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From "Uwe Schindler (JIRA)" <>
Subject [jira] Commented: (LUCENE-1693) AttributeSource/TokenStream API improvements
Date Wed, 15 Jul 2009 16:55:15 GMT


Uwe Schindler commented on LUCENE-1693:

After the whole day thinking about a solution for overriding deprecated methods, I came to
one conclusion/solution, that would create a "visible" backwards break (to be noted in CHANGES.txt).

Mike's idea from LUCENE-1678 is good, but very complicated for this issue and may lead to
unpredicted behavior. And what makes me think, that this will not be a problem for developers
is the fact that there is no JIRA issue about a similar break. When Lucene switched from next()
to next(reusableToken), we also had a compatibility method in TokenStream that delegates to
next(new Token()). Core streams did *not* implement the old method and the indexer code only
called next(Token). If somebody would have overridden only the old next() method of a core
tokenstream, this method would have been never called -> bumm we had a break, but nobody
realized it. With the new patch, we have the same in 2.9 for incrementToken vs. next(Token)
and also next(). In principle the same issue like in LUCENE-1678.

The good thing is, that most TokenStreams in core are final, except the following ones:
- ISOLatin1Filter
- KeywordTokenizer
- StandardTokenizer
and last but not least the whole structure of subclasses of CharTokenizer. The good thing
and thanks to the developer, they are correctly implemented, making their methods incrementToken,
next(Token) *final*. Haha, nobody could override them, so the class is not final, but the
affected methods. So all subclasses of CharTokenizer are also not affected.

My latest patch also includes this *final* modifier for the abstract CharTokenizer:
public final Token next(final Token reusableToken) throws IOException {
 // Overriding this method to make it final as before has no effect for the reflection-wrapper
in TokenStream.
 // TokenStream.hasReusableNext is true because of this, but it is never used, as incrementToken()
has preference.

So it is not overrideable and is still compatible (code calling next(Token) will be delegated
to incrementToken() by the superclass). For complete correctness also next() should be similar
overridden. In both cases the super's method always delegates preferably to incrementToken()
so iven that a subclass of TokenStream overrides this method and so hasNext == true and hasReusableNext
== true, incrementToken() is still preferred, so everything works.

To prevent users from overriding next() or next(Token) of core or contrib tokenstreams (which
in my opinion nobody has ever done, because if yes, we would have a bug report regarding the
last transition). For those people, that really have done it (they used one of the tree classes
above as super for their own class, the error would not be to detect. Their TokenStream would
simply not work, as next()/next(Token) is never called. To produce a compile error for them
(or a runtime error, when they instantiate such a class), I suggest to include a backwards-break
(which is better than failing silently). All non-final TokenStreams/Tokenizers/TokenFilters
should simply include the code snipplet above to redeclare next() *and* next(Token) as final
(only delegating to super). Instead of failing silently, users will get runtime linker errors
(when they replace the lucene jar) or compile errors. We have done a similar change in TokenFilter,
because we made the delegate stream final to prevent disturbing the attributes (Mike have
done this in LUCENE-1636).

CHANGES.txt would contain this as BW-break together with the other breaks.

Any comments? Michael, what do you think?

> AttributeSource/TokenStream API improvements
> --------------------------------------------
>                 Key: LUCENE-1693
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Analysis
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: 2.9
>         Attachments: LUCENE-1693.patch, LUCENE-1693.patch, LUCENE-1693.patch, LUCENE-1693.patch,
LUCENE-1693.patch, LUCENE-1693.patch, LUCENE-1693.patch, LUCENE-1693.patch, LUCENE-1693.patch,
LUCENE-1693.patch, LUCENE-1693.patch, lucene-1693.patch,,,,,
> This patch makes the following improvements to AttributeSource and
> TokenStream/Filter:
> - removes the set/getUseNewAPI() methods (including the standard
>   ones). Instead by default incrementToken() throws a subclass of
>   UnsupportedOperationException. The indexer tries to call
>   incrementToken() initially once to see if the exception is thrown;
>   if so, it falls back to the old API.
> - introduces interfaces for all Attributes. The corresponding
>   implementations have the postfix 'Impl', e.g. TermAttribute and
>   TermAttributeImpl. AttributeSource now has a factory for creating
>   the Attribute instances; the default implementation looks for
>   implementing classes with the postfix 'Impl'. Token now implements
>   all 6 TokenAttribute interfaces.
> - new method added to AttributeSource:
>   addAttributeImpl(AttributeImpl). Using reflection it walks up in the
>   class hierarchy of the passed in object and finds all interfaces
>   that the class or superclasses implement and that extend the
>   Attribute interface. It then adds the interface->instance mappings
>   to the attribute map for each of the found interfaces.
> - AttributeImpl now has a default implementation of toString that uses
>   reflection to print out the values of the attributes in a default
>   formatting. This makes it a bit easier to implement AttributeImpl,
>   because toString() was declared abstract before.
> - Cloning is now done much more efficiently in
>   captureState. The method figures out which unique AttributeImpl
>   instances are contained as values in the attributes map, because
>   those are the ones that need to be cloned. It creates a single
>   linked list that supports deep cloning (in the inner class
>   AttributeSource.State). AttributeSource keeps track of when this
>   state changes, i.e. whenever new attributes are added to the
>   AttributeSource. Only in that case will captureState recompute the
>   state, otherwise it will simply clone the precomputed state and
>   return the clone. restoreState(AttributeSource.State) walks the
>   linked list and uses the copyTo() method of AttributeImpl to copy
>   all values over into the attribute that the source stream
>   (e.g. SinkTokenizer) uses. 
> The cloning performance can be greatly improved if not multiple
> AttributeImpl instances are used in one TokenStream. A user can
> e.g. simply add a Token instance to the stream instead of the individual
> attributes. Or the user could implement a subclass of AttributeImpl that
> implements exactly the Attribute interfaces needed. I think this
> should be considered an expert API (addAttributeImpl), as this manual
> optimization is only needed if cloning performance is crucial. I ran
> some quick performance tests using Tee/Sink tokenizers (which do
> cloning) and the performance was roughly 20% faster with the new
> API. I'll run some more performance tests and post more numbers then.
> Note also that when we add serialization to the Attributes, e.g. for
> supporting storing serialized TokenStreams in the index, then the
> serialization should benefit even significantly more from the new API
> than cloning. 
> Also, the TokenStream API does not change, except for the removal 
> of the set/getUseNewAPI methods. So the patches in LUCENE-1460
> should still work.
> All core tests pass, however, I need to update all the documentation
> and also add some unit tests for the new AttributeSource
> functionality. So this patch is not ready to commit yet, but I wanted
> to post it already for some feedback. 

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