Curator zookeeper 分布式锁实现

curator版本：

        <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-recipes</artifactId>
            <version>2.5.0</version>
        </dependency>

 

 

代码实现

 

   private boolean internalLock(long time, TimeUnit unit) throws Exception
    {
        /*
           Note on concurrency: a given lockData instance
           can be only acted on by a single thread so locking isn't necessary
        */

        Thread          currentThread = Thread.currentThread();

        LockData        lockData = threadData.get(currentThread);
        if ( lockData != null )//当前线程已经获得锁（重入），对线程数据的锁+1
        {
            // re-entering
            lockData.lockCount.incrementAndGet();
            return true;
        }
        //尝试获取锁，获取锁成功返回zk路径
        String lockPath = internals.attemptLock(time, unit, getLockNodeBytes());
        if ( lockPath != null )
        {
            LockData        newLockData = new LockData(currentThread, lockPath);
            threadData.put(currentThread, newLockData);
            return true;
        }

        return false;
    }

 

代码调用

 

    String attemptLock(long time, TimeUnit unit, byte[] lockNodeBytes) throws Exception
    {
        final long      startMillis = System.currentTimeMillis();
        final Long      millisToWait = (unit != null) ? unit.toMillis(time) : null;
        final byte[]    localLockNodeBytes = (revocable.get() != null) ? new byte[0] : lockNodeBytes;
        int             retryCount = 0;

        String          ourPath = null;
        boolean         hasTheLock = false;
        boolean         isDone = false;
        while ( !isDone )
        {
            isDone = true;

            try
            {
                //创建节点，模式：EPHEMERAL_SEQUENTIAL 直译：瞬态的-有序的，不会持久化，client与zk服务断掉链接的时候，此节点会自动删除，这样在服务停机时再重启时，不会出现锁无法释放
                if ( localLockNodeBytes != null )
                {
                    ourPath = client.create().creatingParentsIfNeeded().withProtection().withMode(CreateMode.EPHEMERAL_SEQUENTIAL).forPath(path, localLockNodeBytes);
                }
                else
                {
                    ourPath = client.create().creatingParentsIfNeeded().withProtection().withMode(CreateMode.EPHEMERAL_SEQUENTIAL).forPath(path);
                }
                //循环尝试获取锁
                hasTheLock = internalLockLoop(startMillis, millisToWait, ourPath);
            }
            catch ( KeeperException.NoNodeException e )
            {
                // gets thrown by StandardLockInternalsDriver when it can't find the lock node
                // this can happen when the session expires, etc. So, if the retry allows, just try it all again
                if ( client.getZookeeperClient().getRetryPolicy().allowRetry(retryCount++, System.currentTimeMillis() - startMillis, RetryLoop.getDefaultRetrySleeper()) )
                {
                    isDone = false;
                }
                else
                {
                    throw e;
                }
            }
        }

        if ( hasTheLock )
        {
            return ourPath;
        }

        return null;
    }

  代码调用

 

 

    private boolean internalLockLoop(long startMillis, Long millisToWait, String ourPath) throws Exception
    {
        boolean     haveTheLock = false;
        boolean     doDelete = false;
        try
        {
            if ( revocable.get() != null )
            {
                client.getData().usingWatcher(revocableWatcher).forPath(ourPath);
            }

            while ( (client.getState() == CuratorFrameworkState.STARTED) && !haveTheLock )
            {
                List<String>        children = getSortedChildren();
                String              sequenceNodeName = ourPath.substring(basePath.length() + 1); // +1 to include the slash

                //验证当前线程持有的node是不是路径下最靠前的node，如果是，获取到锁，否则没有获取到
                PredicateResults    predicateResults = driver.getsTheLock(client, children, sequenceNodeName, maxLeases);
                if ( predicateResults.getsTheLock() )
                {
                    haveTheLock = true;
                }
                else
                {
                    //获取到路径下排在当前节点前面的节点，watch此节点，防止大规模的watch
                    String  previousSequencePath = basePath + "/" + predicateResults.getPathToWatch();

                    synchronized(this)
                    {
                        try 
                        {
                            // use getData() instead of exists() to avoid leaving unneeded watchers which is a type of resource leak
                            client.getData().usingWatcher(watcher).forPath(previousSequencePath);
                            if ( millisToWait != null )
                            {
                                millisToWait -= (System.currentTimeMillis() - startMillis);
                                startMillis = System.currentTimeMillis();
                                if ( millisToWait <= 0 )
                                {
                                    doDelete = true;    // timed out - delete our node
                                    break;
                                }
                                //获取不到，等待指定时间，等待watcher叫醒
                                wait(millisToWait);
                            }
                            else
                            {
                                //获取不到锁，等待watcher叫醒
                                wait();
                            }
                        }
                        catch ( KeeperException.NoNodeException e ) 
                        {
                            // it has been deleted (i.e. lock released). Try to acquire again
                        }
                    }
                }
            }
        }
        catch ( Exception e )
        {
            doDelete = true;
            throw e;
        }
        finally
        {
            if ( doDelete )
            {
                deleteOurPath(ourPath);
            }
        }
        return haveTheLock;
    }


    @Override
    public PredicateResults getsTheLock(CuratorFramework client, List<String> children, String sequenceNodeName, int maxLeases) throws Exception
    {
        int             ourIndex = children.indexOf(sequenceNodeName);
        validateOurIndex(sequenceNodeName, ourIndex);
        //判断是否是第一个节点
        boolean         getsTheLock = ourIndex < maxLeases;
        String          pathToWatch = getsTheLock ? null : children.get(ourIndex - maxLeases);

        return new PredicateResults(pathToWatch, getsTheLock);
    }

 

 zookeeper四种模式的节点：

 

    /**
     * The znode will not be automatically deleted upon client's disconnect.
     */
    PERSISTENT (0, false, false),
    /**
    * The znode will not be automatically deleted upon client's disconnect,
    * and its name will be appended with a monotonically increasing number.
    */
    PERSISTENT_SEQUENTIAL (2, false, true),
    /**
     * The znode will be deleted upon the client's disconnect.
     */
    EPHEMERAL (1, true, false),
    /**
     * The znode will be deleted upon the client's disconnect, and its name
     * will be appended with a monotonically increasing number.
     */
    EPHEMERAL_SEQUENTIAL (3, true, true);

 curator分布式锁实现创建的是EPHEMERAL_SEQUENTIAL，使用时不用担心服务停掉，锁无法释放的问题