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/*
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.apache.tomcat.util.collections;
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import java.lang.ref.Reference;
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import java.lang.ref.ReferenceQueue;
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import java.lang.ref.WeakReference;
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import java.util.AbstractMap;
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import java.util.AbstractSet;
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import java.util.Collection;
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import java.util.Iterator;
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import java.util.Map;
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import java.util.Objects;
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import java.util.Set;
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import java.util.concurrent.ConcurrentHashMap;
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import java.util.concurrent.ConcurrentMap;
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/**
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* Concurrent hash map that holds its keys via weak references. Unlike
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* <code>WeakHashMap</code> this class does not handle dead keys during common
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* access operations, but expects you to call its {@link #maintain()} method
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* periodically. Both keys and values are expected to be not-<code>null</code>.
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*
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* @param <K> The type of keys used with the Map instance
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* @param <V> The type of values used with the Map instance
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*/
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public class ManagedConcurrentWeakHashMap<K, V> extends AbstractMap<K, V> implements
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ConcurrentMap<K, V> {
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private final ConcurrentMap<Key, V> map = new ConcurrentHashMap<>();
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private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
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/**
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* Method, that has to be invoked periodically to clean dead keys from the
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* map.
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*/
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public void maintain() {
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Key key;
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while ((key = (Key) queue.poll()) != null) {
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if (key.isDead()) {
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// No need to lookup if the key is not in the map
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continue;
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}
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key.ackDeath();
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map.remove(key);
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}
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}
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private static class Key extends WeakReference<Object> {
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private final int hash;
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private boolean dead;
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public Key(Object key, ReferenceQueue<Object> queue) {
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super(key, queue);
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hash = key.hashCode();
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}
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@Override
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public int hashCode() {
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return hash;
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}
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@Override
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public boolean equals(Object obj) {
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if (this == obj) {
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return true;
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}
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if (dead) {
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// Post-mortem cleanup looks for this specific Reference
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// instance
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return false;
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}
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if (!(obj instanceof Reference<?>)) {
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return false;
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}
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Object oA = get();
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Object oB = ((Reference<?>) obj).get();
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if (oA == oB) {
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return true;
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}
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if (oA == null || oB == null) {
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return false;
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}
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return oA.equals(oB);
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}
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public void ackDeath() {
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this.dead = true;
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}
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public boolean isDead() {
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return dead;
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}
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}
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/**
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* Creates Key instance to be used to store values in the map. It is
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* registered with the ReferenceQueue.
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*/
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private Key createStoreKey(Object key) {
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return new Key(key, queue);
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}
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/**
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* Creates Key instance to be used only to lookup values in the map. It is
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* not registered with the ReferenceQueue.
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*/
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private Key createLookupKey(Object key) {
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return new Key(key, null);
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}
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@Override
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public int size() {
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return map.size();
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}
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@Override
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public boolean isEmpty() {
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return map.isEmpty();
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}
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@Override
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public boolean containsValue(Object value) {
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if (value == null) {
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return false;
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}
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return map.containsValue(value);
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}
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@Override
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public boolean containsKey(Object key) {
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if (key == null) {
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return false;
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}
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return map.containsKey(createLookupKey(key));
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}
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@Override
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public V get(Object key) {
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if (key == null) {
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return null;
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}
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return map.get(createLookupKey(key));
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}
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@Override
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public V put(K key, V value) {
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Objects.requireNonNull(value);
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return map.put(createStoreKey(key), value);
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}
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@Override
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public V remove(Object key) {
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return map.remove(createLookupKey(key));
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}
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@Override
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public void clear() {
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map.clear();
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// maintain() clears the queue, though it is not 100% reliable,
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// as queue is populated by GC asynchronously
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maintain();
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}
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@Override
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public V putIfAbsent(K key, V value) {
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Objects.requireNonNull(value);
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Key storeKey = createStoreKey(key);
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V oldValue = map.putIfAbsent(storeKey, value);
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if (oldValue != null) { // ack that key has not been stored
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storeKey.ackDeath();
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}
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return oldValue;
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}
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@Override
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public boolean remove(Object key, Object value) {
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if (value == null) {
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return false;
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}
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return map.remove(createLookupKey(key), value);
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}
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@Override
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public boolean replace(K key, V oldValue, V newValue) {
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Objects.requireNonNull(newValue);
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return map.replace(createLookupKey(key), oldValue, newValue);
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}
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@Override
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public V replace(K key, V value) {
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Objects.requireNonNull(value);
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return map.replace(createLookupKey(key), value);
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}
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@Override
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public Collection<V> values() {
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return map.values();
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}
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@Override
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public Set<Map.Entry<K, V>> entrySet() {
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return new AbstractSet<Map.Entry<K, V>>() {
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@Override
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public boolean isEmpty() {
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return map.isEmpty();
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}
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@Override
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public int size() {
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return map.size();
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}
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@Override
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public Iterator<Map.Entry<K, V>> iterator() {
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return new Iterator<Map.Entry<K, V>>() {
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private final Iterator<Map.Entry<Key, V>> it = map
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.entrySet().iterator();
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@Override
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public boolean hasNext() {
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return it.hasNext();
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}
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@Override
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public Map.Entry<K, V> next() {
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return new Map.Entry<K, V>() {
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private final Map.Entry<Key, V> en = it.next();
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@SuppressWarnings("unchecked")
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@Override
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public K getKey() {
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return (K) en.getKey().get();
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}
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@Override
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public V getValue() {
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return en.getValue();
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}
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@Override
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public V setValue(V value) {
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Objects.requireNonNull(value);
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return en.setValue(value);
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}
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};
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}
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@Override
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public void remove() {
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it.remove();
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}
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};
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}
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};
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}
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}
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