TokenStream.java

package com.antbrains.wordseg.luceneanalyzer;

/**
 * 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;

/**
 * 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}, 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</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 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 {
  }

}