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Lucene example source code file (TokenStream.java)

This example Lucene source code file (TokenStream.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

attributesource, class, closeable, closeable, io, ioexception, ioexception, nosuchmethodexception, nosuchmethodexception, reflection, tokenstream, tokenstream

The Lucene TokenStream.java source code

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 java.lang.reflect.Modifier;

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 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 TokenStream whose input is a Reader; and
 * <li>{@link TokenFilter}, a TokenStream whose input is another
 * <code>TokenStream.
 * </ul>
 * A new <code>TokenStream 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 now extends {@link AttributeSource}, which provides
 * access to all of the token {@link Attribute}s for the <code>TokenStream.
 * 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 TokenStream API is as follows:
 * <ol>
 * <li>Instantiation of TokenStream/{@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.
 * </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,
 * e.g., for buffering purposes (see {@link CachingTokenFilter},
 * {@link TeeSinkTokenFilter}). For this usecase
 * {@link AttributeSource#captureState} and {@link AttributeSource#restoreState}
 * can be used.
 * <p>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 {@link #incrementToken}! This is checked when Java
 * assertions are enabled.
 */
public abstract class TokenStream extends AttributeSource implements Closeable {

  /**
   * A TokenStream using the default attribute factory.
   */
  protected TokenStream() {
    super();
    assert assertFinal();
  }
  
  /**
   * A TokenStream that uses the same attributes as the supplied one.
   */
  protected TokenStream(AttributeSource input) {
    super(input);
    assert assertFinal();
  }
  
  /**
   * A TokenStream using the supplied AttributeFactory for creating new {@link Attribute} instances.
   */
  protected TokenStream(AttributeFactory factory) {
    super(factory);
    assert assertFinal();
  }
  
  private boolean assertFinal() {
    try {
      final Class<?> clazz = getClass();
      assert clazz.isAnonymousClass() ||
        (clazz.getModifiers() & (Modifier.FINAL | Modifier.PRIVATE)) != 0 ||
        Modifier.isFinal(clazz.getMethod("incrementToken").getModifiers()) :
        "TokenStream implementation classes or at least their incrementToken() implementation must be final";
      return true;
    } catch (NoSuchMethodException nsme) {
      return false;
    }
  }
  
  /**
   * 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
   * (using the new <code>TokenStream 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 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 {}
  
}

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