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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();
}
}