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https://issues.apache.org/jira/browse/HBASE-6308?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13431936#comment-13431936
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Andrew Purtell commented on HBASE-6308:
---------------------------------------
I'll take this. Will add a simple test and commit.
> Coprocessors should be loaded in a custom ClassLoader to prevent dependency
> conflicts with HBase
> ------------------------------------------------------------------------------------------------
>
> Key: HBASE-6308
> URL: https://issues.apache.org/jira/browse/HBASE-6308
> Project: HBase
> Issue Type: Improvement
> Components: coprocessors
> Affects Versions: 0.92.1, 0.94.0
> Reporter: James Baldassari
> Assignee: Andrew Purtell
> Fix For: 0.96.0
>
> Attachments: 6308-v2.txt, HBASE-6308-0.92.patch,
> HBASE-6308-trunk.patch
>
>
> Currently each coprocessor is loaded with a URLClassLoader that puts the
> coprocessor's jar at the beginning of the classpath. The URLClassLoader
> always tries to load classes from the parent ClassLoader first and only
> attempts to load from its own configured URLs if the class was not found by
> the parent. This class loading behavior can be problematic for coprocessors
> that have common dependencies with HBase but whose versions are incompatible.
> For example, I have a coprocessor that depends on a different version of
> Avro than the version used by HBase. The current class loading behavior
> results in NoSuchMethodErrors in my coprocessor because some Avro classes
> have already been loaded by HBase, and the ClassLoader for my coprocessor
> picks up HBase's loaded classes first.
> My proposed solution to this problem is to use a custom ClassLoader when
> instantiating coprocessor instances. This custom ClassLoader would always
> attempt to load classes from the coprocessor's jar first and would only
> delegate to the parent ClassLoader if the class were not found in the
> coprocessor jar. However, certain classes would need to be exempt from this
> behavior. As an example, if the Copcoessor interface were loaded by both the
> region server's ClassLoader and the coprocessor's custom ClassLoader, then
> the region server would get a ClassCastException when attempting to cast the
> coprocessor instance to the Coprocessor interface. This problem can be
> avoided by defining a set of class name prefixes that would be exempt from
> loading by the custom ClassLoader. When loading a class, if the class starts
> with any of these prefixes (e.g. "org.apache.hadoop"), then the ClassLoader
> would delegate immediately to the parent ClassLoader.
> I've already implemented a patch to provide this functionality which I'll
> attach shortly.
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