分布式锁源码剖析（4） zookeeper实现分布式锁

zookeeper分布式锁（curator）

maven配置文件：

<dependency>
	<groupId>org.apache.curator</groupId>
	<artifactId>curator-recipes</artifactId>
	<version>2.3.0</version>
</dependency>

代码示例：

RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);
CuratorFramework client = CuratorFrameworkFactory.newClient(
      "192.168.31.184:2181,192.168.31.207:2181,192.168.31.192:2181",
      retryPolicy);
client.start();

InterProcessMutex lock = new InterProcessMutex(client, "/locks/lock_01");

lock.acquire();

核心源码：

创建一个临时有序节点，然后获取到自己在节点中的位置，如果是在第0位，则获取锁成功，把节点信息放入ConcurrentMap<Thread, LockData> threadData这个map中。如果不是第0位，则wait等待，并注册一个watch，对它上一个节点进行监控（监控上一个节点是否存在）。

private boolean internalLock(long time, TimeUnit unit) throws Exception
{
    Thread          currentThread = Thread.currentThread();

    LockData        lockData = threadData.get(currentThread);
    if ( lockData != null )
    {
        // re-entering
        lockData.lockCount.incrementAndGet();
        return true;
    }

    String lockPath = internals.attemptLock(time, unit, getLockNodeBytes());
    if ( lockPath != null )
    {
        LockData        newLockData = new LockData(currentThread, lockPath);
        threadData.put(currentThread, newLockData);
        return true;
    }

    return false;
}

String attemptLock(long time, TimeUnit unit, byte[] lockNodeBytes) {
   ourPath =client.create().creatingParentsIfNeeded().withProtection()
.withMode(CreateMode.EPHEMERAL_SEQUENTIAL).forPath(path, localLockNodeBytes);
      
   asTheLock = internalLockLoop(startMillis, millisToWait, ourPath);
}

释放锁主要逻辑：删除当前持有锁的临时节点，并移除当前线程的LockData。

public void release() throws Exception
{
    Thread      currentThread = Thread.currentThread();
    LockData    lockData = threadData.get(currentThread);

    int newLockCount = lockData.lockCount.decrementAndGet();
    if ( newLockCount > 0 )
    {
        return;
    }
    
    try
    {internals.releaseLock(lockData.lockPath);}
    finally
    {threadData.remove(currentThread);}
}

curator 分布式锁原理图