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https://issues.apache.org/jira/browse/LUCENE-3866?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13395847#comment-13395847
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Uwe Schindler commented on LUCENE-3866:
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I will later also upgrade MIGRATE.txt markdown.
> Make CompositeReader.getSequentialSubReaders() and the corresponding
> IndexReaderContext methods return unmodifiable List<R extends IndexReader>
> -----------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-3866
> URL: https://issues.apache.org/jira/browse/LUCENE-3866
> Project: Lucene - Java
> Issue Type: Improvement
> Reporter: Uwe Schindler
> Assignee: Uwe Schindler
> Fix For: 4.0
>
> Attachments: LUCENE-3866-step1.patch, LUCENE-3866-step2.patch,
> LUCENE-3866-step2.patch
>
>
> Since Lucene 2.9 we have the method getSequentialSubReader() returning
> IndexReader[]. Based on hardly-to-debug errors in user's code, Robert and me
> realized that returning an array from a public API is an anti-pattern. If the
> array is intended to be modifiable (like byte[] in terms,...), it is fine to
> use arrays in public APIs, but not, if the array must be protected from
> modification. As IndexReaders are 100% unmodifiable in trunk code (no
> deletions,...), the only possibility to corrumpt the reader is by modifying
> the array returned by getSequentialSubReaders(). We should prevent this.
> The same theoretically applies to FieldCache, too - but the party that is
> afraid of performance problems is too big to fight against that :(
> For getSequentialSubReaders there is no performance problem at all. The
> binary search of reader-ids inside BaseCompositeReader can still be done with
> the internal protected array, but public APIs should expose only a
> unmodifiable List. The same applies to leaves() and children() in
> IndexReaderContext. This change to list would also allow to make
> CompositeReader and CompositeReaderContext Iterable<R extends IndexReader>,
> so some loops would look nice.
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