org.apache.lucene.analysis
Class TokenStream

java.lang.Object
  org.apache.lucene.util.AttributeSource
      org.apache.lucene.analysis.TokenStream

All Implemented Interfaces:

Closeable

Direct Known Subclasses:

CategoryAttributesStream, EmptyTokenStream, NumericTokenStream, PrefixAndSuffixAwareTokenFilter, PrefixAwareTokenFilter, ShingleMatrixFilter, SingleTokenTokenStream, TeeSinkTokenFilter.SinkTokenStream, TokenFilter, Tokenizer, TokenStreamFromTermPositionVector

public abstract class TokenStream
extends AttributeSource
implements Closeable

A TokenStream enumerates the sequence of tokens, either from Fields of a Document or from query text.

This is an abstract class; concrete subclasses are:

• Tokenizer, a TokenStream whose input is a Reader; and
• TokenFilter, a TokenStream whose input is another TokenStream.

A new 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 AttributeImpls.

TokenStream now extends AttributeSource, which provides access to all of the token Attributes 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 AttributeImpls. See incrementToken() for further details.

The workflow of the new TokenStream API is as follows:

1. Instantiation of TokenStream/TokenFilters which add/get attributes to/from the AttributeSource.
2. The consumer calls reset().
3. The consumer retrieves attributes from the stream and stores local references to all attributes it wants to access.
4. The consumer calls incrementToken() until it returns false consuming the attributes after each call.
5. The consumer calls end() so that any end-of-stream operations can be performed.
6. The consumer calls close() to release any resource when finished using the TokenStream.

To make sure 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().

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

protected	TokenStream() A TokenStream using the default attribute factory.
protected	TokenStream(AttributeSource.AttributeFactory factory) A TokenStream using the supplied AttributeFactory for creating new Attribute instances.
protected	TokenStream(AttributeSource input) A TokenStream that uses the same attributes as the supplied one.

Method Summary

void	close() Releases resources associated with this stream.
void	end() This method is called by the consumer after the last token has been consumed, after incrementToken() returned false (using the new TokenStream API).
abstract boolean	incrementToken() Consumers (i.e., IndexWriter) use this method to advance the stream to the next token.
void	reset() Resets this stream to the beginning.

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

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

TokenStream

protected TokenStream()

A TokenStream using the default attribute factory.

TokenStream

protected TokenStream(AttributeSource input)

A TokenStream that uses the same attributes as the supplied one.

TokenStream

protected TokenStream(AttributeSource.AttributeFactory factory)

A TokenStream using the supplied AttributeFactory for creating new Attribute instances.

Method Detail

incrementToken

public abstract boolean incrementToken()
                                throws IOException

Consumers (i.e., IndexWriter) use this method to advance the stream to the next token. Implementing classes must implement this method and update the appropriate AttributeImpls 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) and AttributeSource.getAttribute(Class), references to all AttributeImpls 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

public void end()
         throws IOException

This method is called by the consumer after the last token has been consumed, after incrementToken() returned false (using the new TokenStream 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.

Throws:

IOException

reset

public void reset()
           throws IOException

Resets this stream to the beginning. This is an optional operation, so subclasses may or may not implement this method. reset() is not needed for the standard indexing process. However, if the tokens of a TokenStream are intended to be consumed more than once, it is necessary to implement reset(). Note that if your TokenStream caches tokens and feeds them back again after a reset, it is imperative that you clone the tokens when you store them away (on the first pass) as well as when you return them (on future passes after reset()).

Throws:

IOException

close

public void close()
           throws IOException

Releases resources associated with this stream.

Specified by:

close in interface Closeable

Throws:

IOException