LRUHashMap.java
/*
* Copyright (c) 2008, Harald Kuhr
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* * Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* * Neither the name of the copyright holder nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
package com.twelvemonkeys.util;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
/**
* Map implementation with size limit, that keeps its entries in LRU
* (least recently used) order, also known as <em>access-order</em>.
* When the size limit is reached, the least recently accessed mappings are
* removed. The number of mappings to be removed from the map, is
* controlled by the trim factor.
* <p>
* <ul>
* <li>Default size limit is 1000 elements.
* See {@link #setMaxSize(int)}/{@link #getMaxSize()}.</li>
* <li>Default trim factor is 1% ({@code 0.01f}).
* See {@link #setTrimFactor(float)}/{@link #getTrimFactor()}.</li>
* </ul>
*
* @author <a href="mailto:harald.kuhr@gmail.com">Harald Kuhr</a>
* @version $Id: //depot/branches/personal/haraldk/twelvemonkeys/release-2/twelvemonkeys-core/src/main/java/com/twelvemonkeys/util/LRUHashMap.java#1 $
*/
public class LRUHashMap<K, V> extends LinkedHashMap<K, V> implements ExpiringMap<K, V> {
private int maxSize = 1000;
private float trimFactor = 0.01f;
/**
* Creates an LRUHashMap with default max size (1000 entries).
*
* <small>This is constructor is here to comply with the reccomendations for
* "standard" constructors in the {@code Map} interface.</small>
*
* @see #LRUHashMap(int)
*/
public LRUHashMap() {
super(16, .75f, true);
}
/**
* Creates an LRUHashMap with the given max size.
*
* @param pMaxSize size limit
*/
public LRUHashMap(int pMaxSize) {
super(16, .75f, true);
setMaxSize(pMaxSize);
}
/**
* Creates an LRUHashMap with initial mappings from the given map,
* and default max size (1000 entries).
*
* <small>This is constructor is here to comply with the reccomendations for
* "standard" constructors in the {@code Map} interface.</small>
*
* @param pContents the map whose mappings are to be placed in this map.
* May be {@code null}.
*
* @see #LRUHashMap(java.util.Map, int)
*/
public LRUHashMap(Map<? extends K, ? extends V> pContents) {
super(16, .75f, true);
putAll(pContents);
}
/**
* Creates an LRUHashMap with initial mappings from the given map,
* and the given max size.
*
* @param pContents the map whose mappings are to be placed in this map.
* May be {@code null}.
* @param pMaxSize size limit
*/
public LRUHashMap(Map<? extends K, ? extends V> pContents, int pMaxSize) {
super(16, .75f, true);
setMaxSize(pMaxSize);
putAll(pContents);
}
/**
* Returns the maximum number of mappings in this map.
*
* @return the size limit
*/
public int getMaxSize() {
return maxSize;
}
/**
* Sets the maximum number of elements in this map.
*
* If the current size is greater than the new max size, the map will be
* trimmed to fit the new max size constraint.
*
* @see #removeLRU()
*
* @param pMaxSize new size limit
*/
public void setMaxSize(int pMaxSize) {
if (pMaxSize < 0) {
throw new IllegalArgumentException("max size must be positive");
}
maxSize = pMaxSize;
while(size() > maxSize) {
removeLRU();
}
}
/**
* Returns the current trim factor.
* <p>
* The trim factor controls how many percent of the maps current size is
* reclaimed, when performing an {@code removeLRU} operation.
* Defaults to 1% ({@code 0.01f}).
*
* @return the current trim factor
*/
public float getTrimFactor() {
return trimFactor;
}
/**
* Sets the trim factor.
* <p>
* The trim factor controls how many percent of the maps current size is
* reclaimed, when performing an {@code removeLRU} operation.
* Defaults to 1% ({@code 0.01f}).
*
* @param pTrimFactor the new trim factor. Acceptable values are between
* 0 (inclusive) and 1 (exclusive).
*
* @see #removeLRU()
*/
public void setTrimFactor(float pTrimFactor) {
if (pTrimFactor < 0f || pTrimFactor >= 1f) {
throw new IllegalArgumentException("trim factor must be between 0 and 1");
}
trimFactor = pTrimFactor;
}
/**
* always returns {@code false}, and instead invokes {@code removeLRU()}
* if {@code size >= maxSize}.
*/
protected boolean removeEldestEntry(Map.Entry<K, V> pEldest) {
// NOTE: As removeLRU() may remove more than one entry, this is better
// than simply removing the eldest entry.
if (size() >= maxSize) {
removeLRU();
}
return false;
}
/**
* Default implementation does nothing.
* May be used by clients as a call-back to notify when mappings expire from
* the map.
*
* @param pRemoved the removed mapping
*/
public void processRemoved(Map.Entry<K, V> pRemoved) {
}
/**
* Removes the least recently used mapping(s) from this map.
* <p>
* How many mappings are removed from the map, is controlled by the
* trim factor.
* In any case, at least one mapping will be removed.
*
* @see #getTrimFactor()
*/
public void removeLRU() {
int removeCount = (int) Math.max((size() * trimFactor), 1);
Iterator<Map.Entry<K, V>> entries = entrySet().iterator();
while ((removeCount--) > 0 && entries.hasNext()) {
entries.next();
entries.remove();
}
}
}