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From "Uwe Schindler (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2345) Make it possible to subclass SegmentReader
Date Fri, 26 Mar 2010 10:56:27 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2345?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12850079#action_12850079
] 

Uwe Schindler commented on LUCENE-2345:
---------------------------------------

Non-final/private method calls from ctors is discouraged, as it creates problems:

- If you override such a init method, you can change the behaviour of the super class' ctor
- During the time the init() method is called, the fields are not yet initialized in the subclass
and also the subclass' ctor was not yet called! This leads to unexspected behaviour (like
final fields changing its contents during the lifetime of the class: during the overridden
init() method they are null and suddenly change to non-null after the subclass' ctor was executed,
which is after the init() method)

Because of this, such init() methods should be private or at least final and never public!

> Make it possible to subclass SegmentReader
> ------------------------------------------
>
>                 Key: LUCENE-2345
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2345
>             Project: Lucene - Java
>          Issue Type: Wish
>          Components: Index
>            Reporter: Tim Smith
>             Fix For: 3.1
>
>         Attachments: LUCENE-2345_3.0.patch
>
>
> I would like the ability to subclass SegmentReader for numerous reasons:
> * to capture initialization/close events
> * attach custom objects to an instance of a segment reader (caches, statistics, so on
and so forth)
> * override methods on segment reader as needed
> currently this isn't really possible
> I propose adding a SegmentReaderFactory that would allow creating custom subclasses of
SegmentReader
> default implementation would be something like:
> {code}
> public class SegmentReaderFactory {
>   public SegmentReader get(boolean readOnly) {
>     return readOnly ? new ReadOnlySegmentReader() : new SegmentReader();
>   }
>   public SegmentReader reopen(SegmentReader reader, boolean readOnly) {
>     return newSegmentReader(readOnly);
>   }
> }
> {code}
> It would then be made possible to pass a SegmentReaderFactory to IndexWriter (for pooled
readers) as well as to SegmentReader.get() (DirectoryReader.open, etc)
> I could prepare a patch if others think this has merit
> Obviously, this API would be "experimental/advanced/will change in future"

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