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From "Jacob Metcalf (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (AVRO-1103) New AvroDeserializer should Locate Appropriate Classloader
Date Sat, 16 Jun 2012 12:44:42 GMT

     [ https://issues.apache.org/jira/browse/AVRO-1103?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jacob Metcalf updated AVRO-1103:
--------------------------------

    Attachment: AVRO-1103-for 0.23.1.patch
    
> 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
>
>
> 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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