/**
 * 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.hadoop.hbase.util;

import java.lang.ref.Reference;
import java.lang.ref.WeakReference;

import org.apache.hadoop.hbase.classification.InterfaceAudience;
import org.apache.hadoop.hbase.util.ObjectPool.ObjectFactory;

/**
 * A {@code WeakReference} based shared object pool.
 * The objects are kept in weak references and
 * associated with keys which are identified by the {@code equals} method.
 * The objects are created by {@link ObjectFactory} on demand.
 * The object creation is expected to be lightweight,
 * and the objects may be excessively created and discarded.
 * Thread safe.
 */
@InterfaceAudience.Private
public class WeakObjectPool<K,V> extends ObjectPool<K,V> {

  public WeakObjectPool(ObjectFactory<K, V> objectFactory) {
    super(objectFactory);
  }

  public WeakObjectPool(ObjectFactory<K, V> objectFactory, int initialCapacity) {
    super(objectFactory, initialCapacity);
  }

  public WeakObjectPool(ObjectFactory<K, V> objectFactory, int initialCapacity,
      int concurrencyLevel) {
    super(objectFactory, initialCapacity, concurrencyLevel);
  }

  @Override
  public void purge() {
    // This method is lightweight while there is no stale reference
    // with the Oracle (Sun) implementation of {@code ReferenceQueue},
    // because {@code ReferenceQueue.poll} just checks a volatile instance
    // variable in {@code ReferenceQueue}.
    while (true) {
      @SuppressWarnings("unchecked")
      WeakObjectReference ref = (WeakObjectReference) staleRefQueue.poll();
      if (ref == null) {
        break;
      }
      referenceCache.remove(ref.key, ref);
    }
  }

  @Override
  public Reference<V> createReference(K key, V obj) {
    return new WeakObjectReference(key, obj);
  }

  private class WeakObjectReference extends WeakReference<V> {
    final K key;

    WeakObjectReference(K key, V obj) {
      super(obj, staleRefQueue);
      this.key = key;
    }
  }

}