org.apache.lucene.facet.taxonomy.LRUHashMap.java Source code

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Here is the source code for org.apache.lucene.facet.taxonomy.LRUHashMap.java

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/*
 * 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.facet.taxonomy;

import java.util.LinkedHashMap;
import java.util.Map;

/**
 * LRUHashMap is an extension of Java's HashMap, which has a bounded size();
 * When it reaches that size, each time a new element is added, the least
 * recently used (LRU) entry is removed.
 * <p>
 * Java makes it very easy to implement LRUHashMap - all its functionality is
 * already available from {@link java.util.LinkedHashMap}, and we just need to
 * configure that properly.
 * <p>
 * Note that like HashMap, LRUHashMap is unsynchronized, and the user MUST
 * synchronize the access to it if used from several threads. Moreover, while
 * with HashMap this is only a concern if one of the threads is modifies the
 * map, with LURHashMap every read is a modification (because the LRU order
 * needs to be remembered) so proper synchronization is always necessary.
 * <p>
 * With the usual synchronization mechanisms available to the user, this
 * unfortunately means that LRUHashMap will probably perform sub-optimally under
 * heavy contention: while one thread uses the hash table (reads or writes), any
 * other thread will be blocked from using it - or even just starting to use it
 * (e.g., calculating the hash function). A more efficient approach would be not
 * to use LinkedHashMap at all, but rather to use a non-locking (as much as
 * possible) thread-safe solution, something along the lines of
 * java.util.concurrent.ConcurrentHashMap (though that particular class does not
 * support the additional LRU semantics, which will need to be added separately
 * using a concurrent linked list or additional storage of timestamps (in an
 * array or inside the entry objects), or whatever).
 * 
 * @lucene.experimental
 */
public class LRUHashMap<K, V> extends LinkedHashMap<K, V> {

    private int maxSize;

    /**
     * Create a new hash map with a bounded size and with least recently
     * used entries removed.
     * @param maxSize
     *     the maximum size (in number of entries) to which the map can grow
     *     before the least recently used entries start being removed.<BR>
     *      Setting maxSize to a very large value, like
     *      {@link Integer#MAX_VALUE} is allowed, but is less efficient than
     *      using {@link java.util.HashMap} because our class needs
     *      to keep track of the use order (via an additional doubly-linked
     *      list) which is not used when the map's size is always below the
     *      maximum size. 
     */
    public LRUHashMap(int maxSize) {
        super(16, 0.75f, true);
        this.maxSize = maxSize;
    }

    /**
     * Return the max size
     */
    public int getMaxSize() {
        return maxSize;
    }

    /**
     * setMaxSize() allows changing the map's maximal number of elements
     * which was defined at construction time.
     * <P>
     * Note that if the map is already larger than maxSize, the current 
     * implementation does not shrink it (by removing the oldest elements);
     * Rather, the map remains in its current size as new elements are
     * added, and will only start shrinking (until settling again on the
     * give maxSize) if existing elements are explicitly deleted.  
     */
    public void setMaxSize(int maxSize) {
        this.maxSize = maxSize;
    }

    // We override LinkedHashMap's removeEldestEntry() method. This method
    // is called every time a new entry is added, and if we return true
    // here, the eldest element will be deleted automatically. In our case,
    // we return true if the size of the map grew beyond our limit - ignoring
    // what is that eldest element that we'll be deleting.
    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return size() > maxSize;
    }

    @SuppressWarnings("unchecked")
    @Override
    public LRUHashMap<K, V> clone() {
        return (LRUHashMap<K, V>) super.clone();
    }

}