package org.apache.lucene.analysis;
/**
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import java.io.IOException;
import java.io.Closeable;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.util.Attribute;
import org.apache.lucene.util.AttributeImpl;
import org.apache.lucene.util.AttributeSource;
/**
* A <code>TokenStream</code> enumerates the sequence of tokens, either from
* {@link Field}s of a {@link Document} or from query text.
* <p>
* This is an abstract class; concrete subclasses are:
* <ul>
* <li>{@link Tokenizer}, a <code>TokenStream</code> whose input is a Reader; and
* <li>{@link TokenFilter}, a <code>TokenStream</code> whose input is another
* <code>TokenStream</code>.
* </ul>
* A new <code>TokenStream</code> API has been introduced with Lucene 2.9. This API
* has moved from being {@link Token}-based to {@link Attribute}-based. While
* {@link Token} still exists in 2.9 as a convenience class, the preferred way
* to store the information of a {@link Token} is to use {@link AttributeImpl}s.
* <p>
* <code>TokenStream</code> now extends {@link AttributeSource}, which provides
* access to all of the token {@link Attribute}s for the <code>TokenStream</code>.
* Note that only one instance per {@link AttributeImpl} is created and reused
* for every token. This approach reduces object creation and allows local
* caching of references to the {@link AttributeImpl}s. See
* {@link #incrementToken()} for further details.
* <p>
* <b>The workflow of the new <code>TokenStream</code> API is as follows:</b>
* <ol>
* <li>Instantiation of <code>TokenStream</code>/{@link TokenFilter}s which add/get
* attributes to/from the {@link AttributeSource}.
* <li>The consumer calls {@link TokenStream#reset()}.
* <li>The consumer retrieves attributes from the stream and stores local
* references to all attributes it wants to access.
* <li>The consumer calls {@link #incrementToken()} until it returns false
* consuming the attributes after each call.
* <li>The consumer calls {@link #end()} so that any end-of-stream operations
* can be performed.
* <li>The consumer calls {@link #close()} to release any resource when finished
* using the <code>TokenStream</code>.
* </ol>
* 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
* {@link #incrementToken()}.
* <p>
* You can find some example code for the new API in the analysis package level
* Javadoc.
* <p>
* Sometimes it is desirable to capture a current state of a <code>TokenStream</code>,
* e.g., for buffering purposes (see {@link CachingTokenFilter},
* {@link TeeSinkTokenFilter}). For this usecase
* {@link AttributeSource#captureState} and {@link AttributeSource#restoreState}
* can be used.
*/
public abstract class TokenStream extends AttributeSource implements Closeable {
/**
* A TokenStream using the default attribute factory.
*/
protected TokenStream() {
super();
}
/**
* A TokenStream that uses the same attributes as the supplied one.
*/
protected TokenStream(AttributeSource input) {
super(input);
}
/**
* A TokenStream using the supplied AttributeFactory for creating new {@link Attribute} instances.
*/
protected TokenStream(AttributeFactory factory) {
super(factory);
}
/**
* Consumers (i.e., {@link IndexWriter}) use this method to advance the stream to
* the next token. Implementing classes must implement this method and update
* the appropriate {@link AttributeImpl}s with the attributes of the next
* token.
* <P>
* 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
* {@link #captureState} to create a copy of the current attribute state.
* <p>
* This method is called for every token of a document, so an efficient
* implementation is crucial for good performance. To avoid calls to
* {@link #addAttribute(Class)} and {@link #getAttribute(Class)},
* references to all {@link AttributeImpl}s that this stream uses should be
* retrieved during instantiation.
* <p>
* 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
* {@link #incrementToken()}.
*
* @return false for end of stream; true otherwise
*/
public abstract boolean incrementToken() throws IOException;
/**
* This method is called by the consumer after the last token has been
* consumed, after {@link #incrementToken()} returned <code>false</code>
* (using the new <code>TokenStream</code> API). Streams implementing the old API
* should upgrade to use this feature.
* <p/>
* 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 {@link WhitespaceTokenizer} was used.
*
* @throws IOException
*/
public void end() throws IOException {
// do nothing by default
}
/**
* Resets this stream to the beginning. This is an optional operation, so
* subclasses may or may not implement this method. {@link #reset()} is not needed for
* the standard indexing process. However, if the tokens of a
* <code>TokenStream</code> are intended to be consumed more than once, it is
* necessary to implement {@link #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 {@link #reset()}).
*/
public void reset() throws IOException {}
/** Releases resources associated with this stream. */
public void close() throws IOException {}
}