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/*
* Copyright 2006-2010 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.util.Collection;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.locks.Lock;
/**
* A concurrent pool of strongly referenced values mapped by key. Values are
* lazily created and pooled.
*
* @author Brian S O'Neill
* @since 1.2
*/
public abstract class AbstractPool<K, V, E extends Exception> {
private final ConcurrentMap<K, V> mValues;
private final WeakReentrantLockPool<K> mLockPool;
protected AbstractPool() {
mValues = new ConcurrentHashMap<K, V>();
mLockPool = new WeakReentrantLockPool<K>();
}
/**
* Returns a value for the given key, which is lazily created and
* pooled. If multiple threads are requesting upon the same key
* concurrently, at most one thread attempts to lazily create the
* value. The others wait for it to become available.
*/
public V get(K key) throws E {
// Quick check without locking.
V value = mValues.get(key);
if (value != null) {
return value;
}
// Check again with key lock held.
Lock lock = mLockPool.get(key);
lock.lock();
try {
value = mValues.get(key);
if (value == null) {
try {
value = create(key);
mValues.put(key, value);
} catch (Exception e) {
// Workaround compiler bug.
org.cojen.util.ThrowUnchecked.fire(e);
}
}
} finally {
lock.unlock();
}
return value;
}
/**
* Remove a value, returning the old value.
*/
public V remove(Object key) {
return mValues.remove(key);
}
/**
* Returns the pool values, which may be concurrently modified.
*/
public Collection<V> values() {
return mValues.values();
}
/**
* Return a new value instance.
*/
protected abstract V create(K key) throws E;
}
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