C# Class Lucene.Net.Analysis.TokenStream

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 TokenStream#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 can be used.

The {@code 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.

Inheritance: Lucene.Net.Util.AttributeSource, IDisposable
Datei anzeigen Open project: synhershko/lucene.net Class Usage Examples

Public Methods

Method Description
Dispose ( ) : void
End ( ) : void

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.
IncrementToken ( ) : bool

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 Util.Attributes 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(Type) and AttributeSource.GetAttribute(Type), references to all Util.Attributes 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().
Reset ( ) : void

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()).

Protected Methods

Method Description
Dispose ( bool disposing ) : void
TokenStream ( ) : System

A TokenStream using the default attribute factory.
TokenStream ( Lucene.Net.Util.AttributeSource.AttributeFactory factory ) : System

A TokenStream using the supplied AttributeFactory for creating new IAttribute instances.
TokenStream ( Lucene.Net.Util.AttributeSource input ) : System

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

Private Methods

Method Description
Close ( ) : void

Method Details

Dispose() public method

public Dispose ( ) : void
return void

Dispose() protected abstract method

protected abstract Dispose ( bool disposing ) : void
disposing bool
return void

End() public method

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.

public End ( ) : void
return void

IncrementToken() public abstract method

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 Util.Attributes 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(Type) and AttributeSource.GetAttribute(Type), references to all Util.Attributes 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().
public abstract IncrementToken ( ) : bool
return bool

Reset() public method

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()).
public Reset ( ) : void
return void

TokenStream() protected method

A TokenStream using the default attribute factory.
protected TokenStream ( ) : System
return System

TokenStream() protected method

A TokenStream using the supplied AttributeFactory for creating new IAttribute instances.
protected TokenStream ( Lucene.Net.Util.AttributeSource.AttributeFactory factory ) : System
factory Lucene.Net.Util.AttributeSource.AttributeFactory
return System

TokenStream() protected method

A TokenStream that uses the same attributes as the supplied one.
protected TokenStream ( Lucene.Net.Util.AttributeSource input ) : System
input Lucene.Net.Util.AttributeSource
return System