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");        }    }