zkClient的并发控制

1.ZkConnection实现IZkConnection，是org.apache.zookeeper.ZooKeeper的代理类，通过ReentrantLock实现connect和close的并发控制。因为connect方法和close方法对成员变量_zk进行了赋值和判空操作，所有要进行并发控制。代码如下：

  @Override
    public void connect(Watcher watcher) {
        _zookeeperLock.lock();
        try {
            if (_zk != null) {
                throw new IllegalStateException("zk client has already been started");
            }
            try {
                LOG.debug("Creating new ZookKeeper instance to connect to " + _servers + ".");
                _zk = new ZooKeeper(_servers, _sessionTimeOut, watcher);
            } catch (IOException e) {
                throw new ZkException("Unable to connect to " + _servers, e);
            }
        } finally {
            _zookeeperLock.unlock();
        }
    }

    public void close() throws InterruptedException {
        _zookeeperLock.lock();
        try {
            if (_zk != null) {
                LOG.debug("Closing ZooKeeper connected to " + _servers);
                _zk.close();
                _zk = null;
            }
        } finally {
            _zookeeperLock.unlock();
        }
    }

2.org.I0Itec.zkclient.ZkLock继承jdk自旋锁ReentrantLock，内部定义三个java.util.concurrent.locks.Condition的成员变量，分别实现节点数据改变，节点状态改变和节点改变三种condition，java的condition介绍可以参考

http://blog.sina.com.cn/s/blog_87fc744801018q7l.html，在ZkClient里，包含一个ZkLock的成员变量，用于实现zkClient的并发控制，比如zkClient的waitUntilExists方法：

 public boolean waitUntilExists(String path, TimeUnit timeUnit, long time) throws ZkInterruptedException {
        Date timeout = new Date(System.currentTimeMillis() + timeUnit.toMillis(time));
        LOG.debug("Waiting until znode '" + path + "' becomes available.");
        if (exists(path)) {
            return true;
        }
        acquireEventLock();
        try {
            while (!exists(path, true)) {
                boolean gotSignal = getEventLock().getZNodeEventCondition().awaitUntil(timeout);
                if (!gotSignal) {
                    return false;
                }
            }
            return true;
        } catch (InterruptedException e) {
            throw new ZkInterruptedException(e);
        } finally {
            getEventLock().unlock();
        }
    }

如果path未创建成功，zkNodeEventCondition会wait一段时间，然后process方法负责释放condition，如下：

 public void process(WatchedEvent event) {
        LOG.debug("Received event: " + event);
        _zookeeperEventThread = Thread.currentThread();

        boolean stateChanged = event.getPath() == null;
        boolean znodeChanged = event.getPath() != null;
        boolean dataChanged = event.getType() == EventType.NodeDataChanged || event.getType() == EventType.NodeDeleted || event.getType() == EventType.NodeCreated
                || event.getType() == EventType.NodeChildrenChanged;

        getEventLock().lock();
        try {

            // We might have to install child change event listener if a new node was created
            if (getShutdownTrigger()) {
                LOG.debug("ignoring event '{" + event.getType() + " | " + event.getPath() + "}' since shutdown triggered");
                return;
            }
            if (stateChanged) {
                processStateChanged(event);
            }
            if (dataChanged) {
                processDataOrChildChange(event);
            }
        } finally {
            if (stateChanged) {
                getEventLock().getStateChangedCondition().signalAll();

                // If the session expired we have to signal all conditions, because watches might have been removed and
                // there is no guarantee that those
                // conditions will be signaled at all after an Expired event
                // TODO PVo write a test for this
                if (event.getState() == KeeperState.Expired) {
                    getEventLock().getZNodeEventCondition().signalAll();
                    getEventLock().getDataChangedCondition().signalAll();
                    // We also have to notify all listeners that something might have changed
                    fireAllEvents();
                }
            }
            if (znodeChanged) {
                getEventLock().getZNodeEventCondition().signalAll();
            }
            if (dataChanged) {
                getEventLock().getDataChangedCondition().signalAll();
            }
            getEventLock().unlock();
            LOG.debug("Leaving process event");
        }
    }