https://lucene.apache.org/lucene-parent/lucene-core: Apache Lucene Java Core (The Apache Software Foundation) 
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 *     http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.lucene.analysis;


import java.io.IOException;
import java.io.Closeable;
import java.lang.reflect.Modifier;

import org.apache.lucene.analysis.tokenattributes.PackedTokenAttributeImpl;
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.AttributeFactory;
import org.apache.lucene.util.AttributeImpl;
import org.apache.lucene.util.AttributeSource;


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.


TokenStream 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 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. 
/**
 * 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>
 * <code>TokenStream</code> 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 {
  
  
Default AttributeFactory instance that should be used for TokenStreams. 
/** Default {@link AttributeFactory} instance that should be used for TokenStreams. */
  public static final AttributeFactory DEFAULT_TOKEN_ATTRIBUTE_FACTORY =
    AttributeFactory.getStaticImplementation(AttributeFactory.DEFAULT_ATTRIBUTE_FACTORY, PackedTokenAttributeImpl.class);

  
A TokenStream using the default attribute factory.

/**
   * A TokenStream using the default attribute factory.
   */
  protected TokenStream() {
    super(DEFAULT_TOKEN_ATTRIBUTE_FACTORY);
    assert assertFinal();
  }
  
  
A TokenStream that uses the same attributes as the supplied one.

/**
   * 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 Attribute instances. 
/**
   * 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., 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<Attribute>) and AttributeSource.getAttribute(Class<Attribute>), 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
/**
   * 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 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:
/**
   * 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.
   * <p>
   * 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.
   * <p>
   * If you override this method, always call {@code super.end()}.
   * 
   * @throws IOException If an I/O error occurs
   */
  public void end() throws IOException {
    endAttributes(); // LUCENE-3849: don't consume dirty atts
  }

  
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). 
/**
   * This method is called by a consumer before it begins consumption using
   * {@link #incrementToken()}.
   * <p>
   * 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.
   * <p>
   * If you override this method, always call {@code super.reset()}, otherwise
   * some internal state will not be correctly reset (e.g., {@link Tokenizer} will
   * throw {@link IllegalStateException} on further usage).
   */
  public void reset() throws IOException {}
  
  
Releases resources associated with this stream.

 If you override this method, always call super.close(), otherwise some internal state will not be correctly reset (e.g., Tokenizer will throw IllegalStateException on reuse). 
/** Releases resources associated with this stream.
   * <p>
   * If you override this method, always call {@code super.close()}, otherwise
   * some internal state will not be correctly reset (e.g., {@link Tokenizer} will
   * throw {@link IllegalStateException} on reuse).
   */
  @Override
  public void close() throws IOException {}
  
}