src/java/org/apache/lucene/search/CachingWrapperFilter.java

package org.apache.lucene.search;

/**
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed 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 org.apache.lucene.index.IndexReader;
import java.util.BitSet;
import java.util.WeakHashMap;
import java.util.Map;
import java.io.IOException;

/**
 * Wraps another filters result and caches it.  The caching
 * behavior is like {@link QueryFilter}.  The purpose is to allow
 * filters to simply filter, and then wrap with this class to add
 * caching, keeping the two concerns decoupled yet composable.
 */
public class CachingWrapperFilter extends Filter {
  private Filter filter;

  /**
   * @todo What about serialization in RemoteSearchable?  Caching won't work.
   *       Should transient be removed?
   */
  private transient Map cache;

  /**
   * @param filter Filter to cache results of
   */
  public CachingWrapperFilter(Filter filter) {
    this.filter = filter;
  }

  public BitSet bits(IndexReader reader) throws IOException {
    if (cache == null) {
      cache = new WeakHashMap();
    }

    synchronized (cache) {  // check cache
      BitSet cached = (BitSet) cache.get(reader);
      if (cached != null) {
        return cached;
      }
    }

    final BitSet bits = filter.bits(reader);

    synchronized (cache) {  // update cache
      cache.put(reader, bits);
    }

    return bits;
  }

  public String toString() {
    return "CachingWrapperFilter("+filter+")";
  }
}