Curator作为操作Zookeeper的优秀工具，网上有很多的文章介绍和实例，我这里就不再介绍了。

我这篇主要想写写最近看Curator源码一点总结，关于ZooKeeper连接不上时，如何控制连接状态的源码。

Curator大体类结构(主要是和重试相关的类)：

（家里电脑没有UML工具，不想装，欠着）

CuratorFrameworkImpl->CuratorZookeeperClient->ConnectionState->HandleHolder->Helper->ZooKeeper(zk客户端原生)

先看如何在连接断开后，更新连接状态。

ZooKeeper连接建立时使用ConnectionState作为watcher监听zk状态，当连接断开后，zk通知watcher(ConnectionState),执行process，更新状态。

import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; class ConnectionState implements Watcher, Closeable{     @Override     public void process(WatchedEvent event){         if ( LOG_EVENTS )         {             log.debug("ConnectState watcher: " + event);         }         for ( Watcher parentWatcher : parentWatchers )         {             TimeTrace timeTrace = new TimeTrace("connection-state-parent-process", tracer.get());             parentWatcher.process(event);             timeTrace.commit();         }         boolean wasConnected = isConnected.get();         boolean newIsConnected = wasConnected;         if ( event.getType() == Watcher.Event.EventType.None )         {             newIsConnected = checkState(event.getState(), wasConnected);         }         if ( newIsConnected != wasConnected )         {             isConnected.set(newIsConnected);             connectionStartMs = System.currentTimeMillis();         }     }    private boolean checkState(Event.KeeperState state, boolean wasConnected)   {         boolean isConnected = wasConnected;         boolean checkNewConnectionString = true;         switch ( state )         {         default:         case Disconnected:         {             isConnected = false;             break;         }         case SyncConnected:         case ConnectedReadOnly:         {             isConnected = true;             break;         }         case AuthFailed:         {             isConnected = false;             log.error("Authentication failed");             break;         }         case Expired:         {             isConnected = false;             checkNewConnectionString = false;             handleExpiredSession();             break;         }         case SaslAuthenticated:         {             // NOP             break;         }         }         if ( checkNewConnectionString && zooKeeper.hasNewConnectionString() )         {             handleNewConnectionString();         }         return isConnected;     }     ...... }

下面来看一个getData操作

org.apache.curator.framework.imps.CuratorFrameworkImpl.getData()

@Override     public GetDataBuilder getData()     {         Preconditions.checkState(isStarted(), "instance must be started before calling this method");         return new GetDataBuilderImpl(this);     }

无论GetData还是SetData都有对应Builder实现类，实现相应的操作。

我们来看看GetDataBuilderImpl的代码。

org.apache.curator.framework.imps.GetDataBuilderImpl

@Override     public byte[] forPath(String path) throws Exception     {         path = client.fixForNamespace(path);         byte[]      responseData = null;         if ( backgrounding.inBackground() )         {             client.processBackgroundOperation(new OperationAndData<String>(this, path, backgrounding.getCallback(), null, backgrounding.getContext()), null);         }         else         {             responseData = pathInForeground(path);         }         return responseData;     }

我这里只是看看非后台执行的逻辑，后台执行代码，请小伙伴自己查看。

private byte[] pathInForeground(final String path) throws Exception     {         TimeTrace   trace = client.getZookeeperClient().startTracer("GetDataBuilderImpl-Foreground");         byte[]      responseData = RetryLoop.callWithRetry         (             client.getZookeeperClient(),             new Callable<byte[]>()             {                 @Override                 public byte[] call() throws Exception                 {                     byte[]      responseData;                     if ( watching.isWatched() )                     {                         responseData = client.getZooKeeper().getData(path, true, responseStat);                     }                     else                     {                         responseData = client.getZooKeeper().getData(path, watching.getWatcher(), responseStat);                     }                     return responseData;                 }             }         );         trace.commit();         return decompress ? client.getCompressionProvider().decompress(path, responseData) : responseData;     }

