content: 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</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.
   * <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();
        if (!clazz.desiredAssertionStatus())
          return true;
        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 {@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 {}
    
  }
  