在Spark Standalone中我们所谓的Client，它的任务其实是由AppClient和DriverClient共同完成的。AppClient是一个允许app（Client）和Spark集群通信的中间人，接受master URL、app的信息、一个集群事件的监听器以及事件监听的回调函数，主要和Master交互App相关的信息，DriverClient主要用于和Master交互Driver相关的信息，比如启动、停止及运行状况等，本篇先介绍AppClient。

1.AppClient类主要字段、方法如下：

AppClient类成员

2.ClientEndpoint

2.1属性

（1）//设置一个boolean标识，用于避免多次调用listener.disconnected()
private var alreadyDisconnected = false
（2）//app向master申请注册的线程池，因为被maser注册是一个阻塞操作，所以线程池的个数是"masterRpcAddresses.size"，这样app就能同时被所有的master注册
private val registerMasterThreadPool = new ThreadPoolExecutor(
0,
masterRpcAddresses.size, // Make sure we can register with all masters at the same time
60L, TimeUnit.SECONDS,
new SynchronousQueueRunnable,
ThreadUtils.namedThreadFactory("appclient-register-master-threadpool"))
（3）一个守护单线程用于申请注册操作
private val registrationRetryThread =
ThreadUtils.newDaemonSingleThreadScheduledExecutor("appclient-registration-retry-thread")

2.2方法

（1）构造函数为ClientEndpoint的主构造器。
（2）onStart方法，用于将App注册到所有的Master上
<code>
override def onStart(): Unit = {
try {
//“1”表示第几次注册，最大次数不超过3次，第n次申请注册到master上
registerWithMaster(1)详见下①
} catch {
case e: Exception =>
logWarning("Failed to connect to master", e)
//监听器停止并将boolen状态标识设置为true
markDisconnected()
//停止rpcendpoint
stop()
}
}
</code>
①registerWithMaster方法如下,用于异步注册到所有的master上，如果没有超过再次注册的次数（3次），那么每20s将会重新调用该方法申请注册,如果注册成功，所有的调用work和futures将会被取消。
<code>
private def registerWithMaster(nthRetry: Int) {
registerMasterFutures = tryRegisterAllMasters()
registrationRetryTimer = registrationRetryThread.scheduleAtFixedRate(new Runnable {
override def run(): Unit = {
Utils.tryOrExit {
if (registered) {
registerMasterFutures.foreach(.cancel(true))
registerMasterThreadPool.shutdownNow()
} else if (nthRetry >= REGISTRATION_RETRIES) {
markDead("All masters are unresponsive! Giving up.")
} else {
registerMasterFutures.foreach(.cancel(true))
registerWithMaster(nthRetry + 1)
}
}
}
}, REGISTRATION_TIMEOUT_SECONDS, REGISTRATION_TIMEOUT_SECONDS, TimeUnit.SECONDS)
}
</code>
（3）onstop方法如下，释放资源。
<code>
override def onStop(): Unit = {
if (registrationRetryTimer != null) {
registrationRetryTimer.cancel(true)
}
registrationRetryThread.shutdownNow()
registerMasterFutures.foreach(_.cancel(true))
registerMasterThreadPool.shutdownNow()
}
</code>
（4）receive方法，receive接受到的消息分为5种，分别为

• （1）app被master成功注册，并将注册成功的app添加到监听器中
  <code>
  case RegisteredApplication(appId_, masterRef) => appId = appId_ registered = true master = Some(masterRef) listener.connected(appId)
  </code>
• （2）移除app，停止rpcendpoint
  <code>
  case ApplicationRemoved(message) => markDead("Master removed our application: %s".format(message))
  stop()
  </code>
• （3）向master申请为app添加executor，并添加到监听器中
  <code>
  case ExecutorAdded(id: Int, workerId: String, hostPort: String, cores: Int, memory: Int) =>val fullId = appId + "/" + id
  logInfo("Executor added: %s on %s (%s) with %d cores".format(fullId, workerId, hostPort, cores))
  sendToMaster(ExecutorStateChanged(appId, id, ExecutorState.RUNNING, None, None))
  listener.executorAdded(fullId, workerId, hostPort, cores, memory)</code>
• （4）Executor的信息发生改变，记录到日志中<code>
  case ExecutorUpdated(id, state, message, exitStatus) =>
  val fullId = appId + "/" + id
  val messageText = message.map(s => " (" + s + ")").getOrElse("")
  logInfo("Executor updated: %s is now %s%s".format(fullId, state, messageText))
  if (ExecutorState.isFinished(state)) {
  listener.executorRemoved(fullId, message.getOrElse(""), exitStatus) }
  </code>
• （5）HA机制，为app更换master
  <code>
  case MasterChanged(masterRef, masterWebUiUrl) =>
  logInfo("Master has changed, new master is at " + masterRef.address.toSparkURL)
  master = Some(masterRef)
  alreadyDisconnected = false
  masterRef.send(MasterChangeAcknowledged(appId))
  </code>