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From "Shai Erera (JIRA)" <j...@apache.org>
Subject [jira] Created: (LUCENE-2320) Add MergePolicy to IndexWriterConfig
Date Sun, 14 Mar 2010 07:33:27 GMT
Add MergePolicy to IndexWriterConfig
------------------------------------

                 Key: LUCENE-2320
                 URL: https://issues.apache.org/jira/browse/LUCENE-2320
             Project: Lucene - Java
          Issue Type: Improvement
          Components: Index
            Reporter: Shai Erera
             Fix For: 3.1


Now that IndexWriterConfig is in place, I'd like to move MergePolicy to it as well. The change
is not straightforward and so I've kept it for a separate issue. MergePolicy requires in its
ctor an IndexWriter, however none can be passed to it before an IndexWriter actually exists.
And today IW may create an MP just for it to be overridden by the application one line afterwards.
I don't want to make iw member of MP non-final, or settable by extending classes, however
it needs to remain protected so they can access it directly. So the proposed changes are:

* Add a SetOnce object (to o.a.l.util), or Immutable, which can only be set once (hence its
name). It'll have the signature SetOnce<T> w/ *synchronized set<T>* and *T get()*.
T will be declared volatile, so that get() won't be synchronized.
* MP will define a *protected final SetOnce<IndexWriter> writer* instead of the current
writer. *NOTE: this is a bw break*. any suggestions are welcomed.
* MP will offer a public default ctor, together with a set(IndexWriter).
* IndexWriter will set itself on MP using set(this). Note that if set will be called more
than once, it will throw an exception (AlreadySetException - or does someone have a better
suggestion, preferably an already existing Java exception?).

That's the core idea. I'd like to post a patch soon, so I'd appreciate your review and proposals.

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