronization/lookup probably costs more then creating the exception itself.
> Another is to prevent the leak from occurring, by inserting the loadedclass into the WeakHashMap wrapped in a WeakReference. Otherwise the class has a strong reference to its classloader (the key) meaning neither gets GC'ed.
> And since the cache_class is static, even if the originating Configuration instance gets GC'ed, its classloader won't.

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