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https://issues.apache.org/jira/browse/AVRO-1103?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13405071#comment-13405071
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Jacob Metcalf commented on AVRO-1103:
-------------------------------------
If you are okay with it I am going to close this issue. I have had to switch my
environment to Hadoop CDH4 under which I cannot reproduce the problem (reasons
below). I have also attached my latest patch for building Avro 1.7 with CDH4 in
case its of any use when you come to supporting Hadoop 2.
In terms of why I cannot reproduce it on CDH4. I was getting this problem
because the core Avro jar and the jar with my Avro specific classes in were
being loaded by different class loaders. This was because I was distributing my
Avro Specific classes via the /lib mechanism - option 2 on
http://www.cloudera.com/blog/2011/01/how-to-include-third-party-libraries-in-your-map-reduce-job/.
However as this page alludes to Cloudera are phasing this option out so now I
have now switched to option 3 - just dropping all my jars directly into the lib
directory on all nodes. Hence they all get loaded by the same classloader and
everything works.
> New AvroDeserializer should Locate Appropriate Classloader
> ----------------------------------------------------------
>
> Key: AVRO-1103
> URL: https://issues.apache.org/jira/browse/AVRO-1103
> Project: Avro
> Issue Type: Improvement
> Components: java
> Affects Versions: 1.7.0
> Environment: Hadoop 0.23.1 with Avro jars replaced by 1.7 jars
> Specific data classes assembled into JAR with mapper/reducer
> Reporter: Jacob Metcalf
> Assignee: Doug Cutting
> Fix For: 1.7.1
>
> Attachments: AVRO-1103-for 0.23.1.patch, AVRO-1103.patch,
> AVRO-1103.patch, AVRO-1103.patch, AvroCDH4.patch
>
>
> Continuing on from AVRO-873 I believe some more work needs to be done to get
> the MapReduce 2 APIs in Avro 1.7 working with Hadoop 0.23. Since it revolves
> around classloaders it is complex to present a unit test which fails so I
> will explain the problem:
> - By default SpecificDatumReader will use the classloader it was loaded from
> to find a Specific class to deserialize into.
> - In earlier versions of Hadoop e.g. 0.20.2 Avro was not included so
> typically you would bundle Avro into your job jar along with the Specific
> classes so they would be on the same classpath.
>
> - However later versions of Hadoop such as 0.23 ship with Avro. Thus you find
> that the SpecificData.class.getClassloader() is typically a parent loader
> which just contains Hadoop components.
> - Thus when SpecificData goes to construct a Specific class from the schema
> it cannot locate it and silently defaults to creating a GenericData.
> In AVRO-873 an additional constructor was added to SpecificData to force it
> to use a different classloader. Thus to extend this fix to the new MR2 APIs:
> - AvroDeserializer could attempt to instantiate the class using
> Class.forName() and from this get the appropriate Classloader and pass this
> into the constructor of SpecificDatumReader.
> - Line 2771 of SpecificData.java is:
> bq. Class c = SpecificData.get().getClass(schema);
> - This would need to be changed to:
> bq. Class c = this.getClass(schema);
> I have raised this in the mail groups here:
> http://search-hadoop.com/m/wVUf1aLCwd/classloader/v=threaded so apologies if
> this is already being thought about.
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