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From "Robert Muir (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3560) add extra safety to concrete codec implementations
Date Sat, 05 Nov 2011 15:30:51 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3560?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13144721#comment-13144721
] 

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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