MessageQueueLock.java

/**
 * Copyright (C) 2010-2013 Alibaba Group Holding Limited
 *
 * 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.alibaba.rocketmq.client.impl.consumer;

import java.util.concurrent.ConcurrentHashMap;

import com.alibaba.rocketmq.common.message.MessageQueue;


/**
 * 严格保证单个队列同一时刻只有一个线程消费
 * 
 * @author shijia.wxr<vintage.wang@gmail.com>
 * @since 2013-6-25
 */
public class MessageQueueLock {
    private ConcurrentHashMap<MessageQueue, Object> mqLockTable =
            new ConcurrentHashMap<MessageQueue, Object>();


    public Object fetchLockObject(final MessageQueue mq) {
        Object objLock = this.mqLockTable.get(mq);
        if (null == objLock) {
            objLock = new Object();
            Object prevLock = this.mqLockTable.putIfAbsent(mq, objLock);
            if (prevLock != null) {
                objLock = prevLock;
            }
        }

        return objLock;
    }
}