Mega Code Archive
Implements a combination of WeakHashMap and IdentityHashMap
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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.
*/
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
/**
* Implements a combination of WeakHashMap and IdentityHashMap.
* Useful for caches that need to key off of a == comparison
* instead of a .equals.
*
* <b>
* This class is not a general-purpose Map implementation! While
* this class implements the Map interface, it intentionally violates
* Map's general contract, which mandates the use of the equals method
* when comparing objects. This class is designed for use only in the
* rare cases wherein reference-equality semantics are required.
*
* Note that this implementation is not synchronized.
* </b>
*/
public class WeakIdentityHashMap<K, V> implements Map<K, V> {
private final ReferenceQueue<K> queue = new ReferenceQueue<K>();
private Map<IdentityWeakReference, V> backingStore = new HashMap<IdentityWeakReference, V>();
public WeakIdentityHashMap() {
}
public void clear() {
backingStore.clear();
reap();
}
public boolean containsKey(Object key) {
reap();
return backingStore.containsKey(new IdentityWeakReference(key));
}
public boolean containsValue(Object value) {
reap();
return backingStore.containsValue(value);
}
public Set<Map.Entry<K, V>> entrySet() {
reap();
Set<Map.Entry<K, V>> ret = new HashSet<Map.Entry<K, V>>();
for (Map.Entry<IdentityWeakReference, V> ref : backingStore.entrySet()) {
final K key = ref.getKey().get();
final V value = ref.getValue();
Map.Entry<K, V> entry = new Map.Entry<K, V>() {
public K getKey() {
return key;
}
public V getValue() {
return value;
}
public V setValue(V value) {
throw new UnsupportedOperationException();
}
};
ret.add(entry);
}
return Collections.unmodifiableSet(ret);
}
public Set<K> keySet() {
reap();
Set<K> ret = new HashSet<K>();
for (IdentityWeakReference ref : backingStore.keySet()) {
ret.add(ref.get());
}
return Collections.unmodifiableSet(ret);
}
public boolean equals(Object o) {
return backingStore.equals(((WeakIdentityHashMap)o).backingStore);
}
public V get(Object key) {
reap();
return backingStore.get(new IdentityWeakReference(key));
}
public V put(K key, V value) {
reap();
return backingStore.put(new IdentityWeakReference(key), value);
}
public int hashCode() {
reap();
return backingStore.hashCode();
}
public boolean isEmpty() {
reap();
return backingStore.isEmpty();
}
public void putAll(Map t) {
throw new UnsupportedOperationException();
}
public V remove(Object key) {
reap();
return backingStore.remove(new IdentityWeakReference(key));
}
public int size() {
reap();
return backingStore.size();
}
public Collection<V> values() {
reap();
return backingStore.values();
}
private synchronized void reap() {
Object zombie = queue.poll();
while (zombie != null) {
IdentityWeakReference victim = (IdentityWeakReference)zombie;
backingStore.remove(victim);
zombie = queue.poll();
}
}
class IdentityWeakReference extends WeakReference<K> {
int hash;
@SuppressWarnings("unchecked")
IdentityWeakReference(Object obj) {
super((K)obj, queue);
hash = System.identityHashCode(obj);
}
public int hashCode() {
return hash;
}
public boolean equals(Object o) {
if (this == o) {
return true;
}
IdentityWeakReference ref = (IdentityWeakReference)o;
if (this.get() == ref.get()) {
return true;
}
return false;
}
}
}
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