[jira] [Commented] (HADOOP-9295) AbstractMapWritable throws exception when calling readFields() multiple times when the maps contain different class types

["Karthik Kambatla (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-9295?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13605402#comment-13605402 ] 

Karthik Kambatla commented on HADOOP-9295:
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I thought about this further and wonder if there is really a problem here. I uploaded a test (test-hadoop-9295.patch) to capture valid use of MapWritable/SortedMapWritable and the test passes. I might be missing something.

[~davidparks21], can you please point out what intended behavior is not being captured in the test.
                
> AbstractMapWritable throws exception when calling readFields() multiple times when the maps contain different class types
> -------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-9295
>                 URL: https://issues.apache.org/jira/browse/HADOOP-9295
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: io
>    Affects Versions: 1.0.3
>            Reporter: David Parks
>            Assignee: Karthik Kambatla
>            Priority: Critical
>         Attachments: MapWritableBugTest.java, test-hadoop-9295.patch
>
>
> Verified the trunk looks the same as 1.0.3 for this issue.
> When mappers output MapWritables with different class types, then they are read in on the Reducer via an iterator (multiple calls to readFields without instantiating a new object) you'll get this:
> java.lang.IllegalArgumentException: Id 1 exists but maps to org.me.ClassTypeOne and not org.me.ClassTypeTwo
>         at org.apache.hadoop.io.AbstractMapWritable.addToMap(AbstractMapWritable.java:73)
>         at org.apache.hadoop.io.AbstractMapWritable.readFields(AbstractMapWritable.java:201)
> It happens because AbstractMapWritable accumulates class type entries in its ClassType to ID (and vice versa) hashmaps.
> Those accumulating classtype-to-id hashmaps need to be cleared to support multiple calls to readFields().
> I've attached a JUnit test that both demonstrates the problem and contains an embedded, fixed version of MapWritable and ArrayMapWritable (note the //TODO comments in the code where it was fixed in 2 places).
> If there's a better way to submit this recommended bug fix, someone please feel free to let me know.

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