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package com.c2kernel.utils;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;
/*******************************************************************************
* WeakReferences are reaped if no strong references are left next time the gc has a chance.
* The ClusterStorageManager caches can optionally use this one, for high volume imports etc
*
* $Revision: 1.5 $ $Date: 2004/10/29 13:29:09 $
******************************************************************************/
public class WeakCache<K, V> extends AbstractMap<K, V> implements NonStrongRefCache<K, V> {
private final Map<K, WeakValue<V>> hash = new HashMap<K, WeakValue<V>>();
private final int minSize;
private final LinkedList<V> hardCache = new LinkedList<V>();
private final ReferenceQueue<V> queue = new ReferenceQueue<V>();
public WeakCache() {
this(0);
}
public WeakCache(int minSize) {
this.minSize = minSize;
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#get(java.lang.Object)
*/
@Override
public V get(Object key) {
V result = null;
WeakValue<V> weak_ref = hash.get(key);
if (weak_ref != null) {
result = weak_ref.get();
if (result == null)
hash.remove(key);
else
if (minSize > 0) { // add to hard cache so it's not reaped for a while
hardCache.addFirst(result);
if (hardCache.size() > minSize) // trim last one off
hardCache.removeLast();
}
}
return result;
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#put(K, V)
*/
@Override
public V put(K key, V value) {
processQueue();
if (minSize > 0) {
hardCache.addFirst(value);
if (hardCache.size() > minSize)
hardCache.removeLast();
}
hash.put(key, new WeakValue<V>(key, value, queue));
return value;
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#remove(java.lang.Object)
*/
@Override
public V remove(Object key) {
processQueue();
if (hash.containsKey(key)) return hash.remove(key).get();
return null;
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#clear()
*/
@Override
public void clear() {
hardCache.clear();
while(queue.poll()!=null);
hash.clear();
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#size()
*/
@Override
public int size() {
processQueue();
return hash.size();
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#keySet()
*/
@Override
public Set<K> keySet() {
processQueue();
return hash.keySet();
}
/* (non-Javadoc)
* @see com.c2kernel.utils.NonStrongRefCache#entrySet()
*/
@Override
public Set<Map.Entry<K, V>> entrySet() {
// Would have to create another Map to do this - too expensive
// Throwing runtime expensive is dangerous, but better than nulls
throw new UnsupportedOperationException();
}
private static class WeakValue<V> extends WeakReference<V> {
private final Object key;
private WeakValue(Object key, V value, ReferenceQueue<V> q) {
super(value, q);
this.key = key;
}
}
/**
* Look for values that have been reaped, and remove their keys from the cache
*/
private void processQueue() {
WeakValue<V> sv;
while ((sv = (WeakValue<V>) queue.poll()) != null) {
hash.remove(sv.key);
}
}
}
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