This is the initial step of MAPREDUCE-6823, which proposes a factory behind FileOutputFormat to create different committers for different filesystems, if so configured..
This patch simply adds the new abstract superclass of FileOutputCommitter, PathOutputCommitter extends OutputCommitter. This abstract class adds the getWorkPath() method as an abstract method, with FIleOutputCommitter being the implementation..
FileOutputFormat then relaxes its requirement of any committer returned by getOutputCommitter(), so that instead of requiring a FileOutputCommitter or subclass, it only needs a PathOutputCommitter, using PathOutputCommitter.getWorkPath() to get the work path.
What does that do?
It allows people to implement subclasses of FileOutputFormat which can provide their own committers which don't need to inherit the complexity that FileOutputCommitter has acquired over time
Currently anyone implementing a new committer (example: Netflix S3 committer) needs to subclass FileOutputCommitter, which is too complex to understand except under a debugger with co-recursive routines, lots of methods which need to be overwritten to guarantee a safe subclass, and, because of its critical role and known subclassing, something which isn't ever going to be cleaned up.
A new, lean, parent class which FileOutputFormat can handle allows people to write new committers which don't have to worry about implementation details of FileOutputCommitter, but instead how well they implement the semantics of committing work.
The full MAPREDUCE-6823 goes beyond this with a change to FileOutputFormat for a factory for creating FS-specific PathOutputCommitter instances. This patch doesn't include that, as that is something which needs to be reviewed in the context of HADOOP-13786 and ideally 1+ committer for another store, so people can say "this factory model works".
All I'm proposing here is: tune the committer class hierarchy in MRv2 so that people can more easily implement committers, and when that factory is done, for it to be switched to easily. And I'd like this in branch-3 from the outset, so existing code which calls FileOutputFormat.getCommitter() to get a FileOutputCommitter just to call getWorkPath() can move to the new interface across all of Hadoop 3.