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https://issues.apache.org/jira/browse/LUCENE-2320?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12846451#action_12846451
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Michael McCandless commented on LUCENE-2320:
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Patch looks good Shai! I'd rather go with the SetOnce approach than introduce
a single-use factory for IW to create the MP.
But, I don't think we should keep the MP ctors that take IW? Ie, you make the
MP then call .setIW on it? We can just remove them (and advertise this in the
CHANGES bw break entry) since it's an experimental API...
> 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
> Assignee: Michael McCandless
> Fix For: 3.1
>
> Attachments: LUCENE-2320.patch
>
>
> 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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