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https://issues.apache.org/jira/browse/MAPREDUCE-1700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12857306#action_12857306
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Steve Loughran commented on MAPREDUCE-1700:
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Getting custom classloaders right is one of the hardest things to do in Java.
Whoever volunteers to do this *and I opt to run away from it* had better talk
to the experts in the area. If it is purely for short-lived standalone tasks
things would be simpler (less classloader leakage risks), but you still have to
be very good at handling the problems a CL tree brings to the table
# returning anything loaded by a custom CL means the CL and all loaded classes
hang around in VM, reloading becomes tricky.
# multiple singletons in a single JVM
# object equality tests fail
# wierd errors that you had better log rather than hope don't happen.
I've always felt that the ASF ought to have an "understands classloaders"
qualification; if you don't pass it, you don't get to submit classloaders to
the codebase.
> 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
>
> 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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