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https://issues.apache.org/jira/browse/MAPREDUCE-954?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12756559#action_12756559
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Tom White commented on MAPREDUCE-954:
-------------------------------------
I've gone back to the cases that have motivated this in order to better
understand why this change is needed.
The first motivating case is MAPREDUCE-901, which needs to change the
constructor to ReduceContext due to an internal type change. As noted above,
user applications should never create context objects, so the constructor could
be marked as Private Evolving to solve this problem, couldn't it?
The other motivating case is MAPREDUCE-372 (chain MapReduce), where various
fields of the context need to be replaced with custom versions. The approach
described in [this
comment|https://issues.apache.org/jira/browse/MAPREDUCE-372?focusedCommentId=12752452&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_12752452]
which is to add a new constructor to ReduceContext (a "pseudo copy
constructor"), could again be made Private Evolving to mark its scope. This
approach didn't cause any information leakage as far as I can tell (the
previous version had to add extra getters, which did cause leakage).
Another approach would be to create package private constructors and have a
Private Evolving ContextFactory in the same package if the intent is to keep
the constructors further away from user code (in IDE autocomplete, for
example). The point is that I think we can avoid the problem of leakage with
some changes to visibility annotations.
> The new interface's Context objects should be interfaces
> --------------------------------------------------------
>
> Key: MAPREDUCE-954
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-954
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: client
> Reporter: Owen O'Malley
> Assignee: Arun C Murthy
> Fix For: 0.21.0
>
> Attachments: MAPREDUCE-954.patch, MAPREDUCE-954.patch,
> MAPREDUCE-954.patch
>
>
> When I was doing HADOOP-1230, I was persuaded to make the Context objects as
> classes. I think that was a serious mistake. It caused a lot of information
> leakage into the public classes.
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