维基百科这样定义 Actor 模型：
在计算科学领域，Actor 模型是一个并行计算（Concurrent Computation）模型，它把 actor 作为并行计算的基本元素来对待：为响应一个接收到的消息，一个 actor 能够自己做出一些决策，如创建更多的 actor，或发送更多的消息，或者确定如何去响应接收到的下一个消息。
我们先看一下 Akka 中 Actor 的定义：
trait Actor {
import Actor._
type Receive = Actor.Receive
implicit val context: ActorContext = {
val contextStack = ActorCell.contextStack.get
if ((contextStack.isEmpty) || (contextStack.head eq null))
throw ActorInitializationException(
s"You cannot create an instance of [${getClass.getName}] explicitly using the constructor (new). " +
"You have to use one of the 'actorOf' factory methods to create a new actor. See the documentation.")
val c = contextStack.head
ActorCell.contextStack.set(null :: contextStack)
c
}
implicit final val self = context.self //MUST BE A VAL, TRUST ME
final def sender(): ActorRef = context.sender()
/* 这个是在子类中一定要实现的抽象方法 */
def receive: Actor.Receive
protected[akka] def aroundReceive(receive: Actor.Receive, msg: Any): Unit = receive.applyOrElse(msg, unhandled)
protected[akka] def aroundPreStart(): Unit = preStart()
protected[akka] def aroundPostStop(): Unit = postStop()
protected[akka] def aroundPreRestart(reason: Throwable, message: Option[Any]): Unit = preRestart(reason, message)
protected[akka] def aroundPostRestart(reason: Throwable): Unit = postRestart(reason)
def supervisorStrategy: SupervisorStrategy = SupervisorStrategy.defaultStrategy
/* when changing this you MUST also change UntypedActorDocTest */
/* 启动Actor之前需要执行的操作，默认为空实现，可以重写该方法 */
@throws(classOf[Exception])
def preStart(): Unit = ()
/* when changing this you MUST also change UntypedActorDocTest */
/* 终止Actor之前需要执行的操作，默认为空实现，可以重写该方法 */
@throws(classOf[Exception])
def postStop(): Unit = ()
/* when changing this you MUST also change UntypedActorDocTest */
@throws(classOf[Exception])
def preRestart(reason: Throwable, message: Option[Any]): Unit = {
/* 重启Actor之前需要执行的操作，默认终止该Actor所监督的所有子Actor，
* 然后调用postStop()方法，可以重写该方法
*/
context.children foreach { child ⇒
context.unwatch(child)
context.stop(child)
}
postStop()
}
/* when changing this you MUST also change UntypedActorDocTest */
@throws(classOf[Exception])
def postRestart(reason: Throwable): Unit = {
/* 重启Actor之前需要执行的操作，默认执行preStart()的实现逻辑，
* 可以重写该方法
*/
preStart()
}
def unhandled(message: Any): Unit = {
message match {
case Terminated(dead) ⇒ throw new DeathPactException(dead)
case _ ⇒ context.system.eventStream.publish(UnhandledMessage(message, sender(), self))
}
}
}
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