Akka并发编程——第二节：Actor模型(一）

本节主要内容

1. 定义Actor
2. 创建Actor

1. 定义Actor

通过扩展akka.actor.Actor 特质并实现receive方法来定义Actor，代码示例如下

//通过扩展Actor并实现receive方法来定义Actor
class MyActor extends Actor {
    //获取LoggingAdapter，用于日志输出
    val log = Logging(context.system, this)

    //实现receive方法，定义Actor的行为逻辑，返回的是一个偏函数
    def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }
  }

receive方法被定义在Actor当中，方法标签如下

//Actor中的receive方法定义，
type Receive = PartialFunction[Any, Unit]
def receive: Actor.Receive

下面给出其完整使用代码：

object Example_01 extends App{
  import akka.actor.Actor
  import akka.event.Logging
  import akka.actor.ActorSystem
  import akka.actor.Props

  class MyActor extends Actor {
    val log = Logging(context.system, this)

    def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }
  }
 //创建ActorSystem对象
  val system = ActorSystem("MyActorSystem")
  //返回ActorSystem的LoggingAdpater
  val systemLog=system.log
  //创建MyActor,指定actor名称为myactor
  val myactor = system.actorOf(Props[MyActor], name = "myactor")

  systemLog.info("准备向myactor发送消息")
  //向myactor发送消息
  myactor!"test"
  myactor! 123

  //关闭ActorSystem，停止程序的运行
  system.shutdown()
}

代码运行结果：

[INFO] [04/02/2016 09:29:54.223] [main] [ActorSystem(MyActorSystem)] 准备向myactor发送消息
[INFO] [04/02/2016 09:29:54.224] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/myactor] received test
[INFO] [04/02/2016 09:29:54.224] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/myactor] received unknown message

输出“[INFO] [04/02/2016 09:29:54.224] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/myactor] received test”中的[MyActorSystem-akka.actor.default-dispatcher-3]为对应的线程名，[akka://MyActorSystem/user/myactor]为Actor路径信息， received test为

def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }

方法处理后的输出。关于[akka://MyActorSystem/user/myactor]路径信息，将在后续内容中进行详细阐述。

也可以通过混入ActorLogging来实现日志功能，具体代码如下：

class MyActor extends Actor with ActorLogging{
    def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }
  }

ActorLogging的定义如下：

trait ActorLogging { this: Actor ⇒
  private var _log: LoggingAdapter = _

  def log: LoggingAdapter = {
    // only used in Actor, i.e. thread safe
    if (_log eq null)
      _log = akka.event.Logging(context.system, this)
    _log
  }

}

完整代码如下：

/*
 *定义Actor时混入ActorLogging
 */
object Example_02 extends App{
  import akka.actor.Actor
  import akka.actor.ActorSystem
  import akka.actor.Props

  class MyActor extends Actor with ActorLogging{
    def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }
  }

  //创建ActorSystem对象
  val system = ActorSystem("MyActorSystem")
  //返回ActorSystem的LoggingAdpater
  val systemLog=system.log
  //创建MyActor，指定actor名称为myactor
  val myactor = system.actorOf(Props[MyActor], name = "myactor")

  systemLog.info("准备向myactor发送消息")
  //向myactor发送消息
  myactor!"test"
  myactor! 123

  //关闭ActorSystem，停止程序的运行
  system.shutdown()
}

代码运行结果：

[INFO] [04/02/2016 09:39:21.088] [main] [ActorSystem(MyActorSystem)] 准备向myactor发送消息
[INFO] [04/02/2016 09:39:21.089] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/myactor] received test
[INFO] [04/02/2016 09:39:21.089] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/myactor] received unknown message

代码原理与Example_01类似，这里不再赘述。

2. 创建Actor

在前面两个例子中，通过

 val myactor = system.actorOf(Props[MyActor], name = "myactor")

创建Actor，需要注意的是system.actorOf方法返回的是ActorRef对象，ActorRef为Actor的引用，使用ActorRef对象可以进行消息的发送等操作。Props为配置对象，在创建Actor时使用，它是不可变的对象，因此它是线程案例且完全可共享的。Akka中创建Actor时，也允许直接传入MyActor对象的引用，例如

//直接通过new MyActor的方式传入MyActor对象的引用，注意这里是Props(new MyActor)
val myactor = system.actorOf(Props(new MyActor), name = "myactor")

但是Akka不推荐这么做，官方文档给出的解释是这种方式会导致不可序列化的Props对象且可能会导致竞争条件（破坏Actor的封装性）。另外需要特别注意的是，不允许通过下列代码创建Actor

//下列两行代码编译可以通过，但运行时出抛出异常
  val  myActor=new MyActor
  val myactor = system.actorOf(Props(myActor), name = "myactor")

