/*
* 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.
*/
package org.apache.tomcat.util.collections;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/**
* Concurrent hash map that holds its keys via weak references. Unlike <code>WeakHashMap</code> this class does not
* handle dead keys during common access operations, but expects you to call its {@link #maintain()} method
* periodically. Both keys and values are expected to be not-<code>null</code>.
*
* @param <K> The type of keys used with the Map instance
* @param <V> The type of values used with the Map instance
*/
public class ManagedConcurrentWeakHashMap<K, V> extends AbstractMap<K,V> implements ConcurrentMap<K,V> {
private final ConcurrentMap<Key,V> map = new ConcurrentHashMap<>();
private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
/**
* Method, that has to be invoked periodically to clean dead keys from the map.
*/
public void maintain() {
Key key;
while ((key = (Key) queue.poll()) != null) {
if (key.isDead()) {
// No need to lookup if the key is not in the map
continue;
}
key.ackDeath();
map.remove(key);
}
}
private static class Key extends WeakReference<Object> {
private final int hash;
private boolean dead;
Key(Object key, ReferenceQueue<Object> queue) {
super(key, queue);
hash = key.hashCode();
}
@Override
public int hashCode() {
return hash;
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (dead) {
// Post-mortem cleanup looks for this specific Reference
// instance
return false;
}
if (!(obj instanceof Reference<?>)) {
return false;
}
Object oA = get();
Object oB = ((Reference<?>) obj).get();
if (oA == oB) {
return true;
}
if (oA == null || oB == null) {
return false;
}
return oA.equals(oB);
}
public void ackDeath() {
this.dead = true;
}
public boolean isDead() {
return dead;
}
}
/**
* Creates Key instance to be used to store values in the map. It is registered with the ReferenceQueue.
*/
private Key createStoreKey(Object key) {
return new Key(key, queue);
}
/**
* Creates Key instance to be used only to lookup values in the map. It is not registered with the ReferenceQueue.
*/
private Key createLookupKey(Object key) {
return new Key(key, null);
}
@Override
public int size() {
return map.size();
}
@Override
public boolean isEmpty() {
return map.isEmpty();
}
@Override
public boolean containsValue(Object value) {
if (value == null) {
return false;
}
return map.containsValue(value);
}
@Override
public boolean containsKey(Object key) {
if (key == null) {
return false;
}
return map.containsKey(createLookupKey(key));
}
@Override
public V get(Object key) {
if (key == null) {
return null;
}
return map.get(createLookupKey(key));
}
@Override
public V put(K key, V value) {
Objects.requireNonNull(value);
return map.put(createStoreKey(key), value);
}
@Override
public V remove(Object key) {
return map.remove(createLookupKey(key));
}
@Override
public void clear() {
map.clear();
// maintain() clears the queue, though it is not 100% reliable,
// as queue is populated by GC asynchronously
maintain();
}
@Override
public V putIfAbsent(K key, V value) {
Objects.requireNonNull(value);
Key storeKey = createStoreKey(key);
V oldValue = map.putIfAbsent(storeKey, value);
if (oldValue != null) { // ack that key has not been stored
storeKey.ackDeath();
}
return oldValue;
}
@Override
public boolean remove(Object key, Object value) {
if (value == null) {
return false;
}
return map.remove(createLookupKey(key), value);
}
@Override
public boolean replace(K key, V oldValue, V newValue) {
Objects.requireNonNull(newValue);
return map.replace(createLookupKey(key), oldValue, newValue);
}
@Override
public V replace(K key, V value) {
Objects.requireNonNull(value);
return map.replace(createLookupKey(key), value);
}
@Override
public Collection<V> values() {
return map.values();
}
@Override
public Set<Map.Entry<K,V>> entrySet() {
return new AbstractSet<>() {
@Override
public boolean isEmpty() {
return map.isEmpty();
}
@Override
public int size() {
return map.size();
}
@Override
public Iterator<Map.Entry<K,V>> iterator() {
return new Iterator<>() {
private final Iterator<Map.Entry<Key,V>> it = map.entrySet().iterator();
@Override
public boolean hasNext() {
return it.hasNext();
}
@Override
public Map.Entry<K,V> next() {
return new Map.Entry<>() {
private final Map.Entry<Key,V> en = it.next();
@SuppressWarnings("unchecked")
@Override
public K getKey() {
return (K) en.getKey().get();
}
@Override
public V getValue() {
return en.getValue();
}
@Override
public V setValue(V value) {
Objects.requireNonNull(value);
return en.setValue(value);
}
};
}
@Override
public void remove() {
it.remove();
}
};
}
};
}
}