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https://issues.apache.org/jira/browse/MAPREDUCE-1700?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14561603#comment-14561603
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Sangjin Lee commented on MAPREDUCE-1700:
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The idea of the application classloader is that there are two classloaders (the
application classloader and the JVM's classloader) which can keep its copies of
the same class. It is possible for the application classloader to load and use
avro 1.7 (from the app classpath) and for the JVM classloader to load and use
avro 1.4 (from the main classpath). It is essentially achieved by reversing the
standard parent-first loading approach.
> 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
> Fix For: 2.0.3-alpha
>
> Attachments: MAPREDUCE-1700-ccl.patch, MAPREDUCE-1700-ccl.patch,
> MAPREDUCE-1700.patch, MAPREDUCE-1700.patch, MAPREDUCE-1700.patch,
> MAPREDUCE-1700.patch, MAPREDUCE-1700.patch, MAPREDUCE-1700.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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