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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.
*
* 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.
*
*
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.
*
*
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 extends LinkedHashMap {
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.
* 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.
*
* 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 eldest) {
return size() > maxSize;
}
@SuppressWarnings("unchecked")
@Override
public LRUHashMap clone() {
return (LRUHashMap) super.clone();
}
}