snowballfilter.java

lucene2.2.0版本

package org.apache.lucene.analysis.snowball;

/**
 * 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.lang.reflect.Method;

import net.sf.snowball.SnowballProgram;
import net.sf.snowball.ext.*;

import org.apache.lucene.analysis.Token;
import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;

/** A filter that stems words using a Snowball-generated stemmer.
 *
 * Available stemmers are listed in {@link net.sf.snowball.ext}.  The name of a
 * stemmer is the part of the class name before "Stemmer", e.g., the stemmer in
 * {@link EnglishStemmer} is named "English".
 */

public class SnowballFilter extends TokenFilter {
  private static final Object [] EMPTY_ARGS = new Object[0];

  private SnowballProgram stemmer;
  private Method stemMethod;

  /** Construct the named stemming filter.
   *
   * @param in the input tokens to stem
   * @param name the name of a stemmer
   */
  public SnowballFilter(TokenStream in, String name) {
    super(in);
    try {
      Class stemClass =
        Class.forName("net.sf.snowball.ext." + name + "Stemmer");
      stemmer = (SnowballProgram) stemClass.newInstance();
      // why doesn't the SnowballProgram class have an (abstract?) stem method?
      stemMethod = stemClass.getMethod("stem", new Class[0]);
    } catch (Exception e) {
      throw new RuntimeException(e.toString());
    }
  }

  /** Returns the next input Token, after being stemmed */
  public final Token next() throws IOException {
    Token token = input.next();
    if (token == null)
      return null;
    stemmer.setCurrent(token.termText());
    try {
      stemMethod.invoke(stemmer, EMPTY_ARGS);
    } catch (Exception e) {
      throw new RuntimeException(e.toString());
    }
    
    Token newToken = new Token(stemmer.getCurrent(),
                      token.startOffset(), token.endOffset(), token.type());
    newToken.setPositionIncrement(token.getPositionIncrement());
    return newToken;
  }
}