Package org.apache.lucene.analysis

Class TokenStream

  • All Implemented Interfaces:
    Closeable, AutoCloseable
    Direct Known Subclasses:
    NumericTokenStream, TokenFilter, Tokenizer
    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.

    • 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.

    • 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.

        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

        public void reset()
           throws IOException
        This method is called by a consumer before it begins consumption using incrementToken().

        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 throw IllegalStateException on further usage).

        Throws:
        IOException