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/*
* Copyright 2006 Amazon Technologies, Inc. or its affiliates.
* Amazon, Amazon.com and Carbonado are trademarks or registered trademarks
* of Amazon Technologies, Inc. or its affiliates. All rights reserved.
*
* 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.amazon.carbonado.util;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/**
* A concurrent pool of weakly referenced values mapped by key. Values are
* created (and recreated) as needed.
*
* @author Brian S O'Neill
* @see AbstractPool
* @since 1.2
*/
abstract class AbstractWeakPool<K, V, E extends Exception> {
private final ConcurrentMap<K, ValueRef<K, V>> mValues;
private final ReferenceQueue<V> mValueRefQueue;
protected AbstractWeakPool() {
mValues = new ConcurrentHashMap<K, ValueRef<K, V>>();
mValueRefQueue = new ReferenceQueue<V>();
}
/**
* Returns a value for the given key. Unused values are automatically
* cleared to free up memory, even if they are still held. Repeat calls are
* guaranteed to return the same value instance only if the value is
* strongly reachable. The following idiom should be used when using the
* pool for maintaining locks:
*
* <pre>
* // Store lock in local variable to be strongly reachable.
* Lock lock = lockPool.get(key);
* lock.lock();
* try {
* // access the resource protected by this lock
* ...
* } finally {
* lock.unlock();
* }
* </pre>
*/
public V get(K key) throws E {
clean();
ValueRef<K, V> valueRef = mValues.get(key);
V value;
if (valueRef == null || (value = valueRef.get()) == null) {
try {
value = create(key);
} catch (Exception e) {
// Workaround compiler bug.
org.cojen.util.ThrowUnchecked.fire(e);
return null;
}
valueRef = new ValueRef<K, V>(value, mValueRefQueue, key);
while (true) {
ValueRef<K, V> existingRef = mValues.putIfAbsent(key, valueRef);
if (existingRef == null) {
// Newly created value is now the official value.
break;
}
V existing = existingRef.get();
if (existing != null) {
// Someone else just created value before us. Use that
// instead and chuck the new value object.
value = existing;
valueRef.clear();
break;
}
// Reference just got cleared. Try again. Explicitly remove it
// to prevent an infinite loop. Note that the two argument
// remove method is called to ensure that what is being removed
// is not a new value.
mValues.remove(((ValueRef<K, V>) existingRef).mKey, existingRef);
}
}
return value;
}
/**
* Manually remove a value, returning the old value.
*/
public V remove(Object key) {
clean();
ValueRef<K, V> valueRef = mValues.remove(key);
V value;
if (valueRef != null && (value = valueRef.get()) != null) {
valueRef.clear();
return value;
}
return null;
}
/**
* Return a new value instance.
*/
protected abstract V create(K key) throws E;
private void clean() {
// Clean out cleared values.
Reference<? extends V> ref;
while ((ref = mValueRefQueue.poll()) != null) {
// Note that the two argument remove method is called to ensure
// that what is being removed is not a new value.
mValues.remove(((ValueRef<K, V>) ref).mKey, ref);
}
}
private static class ValueRef<K, V> extends WeakReference<V> {
final K mKey;
ValueRef(V value, ReferenceQueue<V> queue, K key) {
super(value, queue);
mKey = key;
}
}
}
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