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From "Uwe Schindler (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1801) Tokenizers (which are the source of Tokens) should call AttributeSource.clearAttributes() first
Date Tue, 11 Aug 2009 18:52:15 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1801?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12742012#action_12742012
] 

Uwe Schindler commented on LUCENE-1801:
---------------------------------------

There is an additional problem (mentioned above):
There is currently defined the clear() in Attribute interface, but this leads to the problem,
that it is really more the task of AttributeImpl. The porblem is, that e.g. one TokenFilter,
that adds a new type of attribute must could clear it. On the other hand, Token, which implements
*all* attributes could also provide clear(). By this, there is an inconsistency: clear shold
be removed from the general Attribute interface and moved downto each separate interface with
a separate name. E.g. if somebody calls TermAttribute.clear, but may think that it only clears
the term attribute may be wrong, if the actual implementation is Token, whic clears everything.

The biggest problem is backwards compatibility. Lucene 2.4.1 states in JavaDocs of Token:
"public void clear(): Resets the term text, payload, flags, and positionIncrement to default.
Other fields such as startOffset, endOffset and the token type are not reset since they are
normally overwritten by the tokenizer."

I would propose to change the whole thing:
- Remove clear() from the superinterface Attribute.
- Let do Token what it is used to (as of 2.4.1)
- Define a clearTerm(), clearPositionIncrement(), clearFlags() method for each attribute type
separate (Token/TokenWrapper must implement it).
- clear() is only defined in AttributeImpl and clears the whole implementation. AttributeSource.clearAttributes
calls this method. Current code calling clear() on the attribute interface will fail to compile,
but these are the places that must be fixed.

The problem of backwards compatibility can be solved the following way:
- TokenWrapper clears the complete delegate Token to be consistent with AttributeSource.clearAttributes()
- complete reset to default values incl offset. A problem only occurs if somebody registers
Token (not the wrapper around as Attribute), then clearAttributes() would not be consistent
with the rest, as it would miss to clear the offset.

How will we handle the clearAttributes() call in Tokenizers then? Should we only clear those
attributes we work on in a Tokenizer/TokenFilter?

> Tokenizers (which are the source of Tokens) should call AttributeSource.clearAttributes()
first
> -----------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-1801
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1801
>             Project: Lucene - Java
>          Issue Type: Task
>    Affects Versions: 2.9
>            Reporter: Uwe Schindler
>            Assignee: Uwe Schindler
>             Fix For: 2.9
>
>
> This is a followup for LUCENE-1796:
> {quote}
> Token.clear() used to be called by the consumer... but then it was switched to the producer
here: LUCENE-1101 
> I don't know if all of the Tokenizers in lucene were ever changed, but in any case it
looks like at least some of these bugs were introduced with the switch to the attribute API
- for example StandardTokenizer did clear it's reusableToken... and now it doesn't.
> {quote}
> As alternative to changing all core/contrib Tokenizers to call clearAttributes first,
we could do this in the indexer, what would be a overhead for old token streams that itsself
clear their reusable token. This issue should also update the Javadocs, to clearly state inside
Tokenizer.java, that the source TokenStream (normally the Tokenizer) should clear *all* Attributes.
If it does not do it and e.g. the positionIncrement is changed to 0 by any TokenFilter, but
the filter does not change it back to 1, the TokenStream would stay with 0. If the TokenFilter
would call PositionIncrementAttribute.clear() (because he is responsible), it could also break
the TokenStream, because clear() is a general method for the whole attribute instance. If
e.g. Token is used as AttributeImpl, a call to clear() would also clear offsets and termLength,
which is not wanted. So the source of the Tokenization should rest the attributes to default
values.
> LUCENE-1796 removed the iterator creation cost, so clearAttributes should run fast, but
is an additional cost during Tokenization, as it was not done consistently before, so a small
speed degradion is caused by this, but has nothing to do with the new TokenStream API.

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