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/**
* Copyright (c) 2012, University of Konstanz, Distributed Systems Group 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 University of Konstanz 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 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 org.jscsi.utils;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
/**
* SoftHashMap
*
* Based on the SoftHashMap implemented by Dr. Heinz Kabutz.
*
*
* Hash map based on soft references. The hash map always makes sure a limited amount of strong references it maintained
* in FIFO order to _simulate_ LRU.
*
*
* Note that the put and remove methods always return null.
*
*
* @param Key object of type K.
* @param Value object of type V.
*
*/
public final class SoftHashMap extends AbstractMap {
/** Default strong reference count. */
private static final int DEFAULT_STRONG_REFERENCE_COUNT = 32;
/** The internal HashMap that will hold the SoftReference. */
private final Map> internalMap;
/** The number of "hard" references to hold internally. */
private final int strongReferenceCount;
/** The FIFO list of strong references, order of last access. */
private V[] strongReferenceArray;
/** Current offset of FIFO list. */
private int currentStrongReferenceOffset;
/** Reference queue for cleared SoftReference objects. */
private final ReferenceQueue> queue;
/**
* Default constructor internally using 32 strong references.
*/
public SoftHashMap () {
this(DEFAULT_STRONG_REFERENCE_COUNT);
}
/**
* Constructor that allows to specify how many strong references should be used internally.
*
* @param initStrongReferenceCount Number of internal strong references.
*/
@SuppressWarnings ("unchecked")
public SoftHashMap (final int initStrongReferenceCount) {
internalMap = new HashMap>();
strongReferenceCount = initStrongReferenceCount;
strongReferenceArray = (V[]) new Object[initStrongReferenceCount];
currentStrongReferenceOffset = 0;
queue = new ReferenceQueue>();
}
/**
* {@inheritDoc}
*/
@Override
public final V get (final Object key) {
V value = null;
final SoftReference softReference = internalMap.get(key);
if (softReference != null) {
// Soft reference was garbage collected.
value = softReference.get();
if (value == null) {
// Reflect garbage collected soft reference in internal hash
// map.
internalMap.remove(key);
} else {
synchronized (strongReferenceArray) {
// FIFO on strong references.
strongReferenceArray[currentStrongReferenceOffset++] = value;
// Assure FIFO does not grow beyond strongReferenceCount.
if (currentStrongReferenceOffset >= strongReferenceCount) {
currentStrongReferenceOffset = 0;
}
}
}
}
return value;
}
/**
* {@inheritDoc}
*/
@Override
public final V put (final K key, final V value) {
processQueue();
internalMap.put(key, new SoftValue(value, key, queue));
return null;
}
/**
* {@inheritDoc}
*/
@Override
public final V remove (final Object key) {
processQueue();
internalMap.remove(key);
return null;
}
/**
* {@inheritDoc}
*/
@Override
@SuppressWarnings ("unchecked")
public final synchronized void clear () {
strongReferenceArray = (V[]) new Object[strongReferenceCount];
processQueue();
internalMap.clear();
}
/**
* {@inheritDoc}
*/
@Override
public final int size () {
processQueue();
return internalMap.size();
}
/**
* {@inheritDoc}
*/
@Override
public final Set> entrySet () {
throw new UnsupportedOperationException();
}
/**
* Remove garbage collected soft values with the help of the reference queue.
*/
@SuppressWarnings ({ "rawtypes", "unchecked" })
private final void processQueue () {
SoftValue softValue;
while ((softValue = (SoftValue) queue.poll()) != null) {
internalMap.remove(softValue.key);
}
}
/**
* Internal subclass to store keys and values for more convenient lookups.
*/
private final class SoftValue extends SoftReference {
private final K key;
/**
* Constructor.
*
* @param initValue Value wrapped as soft reference.
* @param initKey Key for given value.
* @param initReferenceQueue Reference queue for cleanup.
*/
@SuppressWarnings ({ "rawtypes", "unchecked" })
private SoftValue (final V initValue, final K initKey, final ReferenceQueue initReferenceQueue) {
super(initValue, initReferenceQueue);
key = initKey;
}
}
}
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