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https://issues.apache.org/jira/browse/LUCENE-3560?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13144721#comment-13144721
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Robert Muir commented on LUCENE-3560:
-------------------------------------
{quote}
So let's leave extensibility where it's clear that stuff can be extended with
no harm (or "no harm if you read the instructions").
{quote}
And thats exactly what this patch does: it only allows extensibility where
there is no harm. The problem is, in trunk today, no methods on any codecs are
final (and some should be!).
The places where there is no harm: e.g. getPostingsFormatForField, are still
left open. This patch doesn't stop you from doing anything you can't already do
today.
It only stops code that was already doomed to fail at runtime from even
compiling.
> add extra safety to concrete codec implementations
> --------------------------------------------------
>
> Key: LUCENE-3560
> URL: https://issues.apache.org/jira/browse/LUCENE-3560
> Project: Lucene - Java
> Issue Type: Improvement
> Affects Versions: 4.0
> Reporter: Robert Muir
> Attachments: LUCENE-3560.patch
>
>
> In LUCENE-3490, we reorganized the codec model, and a key part of this is
> that Codecs are "safer"
> and don't rely upon client-side configuration: IndexReader doesn't take Codec
> or anything of that
> nature, only IndexWriter.
> Instead for "read" all codecs are initialized from the classpath via a no-arg
> ctor from Java's
> Service Provider Mechanism.
> So, although Codecs can still take parameters in the constructors, be
> subclassable, etc (for passing
> to IndexWriter), this enforces that they must write any configuration
> information they need into
> the index, so that we don't have a flimsy API.
> I think we should go even further, for additional safety. Any methods on our
> concrete codecs that
> are not intended to be subclassed should be final, and we should add
> assertions to verify this.
> For example, SimpleText's files() implementation should be final. If you want
> to make an extension
> of simpletext that has additional files, then this is a different index
> format and should have a
> different name!
> Note: This doesn't stop extensibility, only stupid mistakes.
> For example, this means that Lucene40Codec's postingsFormat() implementation
> is final, even though
> it offers a configurable "hook" (getPostingsFormatForField) for you to
> specify per-field postings
> formats (which it writes into a .per file into the index, so that it knows
> how to read each field).
> {code}
> private final PostingsFormat postingsFormat = new PerFieldPostingsFormat() {
> @Override
> public PostingsFormat getPostingsFormatForField(String field) {
> return Lucene40Codec.this.getPostingsFormatForField(field);
> }
> };
> ...
> @Override
> public final PostingsFormat postingsFormat() {
> return postingsFormat;
> }
> ...
> /** Returns the postings format that should be used for writing
> * new segments of <code>field</code>.
> *
> * The default implementation always returns "Lucene40"
> */
> public PostingsFormat getPostingsFormatForField(String field) {
> return defaultFormat;
> }
> {code}
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