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https://issues.apache.org/jira/browse/HADOOP-3665?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12609381#action_12609381
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Chris Douglas commented on HADOOP-3665:
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* NullWritable should define a RawComparator, so it doesn't need to use the
default WritableComparator rather than special-casing NullWritable
* Your apt use of ReflectionUtils calls out a separate issue with
WritableComparator, i.e. that the keys and values it uses are not Configured,
where in most contexts they are. This should probably be a separate JIRA...
* SequenceFile shouldn't make a special case of NullWritable, either. Does the
rest of the logic still work if the expression is changed from keylength == 0
to keylength < 0?
* ReflectionUtils shouldn't special-case NullWritable, either. Are you certain
this is necessary? Doesn't setAccessible take care of this?
> WritableComparator newKey() fails for NullWritable
> --------------------------------------------------
>
> Key: HADOOP-3665
> URL: https://issues.apache.org/jira/browse/HADOOP-3665
> Project: Hadoop Core
> Issue Type: Bug
> Components: io
> Affects Versions: 0.16.0, 0.16.1, 0.16.2, 0.16.3, 0.16.4, 0.17.0
> Environment: n/a
> Reporter: Lukas Vlcek
> Priority: Minor
> Fix For: 0.19.0
>
> Attachments: HADOOP-3665.path
>
>
> It is not possible to use NullWritable as a key in order to suppress key
> value in output.
> Syndrome exception:
> Caused by: java.lang.IllegalAccessException: Class
> org.apache.hadoop.io.WritableComparator can not access a member of class
> org.apache.hadoop.io.NullWritable with modifiers "private"
> The problem is that NullWritable is a singleton and does not provide public
> non-parametric constructor. The following code in WritableComparator causes
> the exception: return (WritableComparable)keyClass.newInstance();
> Proposed simple solution is to use ReflectionUtils instead (it requires
> modification as well).
> This issue is probably related to HADOOP-2922
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