lucene/analysis/common/src/java/org/apache/lucene/collation/CollationKeyAnalyzer.java

/*
 * 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.
 */
package org.apache.lucene.collation;

import java.text.Collator;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.core.KeywordTokenizer;
import org.apache.lucene.util.AttributeFactory;

/**
 * Configures {@link KeywordTokenizer} with {@link CollationAttributeFactory}.
 *
 * <p>Converts the token into its {@link java.text.CollationKey}, and then encodes the CollationKey
 * directly to allow it to be stored as an index term.
 *
 * <p><strong>WARNING:</strong> Make sure you use exactly the same Collator at index and query time
 * -- CollationKeys are only comparable when produced by the same Collator. Since {@link
 * java.text.RuleBasedCollator}s are not independently versioned, it is unsafe to search against
 * stored CollationKeys unless the following are exactly the same (best practice is to store this
 * information with the index and check that they remain the same at query time):
 *
 * <ol>
 *   <li>JVM vendor
 *   <li>JVM version, including patch version
 *   <li>The language (and country and variant, if specified) of the Locale used when constructing
 *       the collator via {@link Collator#getInstance(java.util.Locale)}.
 *   <li>The collation strength used - see {@link Collator#setStrength(int)}
 * </ol>
 *
 * <p>The <code>ICUCollationKeyAnalyzer</code> in the analysis-icu package uses ICU4J's Collator,
 * which makes its version available, thus allowing collation to be versioned independently from the
 * JVM. ICUCollationKeyAnalyzer is also significantly faster and generates significantly shorter
 * keys than CollationKeyAnalyzer. See <a
 * href="http://site.icu-project.org/charts/collation-icu4j-sun"
 * >http://site.icu-project.org/charts/collation-icu4j-sun</a> for key generation timing and key
 * length comparisons between ICU4J and java.text.Collator over several languages.
 *
 * <p>CollationKeys generated by java.text.Collators are not compatible with those those generated
 * by ICU Collators. Specifically, if you use CollationKeyAnalyzer to generate index terms, do not
 * use ICUCollationKeyAnalyzer on the query side, or vice versa.
 *
 * @since 3.1
 */
public final class CollationKeyAnalyzer extends Analyzer {
  private final CollationAttributeFactory factory;

  /**
   * Create a new CollationKeyAnalyzer, using the specified collator.
   *
   * @param collator CollationKey generator
   */
  public CollationKeyAnalyzer(Collator collator) {
    this.factory = new CollationAttributeFactory(collator);
  }

  @Override
  protected AttributeFactory attributeFactory(String fieldName) {
    return factory;
  }

  @Override
  protected TokenStreamComponents createComponents(String fieldName) {
    KeywordTokenizer tokenizer =
        new KeywordTokenizer(factory, KeywordTokenizer.DEFAULT_BUFFER_SIZE);
    return new TokenStreamComponents(tokenizer, tokenizer);
  }
}