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/*
* Copyright 2000-2013 Vaadin Ltd.
*
* Licensed 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 com.vaadin.util;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractMap;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
/**
* A Map holding weak references to its values. It is internally backed by a
* normal HashMap and all values are stored as WeakReferences. Garbage collected
* entries are removed when touched.
* <p>
* <em>Note</em> this class is not serializable.
*
* @author Vaadin Ltd
* @since 7.1.4
*/
public class WeakValueMap<K, V> implements Map<K, V> {
/**
* This class holds a weak reference to the value and a strong reference to
* the key for efficient removal of stale values.
*/
private static class WeakValueReference<K, V> extends WeakReference<V> {
private final K key;
WeakValueReference(K key, V value, ReferenceQueue<V> refQueue) {
super(value, refQueue);
this.key = key;
}
K getKey() {
return key;
}
}
private final HashMap<K, WeakValueReference<K, V>> backingMap;
private final ReferenceQueue<V> refQueue;
/**
* Constructs a new WeakValueMap, where all values are stored as weak
* references.
*/
public WeakValueMap() {
backingMap = new HashMap<K, WeakValueReference<K, V>>();
refQueue = new ReferenceQueue<V>();
}
/**
* {@inheritDoc}
*/
@Override
public V put(K key, V value) {
if (key == null) {
throw new NullPointerException("key cannot be null");
}
if (value == null) {
throw new NullPointerException("value cannot be null");
}
removeStaleEntries();
backingMap.put(key, new WeakValueReference<K, V>(key, value, refQueue));
return value;
}
/**
* {@inheritDoc}
*/
@Override
public V remove(Object o) {
removeStaleEntries();
WeakReference<V> value = backingMap.remove(o);
return value == null ? null : value.get();
}
/**
* {@inheritDoc}
*/
@Override
public void putAll(Map<? extends K, ? extends V> map) {
if (map != null) {
for (Map.Entry<? extends K, ? extends V> entry : map.entrySet()) {
put(entry.getKey(), entry.getValue());
}
}
}
/**
* {@inheritDoc}
*/
@Override
public void clear() {
backingMap.clear();
}
/**
* {@inheritDoc}
*/
@Override
public Set<K> keySet() {
removeStaleEntries();
return backingMap.keySet();
}
/**
* {@inheritDoc}
*/
@Override
public V get(Object o) {
removeStaleEntries();
WeakReference<V> weakValue = backingMap.get(o);
if (weakValue != null) {
return weakValue.get();
}
return null;
}
/**
* {@inheritDoc}
*/
@Override
public int size() {
removeStaleEntries();
return backingMap.size();
}
/**
* {@inheritDoc}
*/
@Override
public boolean isEmpty() {
removeStaleEntries();
return backingMap.isEmpty();
}
/**
* {@inheritDoc}
*/
@Override
public boolean containsKey(Object o) {
removeStaleEntries();
return backingMap.containsKey(o);
}
/**
* {@inheritDoc}
*/
@Override
public boolean containsValue(Object o) {
removeStaleEntries();
for (V value : values()) {
if (o.equals(value)) {
return true;
}
}
return false;
}
/**
* {@inheritDoc}
*/
@Override
public Collection<V> values() {
removeStaleEntries();
Collection<V> values = new HashSet<V>();
for (WeakReference<V> weakValue : backingMap.values()) {
V value = weakValue.get();
if (value != null) {
// null values have been GC'd, which may happen long before
// anything is enqueued in the ReferenceQueue.
values.add(value);
}
}
return values;
}
/**
* {@inheritDoc}
*/
@Override
public Set<Entry<K, V>> entrySet() {
removeStaleEntries();
Set<Entry<K, V>> entrySet = new HashSet<Entry<K, V>>();
for (Entry<K, WeakValueReference<K, V>> entry : backingMap.entrySet()) {
V value = entry.getValue().get();
if (value != null) {
// null values have been GC'd, which may happen long before
// anything is enqueued in the ReferenceQueue.
entrySet.add(new AbstractMap.SimpleEntry<K, V>(entry.getKey(),
value));
}
}
return entrySet;
}
/**
* Cleans up stale entries by polling the ReferenceQueue.
* <p>
* Depending on the GC implementation and strategy, the ReferenceQueue is
* not necessarily notified immediately when a reference is garbage
* collected, but it will eventually be.
*/
private void removeStaleEntries() {
Reference<? extends V> ref;
while ((ref = refQueue.poll()) != null) {
Object key = ((WeakValueReference<?, ?>) ref).getKey();
backingMap.remove(key);
}
}
}
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