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https://issues.apache.org/jira/browse/HADOOP-8549?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13414687#comment-13414687
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Harsh J commented on HADOOP-8549:
---------------------------------
Hi and thanks for the reply!
bq. I tried it to make as ArrayWritable but it breaks the existing MapWritable
code since implementation class information has to be written into stream. Even
i tried setters/getters but it will not work since in M/R code Writables are
created through default constructor. So protected member was only way i can
think of solving the problem.
{code}
/** {@inheritDoc} */
@Override
public void write(DataOutput out) throws IOException {
super.write(out);
// Write out the number of entries in the map
out.writeInt(instance.size());
// Then write out each key/value pair
for (Map.Entry<Writable, Writable> e: instance.entrySet()) {
out.writeByte(getId(e.getKey().getClass()));
e.getKey().write(out);
out.writeByte(getId(e.getValue().getClass()));
e.getValue().write(out);
}
}
{code}
In the above writer method, I do not see the instance's type being serialized.
The key and value types are indeed stored, but not of the instance's.
Why can we hence not have a {{MapWritable(Class <? extends Map<Writable,
Writable>> mapType)}} that then instantiates the class for the field member
"instance", rather than a dev hacking like that?
bq. Since client has to use different class to get the new behavior (ex
LinkedHashMapWritable) and Writables are not polymorphic, portability should
not be an issue.
Lets still have a test case. Your current test case merely checks the ordering
of a sequential implementation without serializing it. Lets instead serialize
it, deserialize it again with your custom writable, check the ordering. Also
deserialize it with a regular instance, and check the content. Let me know if
this makes sense. This will help catch regressions in future and good tests are
always good to have.
Thanks and let me know if am wrong somewhere or if you have any other
questions! :)
> Allow other implementation's of java.util.Map in MapWritable
> ------------------------------------------------------------
>
> Key: HADOOP-8549
> URL: https://issues.apache.org/jira/browse/HADOOP-8549
> Project: Hadoop Common
> Issue Type: Improvement
> Components: io
> Affects Versions: 2.0.0-alpha
> Reporter: madhukara phatak
> Assignee: madhukara phatak
> Priority: Minor
> Attachments: HADOOP-8549.patch
>
>
> Current implementation of MapWritable uses HashMap as Map implementation. But
> in some of the use cases we need other implementations of Map like
> LinkedHashMap,SortedMap.This jira changes visibility of 'instance' in
> MapWritable from private to protected which allows us to inject custom Map
> implementation through sub classing MapWritable .
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