Class 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:

A new TokenStream API has been introduced with Lucene 2.9. This API has moved from being Token based to Lucene.Net.Util.IAttribute 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 Lucene.Net.Util.Attributes.

TokenStream now extends Lucene.Net.Util.AttributeSource, which provides access to all of the token Lucene.Net.Util.IAttributes for the TokenStream. Note that only one instance per Lucene.Net.Util.Attribute is created and reused for every token. This approach reduces object creation and allows local caching of references to the Lucene.Net.Util.Attributes. See IncrementToken() for further details.

The workflow of the new TokenStream API is as follows:

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 Lucene.Net.Util.AttributeSource.CaptureState and Lucene.Net.Util.AttributeSource.RestoreState(Lucene.Net.Util.AttributeSource.State) can be used.

Inheritance
System.Object
Lucene.Net.Util.AttributeSource
TokenStream
NumericTokenStream
TeeSinkTokenFilter.SinkTokenStream
TokenFilter
Tokenizer
Namespace:
Assembly: Lucene.Net.NetCore.dll
Syntax
public abstract class TokenStream : AttributeSource, IDisposable

Constructors

Name Description
TokenStream()

A TokenStream using the default attribute factory.
TokenStream(AttributeSource)

A TokenStream that uses the same attributes as the supplied one.
TokenStream(AttributeSource.AttributeFactory)

A TokenStream using the supplied AttributeFactory for creating new Lucene.Net.Util.IAttribute instances.

Methods

Name Description
Close()

Releases resources associated with this stream.

Dispose()
Dispose(Boolean)
End()

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()

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 Lucene.Net.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 Lucene.Net.Util.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 Lucene.Net.Util.AttributeSource.AddAttribute``1 and Lucene.Net.Util.AttributeSource.GetAttribute``1, references to all Lucene.Net.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()

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()).