[ http://issues.apache.org/jira/browse/HADOOP-839?page=all ]
Feng Jiang updated HADOOP-839:
------------------------------
Attachment: patch.839.r488924
This is reopened version of issue#689.
The org.apache.hadoop.io.GenericWritable.java (at -r488924) has a little bit
problem, though it works in the most cases. But the registered classes are in
the relationship of Parents-child, it doesn't work correctly.
For example, ClassType1 is super class of ClassType2, the an object that wanted
to be wrapped is ClassType2. But in the set(Writable obj) method, the
ClassType1 may be treated as corresponding Class incorrectly.
Meanwhile, the old GenericWritable uses the O(N) algorithm to look up the
corresponding class, so the performance is not so good especially when set()
method is called frequently.
The updated GenericWritable fixed this bug, and improved the performance. It
has detailed javadoc and sample to show users how to use it.
> CLONE -hadoop should provide a common way to wrap instances with different
> types into one type
> ----------------------------------------------------------------------------------------------
>
> Key: HADOOP-839
> URL: http://issues.apache.org/jira/browse/HADOOP-839
> Project: Hadoop
> Issue Type: Improvement
> Components: io
> Environment: All environment
> Reporter: Feng Jiang
> Fix For: 0.9.0
>
> Attachments: patch.839.r488924
>
>
> When two sequence files, which have same Key type but different Value types,
> are mapped out to reduce, multiple Value types is not allowed. In this case,
> we need a way to wrap instances with different types into one class type to
> reduce.
> In current code, ObjectWritable is a sole choice. but it costs too many
> space, because the class declaration will be appended into output file as a
> string for every Key-value pair.
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