RetryLoop.callWithRetry代码如下：

public static<T> T      callWithRetry(CuratorZookeeperClient client, Callable<T> proc) throws Exception     {         T               result = null;         RetryLoop       retryLoop = client.newRetryLoop();         while ( retryLoop.shouldContinue() )         {             try             {                 client.internalBlockUntilConnectedOrTimedOut();                                  result = proc.call();                 retryLoop.markComplete();             }             catch ( Exception e )             {                 retryLoop.takeException(e);             }         }         return result;     }

上面函数中client.internalBlockUntilConnectedOrTimedOut()这句代码，就是测试当前状态是否处于连接态，如果不是就一直等待，一直到Timeout时间用完.

超过了Timeout时间，状态还是非连接态。这时仍然调用client.getZooKeeper().getData()仿佛，之所以这么设计，我分析原因，如果非连接态直接调用getZooKeeper()会触发线程加锁及重试连接的过程。加锁是一个相对比较重型操作，线程的事前重试比阻塞相对效果更好。

重点是下面这个方法：

org.apache.curator.ConnectionState.getZooKeeper()

ZooKeeper getZooKeeper() throws Exception     {         if ( SessionFailRetryLoop.sessionForThreadHasFailed() )         {             throw new SessionFailRetryLoop.SessionFailedException();         }         Exception exception = backgroundExceptions.poll();         if ( exception != null )         {             log.error("Background exception caught", exception);             tracer.get().addCount("background-exceptions", 1);             throw exception;         }         boolean localIsConnected = isConnected.get();         if ( !localIsConnected )         {             checkTimeouts();         }         return zooKeeper.getZooKeeper();     }

判断如果是非连接状态，就会调用checkTimeouts方法进行检查，重试。

checkTimeouts逻辑：

1.判断是否到了最小的连接过期时间。没有到，先不处理，证明刚才的连接检查可能不准确。

2.判断是否有新的zk地址。有则重新设定环境后进行重连。

3.如果过了最大Session过期时间（证明这个连接，时间太长了，已经过期了），需要重连。如没有大于Session过期时间，连接还是断开的，抛出异常。

private synchronized void checkTimeouts() throws Exception     {         int minTimeout = Math.min(sessionTimeoutMs, connectionTimeoutMs);         long elapsed = System.currentTimeMillis() - connectionStartMs;         if ( elapsed >= minTimeout )         {             if ( zooKeeper.hasNewConnectionString() )             {                 handleNewConnectionString();             }             else             {                 int maxTimeout = Math.max(sessionTimeoutMs, connectionTimeoutMs);                 if ( elapsed > maxTimeout )                 {                     if ( !Boolean.getBoolean(DebugUtils.PROPERTY_DONT_LOG_CONNECTION_ISSUES) )                     {                         log.warn(String.format("Connection attempt unsuccessful after %d (greater than max timeout of %d). Resetting connection and trying again with a new connection.", elapsed, maxTimeout));                     }                     reset();                 }                 else                 {                     KeeperException.ConnectionLossException connectionLossException = new CuratorConnectionLossException();                     if ( !Boolean.getBoolean(DebugUtils.PROPERTY_DONT_LOG_CONNECTION_ISSUES) )                     {                         log.error(String.format("Connection timed out for connection string (%s) and timeout (%d) / elapsed (%d)", zooKeeper.getConnectionString(), connectionTimeoutMs, elapsed), connectionLossException);                     }                     tracer.get().addCount("connections-timed-out", 1);                     throw connectionLossException;                 }             }         }     }

private synchronized void reset() throws Exception     {         log.debug("reset");         instanceIndex.incrementAndGet();         isConnected.set(false);         connectionStartMs = System.currentTimeMillis();         zooKeeper.closeAndReset();         zooKeeper.getZooKeeper();   // initiate connection     }