DoubleCheckCache.java

/*
 * Copyright 2014 mango.jfaster.org
 *
 * The Mango Project 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.jfaster.mango.util.local;

import java.util.concurrent.ConcurrentHashMap;

/**
 * @author ash
 */
public class DoubleCheckCache<K, V> implements LoadingCache<K, V> {

  private final CacheLoader<K, V> loader;
  private final ConcurrentHashMap<K, V> cache = new ConcurrentHashMap<K, V>();
  private final ConcurrentHashMap<K, Object> locks = new ConcurrentHashMap<K, Object>();

  public DoubleCheckCache(CacheLoader<K, V> loader) {
    this.loader = loader;
  }

  @Override
  public V get(K key) {
    V value = cache.get(key);
    if (value == null) {
      synchronized (getLock(key)) {
        value = cache.get(key);
        if (value == null) {
          value = loader.load(key);
          if (value != null) {
            cache.put(key, value);
          }
        }
      }
    }
    return value;
  }

  private Object getLock(K key) {
    Object lock = locks.get(key);
    if (lock == null) {
      lock = new Object();
      Object old = locks.putIfAbsent(key, lock);
      if (old != null) { // 已经存在lock
        lock = old;
      }
    }
    return lock;
  }

}