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https://issues.apache.org/jira/browse/MAPREDUCE-4851?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13511606#comment-13511606
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Radim Kolar commented on MAPREDUCE-4851:
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Actually it can be made backward compatible. Accept old style comparators and 
do runtime check if they are instance of new class and then invoke lifecycle 
methods.
                
> add lifecycle to Comparators
> ----------------------------
>
>                 Key: MAPREDUCE-4851
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4851
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>            Reporter: Radim Kolar
>              Labels: mrv2
>
> current mapreduce api is using RawComparator interface in:
> setGroupingComparatorClass
> setSortComparatorClass
> This interface has no lifecycle support. I propose to change that methods to 
> take argument new class implements RawComparator with setup and cleanup 
> methods. 
> This will leave existing code with RawComparator alone, Providing some 
> backward compatibility.
> new class: 
> class SortComparator implements RawComparator

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