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From "Uwe Schindler (Commented) (JIRA)" <>
Subject [jira] [Commented] (LUCENE-3560) add extra safety to concrete codec implementations
Date Sat, 05 Nov 2011 10:16:51 GMT


Uwe Schindler commented on LUCENE-3560:

Heavy reflecting in the good old TokenStream assertFinal (I was so unhappy when Analyzer was
restructured that it had to go there... *g*).

Some comments:
- We should maybe also add a check that there is at least a default constructor available.
this.getClass().getConstructor() does not throw exception
- In general, subclassing a Codec or a PostingsFormat is wrong (except the Lucene3x hack).
If you subclass a codec/PF, you can no longer  change it's name. So anybody who subclasses
a codec will produce a clone with the same name but perhaps another index format. This is
prevented by Robert's finalness on the format hooks, but what else could a codec do different
if its not final without breaking index format?
- I think even 3x Codec should be final and not subclassed by the RW codec. The RW Preflex
codec in tests should subclass abstract Codec, and simply delegate all "read" methods to the
RO-Codec [I am not sure if this all works as its very complicated... *g* - I only mention:
new Exception().getStackTrace() to inspect call stack... highly sophisticated!].
> add extra safety to concrete codec implementations
> --------------------------------------------------
>                 Key: LUCENE-3560
>                 URL:
>             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
> 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
> 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
> 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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