Akka的优点太多，高性能、高可靠、高并发、分布式、可容错、可扩展、事件驱动，不一一叙述。不同版本的API差异很大，本文代码运行在 Scala 2.10.3 和 Akka 2.3.2 之上。

<dependency>
  <groupId>com.typesafe.akka</groupId>
  <artifactId>akka-actor_2.10</artifactId>
  <version>2.3.2</version>
</dependency>
<dependency>
  <groupId>org.scala-lang</groupId>
  <artifactId>scala-library</artifactId>
  <version>2.10.3</version>
</dependency>

定义

定义Actor很简单，继承 akka.actor.Actor ，实现receive方法即可。

class Hello extends Actor {
  def receive = {
    case msg: String => println("hello " + msg)
    case _ => println("unexpected message.")
  }
}

启动

创建Actor实例需要通过 ActorSystem 。

val system = ActorSystem("HelloSystem")
val hello = system.actorOf(Props[Hello], name = "hello")
val hello1 = system.actorOf(Props[Hello])
val hello2 = system.actorOf(Props(new Hello()))

如果要在 Actor 中继续创建子 Actor，需要使用内置的 ActorContext 对象。

context.actorOf(Props[children], name = "children")

如果要创建远程 Actor，需要通过 actorSelection 方法，原 actorFor 方法不再使用。

context.actorSelection("akka.tcp://HelloRemoteSystem@127.0.0.1:5150/user/RemoteActor")

发消息

巨简单，就是一个!，可以发送任意类型的消息，此消息是异步的。

hello ! "bruce"
hello ! 10086

同步消息的发送需要使用 Future 对象。

implicit val timeout = Timeout(5 seconds)
val future = hello ? "sha"
val result = Await.result(future, timeout.duration).asInstanceOf[String]

停止

有两种方式停止一个Actor。

一种是通过内部 ActorContext.stop() 方法，该方法会将 children actor 逐层杀掉后，再自刎。

def receive = {
    case "stop" => context.stop(self)
    ...
  }

另一种是外部喂毒药，通过 ActorRef.tell() 方法实现。后一个参数是向谁reply，这里显然不需要，传空。

hello.tell(PoisonPill.getInstance, ActorRef.noSender);

哼哈示例

哼哈二将本是两位佛寺的门神俗称，是执金刚神的一种。明代小说《封神演义》作者陈仲琳据此附会两员神将，形象威武凶猛。一名郑伦，能鼻哼白气制敌；一名陈奇，能口哈黄气擒将。

object HengHa extends App {
  val system = ActorSystem("HengHaSystem")
  val ha = system.actorOf(Props[Ha], name = "ha")
  val heng = system.actorOf(Props(new Heng(ha)), name = "heng")

  heng ! "start"
}

class Heng(ha: ActorRef) extends Actor {
  def receive = {
    case "start" => ha ! "heng"
    case "ha" => 
      println("哈")
      ha ! "heng"
    case _ => println("heng what?")
  }
}

class Ha extends Actor {
  def receive = {
    case "heng" => 
      println("哼")
      sender ! "ha"
    case _ => println("ha what?")
  }
}

Run 起来，结果：

哼
哈
哼
哈
哼
...

远程示例

local工程

application.conf

akka {
  loglevel = "DEBUG"
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  }
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = "127.0.0.1"
      port = 5155
    }
 }
}

object Local extends App {
  val system = ActorSystem("LocalSystem")
  val localActor = system.actorOf(Props[LocalActor], name = "LocalActor") // the local actor
  localActor ! "START" // start the action
}

class LocalActor extends Actor {
  // create the remote actor
  val remote = context.actorSelection("akka.tcp://HelloRemoteSystem@127.0.0.1:5150/user/RemoteActor")
  var counter = 0

  def receive = {
    case "START" =>
      remote ! "Hello from the LocalActor"
    case msg: String =>
      println(s"LocalActor received message: ‘$msg‘")

      if (counter < 5) {
        sender ! "Hello back to you"
        counter += 1
      }
  }
}

remote工程

application.conf

akka {
  loglevel = "DEBUG"
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  }
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = "127.0.0.1"
      port = 5150
    }
 }
}

object HelloRemote extends App {
  val system = ActorSystem("HelloRemoteSystem")
  val remoteActor = system.actorOf(Props[RemoteActor], name = "RemoteActor")

  remoteActor ! "The RemoteActor is alive"
}

class RemoteActor extends Actor {
  def receive = {
    case msg: String =>
      println(s"RemoteActor received message ‘$msg‘")
      sender ! "Hello from the RemoteActor"
  }
}