Add MergePolicy to IndexWriterConfig
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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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