完整运行代码如下：

/*
 *创建Actor
 */
object Example_03 extends App{
  import akka.actor.Actor
  import akka.actor.ActorSystem
  import akka.actor.Props

  class MyActor extends Actor with ActorLogging{
    def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }
  }

  val system = ActorSystem("MyActorSystem")
  val systemLog=system.log

  //下列两行代码编译可以通过，但运行时出抛出异常
  val  myActor=new MyActor
  val myactor = system.actorOf(Props(myActor), name = "myactor")

  systemLog.info("准备向myactor发送消息")
  //向myactor发送消息
  myactor!"test"
  myactor! 123

  //关闭ActorSystem，停止程序的运行
  system.shutdown()
}

运行结果如下：

Exception in thread "main" akka.actor.ActorInitializationException: You cannot create an instance of [chapter02.Example_03$MyActor] explicitly using the constructor (new). You have to use one of the 'actorOf' factory methods to create a new actor. See the documentation.
    at akka.actor.ActorInitializationException$.apply(Actor.scala:167)
    at akka.actor.Actor$class.$init$(Actor.scala:423)
    at chapter02.Example_03$MyActor.<init>(MyActor.scala:73)
    at chapter02.Example_03$delayedInit$body.apply(MyActor.scala:84)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
	at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.collection.immutable.List.foreach(List.scala:318)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
    at scala.App$class.main(App.scala:71)
    at chapter02.Example_03$.main(MyActor.scala:68)
    at chapter02.Example_03.main(MyActor.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)

从“You cannot create an instance of [chapter02.Example_03$MyActor] explicitly using the constructor (new). You have to use one of the ‘actorOf’ factory methods to create a new actor.”可以看到，不能通过显式地调用构造函数创建Actor，只能使用actorOf工厂方法创建Actor。

下面介绍2种在实际中经常使用的Actor创建方法
（1）调用system.actorOf创建Actor

val system = ActorSystem("mySystem")
val myActor = system.actorOf(Props[MyActor], "myactor2")

完整代码在Example_01、Example_02中已经演示过了，这里需要说明的是通过system.actorOf工厂方法创建的Actor为顶级Actor

在Akka框架中，每个Akka应用程序都会有一个守卫Actor，名称为user，所有通过system.actorOf工厂方法创建的Actor都为user的子Actor，也是整个Akka程序的顶级Actor。

（2）调用context.actorOf创建Actor
完整代码如下：

/*
 *创建Actor,调用context.actorOf方法
 */
object Example_04 extends App{
  import akka.actor.Actor
  import akka.actor.ActorSystem
  import akka.actor.Props


  class FirstActor extends Actor with ActorLogging{
    //通过context.actorOf方法创建Actor
    val child = context.actorOf(Props[MyActor], name = "myChild")
    def receive = {
      case x => child ! x;log.info("received "+x)
    }

  }


  class MyActor extends Actor with ActorLogging{
    def receive = {
      case "test" => log.info("received test")
      case _      => log.info("received unknown message")
    }
  }

  val system = ActorSystem("MyActorSystem")
  val systemLog=system.log

  //创建FirstActor对象
  val myactor = system.actorOf(Props[FirstActor], name = "firstActor")

  systemLog.info("准备向myactor发送消息")
  //向myactor发送消息
  myactor!"test"
  myactor! 123
  Thread.sleep(5000)
  //关闭ActorSystem，停止程序的运行
  system.shutdown()
}

代码运行结果

[INFO] [04/02/2016 15:05:34.770] [main] [ActorSystem(MyActorSystem)] 准备向myactor发送消息
[INFO] [04/02/2016 15:05:34.771] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/firstActor/myChild] received test
[INFO] [04/02/2016 15:05:34.771] [MyActorSystem-akka.actor.default-dispatcher-2] [akka://MyActorSystem/user/firstActor] received test
[INFO] [04/02/2016 15:05:34.771] [MyActorSystem-akka.actor.default-dispatcher-2] [akka://MyActorSystem/user/firstActor] received 123
[INFO] [04/02/2016 15:05:34.771] [MyActorSystem-akka.actor.default-dispatcher-3] [akka://MyActorSystem/user/firstActor/myChild] received unknown message

通过代码的运行结果可以看到，FirstActor的Actor路径信息为akka://MyActorSystem/user/firstActor，而通过

class FirstActor extends Actor with ActorLogging{
    //通过context.actorOf方法创建Actor
    val child = context.actorOf(Props[MyActor], name = "myChild")
    def receive = {
      case x => child ! x;log.info("received "+x)
    }

  }

代码使用context.actorOf创建的MyActor，其Actor路径信息为[akka://MyActorSystem/user/firstActor/myChild]，这意味着mychild为firstActor的子Actor，层次结构如下图所示

也就是说context.actorOf和system.actorOf的差别是system.actorOf创建的actor为顶级Actor，而context.actorOf方法创建的actor为调用该方法的Actor的子Actor