Class TokenStream
- All Implemented Interfaces:
Closeable
,AutoCloseable
- Direct Known Subclasses:
NumericTokenStream
,TokenFilter
,Tokenizer
TokenStream
enumerates the sequence of tokens, either from
Field
s of a Document
or from query text.
This is an abstract class; concrete subclasses are:
Tokenizer
, aTokenStream
whose input is a Reader; andTokenFilter
, aTokenStream
whose input is anotherTokenStream
.
TokenStream
API has been introduced with Lucene 2.9. This API
has moved from being Token
-based to Attribute
-based. While
Token
still exists in 2.9 as a convenience class, the preferred way
to store the information of a Token
is to use AttributeImpl
s.
TokenStream
now extends AttributeSource
, which provides
access to all of the token Attribute
s for the TokenStream
.
Note that only one instance per AttributeImpl
is created and reused
for every token. This approach reduces object creation and allows local
caching of references to the AttributeImpl
s. See
incrementToken()
for further details.
The workflow of the new TokenStream
API is as follows:
- Instantiation of
TokenStream
/TokenFilter
s which add/get attributes to/from theAttributeSource
. - The consumer calls
reset()
. - The consumer retrieves attributes from the stream and stores local references to all attributes it wants to access.
- The consumer calls
incrementToken()
until it returns false consuming the attributes after each call. - The consumer calls
end()
so that any end-of-stream operations can be performed. - The consumer calls
close()
to release any resource when finished using theTokenStream
.
incrementToken()
.
You can find some example code for the new API in the analysis package level Javadoc.
Sometimes it is desirable to capture a current state of a TokenStream
,
e.g., for buffering purposes (see CachingTokenFilter
,
TeeSinkTokenFilter). For this usecase
AttributeSource.captureState()
and AttributeSource.restoreState(org.apache.lucene.util.AttributeSource.State)
can be used.
The TokenStream
-API in Lucene is based on the decorator pattern.
Therefore all non-abstract subclasses must be final or have at least a final
implementation of incrementToken()
! This is checked when Java
assertions are enabled.
-
Nested Class Summary
Nested classes/interfaces inherited from class org.apache.lucene.util.AttributeSource
AttributeSource.AttributeFactory, AttributeSource.State
-
Constructor Summary
ConstructorsModifierConstructorDescriptionprotected
A TokenStream using the default attribute factory.protected
TokenStream
(AttributeSource input) A TokenStream that uses the same attributes as the supplied one.protected
A TokenStream using the supplied AttributeFactory for creating newAttribute
instances. -
Method Summary
Modifier and TypeMethodDescriptionvoid
close()
Releases resources associated with this stream.void
end()
This method is called by the consumer after the last token has been consumed, afterincrementToken()
returnedfalse
(using the newTokenStream
API).abstract boolean
Consumers (i.e.,IndexWriter
) use this method to advance the stream to the next token.void
reset()
This method is called by a consumer before it begins consumption usingincrementToken()
.Methods inherited from class org.apache.lucene.util.AttributeSource
addAttribute, addAttributeImpl, captureState, clearAttributes, cloneAttributes, copyTo, equals, getAttribute, getAttributeClassesIterator, getAttributeFactory, getAttributeImplsIterator, hasAttribute, hasAttributes, hashCode, reflectAsString, reflectWith, restoreState, toString
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Constructor Details
-
TokenStream
protected TokenStream()A TokenStream using the default attribute factory. -
TokenStream
A TokenStream that uses the same attributes as the supplied one. -
TokenStream
A TokenStream using the supplied AttributeFactory for creating newAttribute
instances.
-
-
Method Details
-
incrementToken
Consumers (i.e.,IndexWriter
) use this method to advance the stream to the next token. Implementing classes must implement this method and update the appropriateAttributeImpl
s with the attributes of the next token.The producer must make no assumptions about the attributes after the method has been returned: the caller may arbitrarily change it. If the producer needs to preserve the state for subsequent calls, it can use
AttributeSource.captureState()
to create a copy of the current attribute state.This method is called for every token of a document, so an efficient implementation is crucial for good performance. To avoid calls to
AttributeSource.addAttribute(Class)
andAttributeSource.getAttribute(Class)
, references to allAttributeImpl
s that this stream uses should be retrieved during instantiation.To ensure that filters and consumers know which attributes are available, the attributes must be added during instantiation. Filters and consumers are not required to check for availability of attributes in
incrementToken()
.- Returns:
- false for end of stream; true otherwise
- Throws:
IOException
-
end
This method is called by the consumer after the last token has been consumed, afterincrementToken()
returnedfalse
(using the newTokenStream
API). Streams implementing the old API should upgrade to use this feature. This method can be used to perform any end-of-stream operations, such as setting the final offset of a stream. The final offset of a stream might differ from the offset of the last token eg in case one or more whitespaces followed after the last token, but a WhitespaceTokenizer was used.Additionally any skipped positions (such as those removed by a stopfilter) can be applied to the position increment, or any adjustment of other attributes where the end-of-stream value may be important.
If you override this method, always call
super.end()
.- Throws:
IOException
- If an I/O error occurs
-
reset
This method is called by a consumer before it begins consumption usingincrementToken()
.Resets this stream to a clean state. Stateful implementations must implement this method so that they can be reused, just as if they had been created fresh.
If you override this method, always call
super.reset()
, otherwise some internal state will not be correctly reset (e.g.,Tokenizer
will throwIllegalStateException
on further usage).- Throws:
IOException
-
close
Releases resources associated with this stream.If you override this method, always call
super.close()
, otherwise some internal state will not be correctly reset (e.g.,Tokenizer
will throwIllegalStateException
on reuse).- Specified by:
close
in interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Throws:
IOException
-