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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-1700) User supplied dependencies may conflict with MapReduce system JARs
Date Thu, 06 Sep 2012 20:51:09 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13450030#comment-13450030
] 

Steve Loughran commented on MAPREDUCE-1700:
-------------------------------------------

no, we don't want to go anywhere near servlet classloaders, because you end up in WAR EAR
and app server trees. The app server takes priority, except in the special case of JBoss in
the past, which shared classes across webapps

https://community.jboss.org/wiki/JBossClassLoaderHistory
http://docs.jboss.org/jbossweb/2.1.x/class-loader-howto.html

people will hit walls when they try to do things like upgrade the XML parser or try and add
a new URL handler. 

I'll look at the patch, but classloaders are a mine of grief. That's the strength of OSGi:
the grief is standardised an someone else has done the grief mining already

                
> User supplied dependencies may conflict with MapReduce system JARs
> ------------------------------------------------------------------
>
>                 Key: MAPREDUCE-1700
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1700
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>            Reporter: Tom White
>            Assignee: Tom White
>         Attachments: MAPREDUCE-1700-ccl.patch, MAPREDUCE-1700.patch, MAPREDUCE-1700.patch
>
>
> If user code has a dependency on a version of a JAR that is different to the one that
happens to be used by Hadoop, then it may not work correctly. This happened with user code
using a different version of Avro, as reported [here|https://issues.apache.org/jira/browse/AVRO-493?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12852081#action_12852081].
> The problem is analogous to the one that application servers have with WAR loading. Using
a specialized classloader in the Child JVM is probably the way to solve this.

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