Scala入门到精通——第二十六节 Scala并发编程基础

本节主要内容

Scala并发编程简介
Scala Actor并发编程模型
react模型
Actor的几种状态
Actor深入使用解析

1. Scala并发编程简介

2003 年，Herb Sutter 在他的文章 “The Free Lunch Is Over” 中揭露了行业中最不可告人的一个小秘密，他明确论证了处理器在速度上的发展已经走到了尽头，并且将由全新的单芯片上的并行 “内核”（虚拟 CPU）所取代。这一发现对编程社区造成了不小的冲击，因为正确创建线程安全的代码，在理论而非实践中，始终会提高高性能开发人员的身价，而让各公司难以聘用他们。看上去，仅有少数人充分理解了 Java 的线程模型、并发 API 以及 “同步” 的含义，以便能够编写同时提供安全性和吞吐量的代码 —— 并且大多数人已经明白了它的困难所在（来源：http://www.ibm.com/developerworks/cn/java/j-scala02049.html）。

在Java中，要编写一个线程安全的程序并不是一件易事，例如：

class Account {  private int balance;  synchronized public int getBalance() {  return balance;  }  synchronized public void incrementBalance() {  balance++;  }
}

上面这段java代码虽然方法前面加了synchronized ，但它仍然不是线程安全的，例如，在执行下面两个语句

account.incrementBalance();
account.getBalance();

时，有可能account.incrementBalance()执行完成后，其它线程可能会获取对象的锁，修改account的balance，从而造成得不到预期结果的问题。解决问题的方法是将两个功能结合起来形成一个方法：

synchronized public int incrementAndGetBalance() {  balance++;  return balance;
}

但这可能并不是我们想要的，每次获取balance都要将balance增加，这显然与实际不符。除此之外，java中的并发编程可能还会经常遇到死锁问题，而这个问题往往难调试，问题可能会随机性的出现。总体上来看，java的并发编程模型相对较复杂，难以驾驭。

Scala很好地解决了java并发编程的问题，要在scala中进行并发编程，有以下几种途径可以实现：
1 actor消息模型、akka actor并发模型。

2 Thread、Runnable

3 java.util.concurennt

4 第三方开源并发框架如Netty，Mina

在上述四种途径当中，利用 actor消息模型、akka actor并发模型是scala并发编程的首先，本节主要介绍actor消息模型，akka actor并发模型我们将放在后面的章节中介绍。
在scala中，通过不变对象来实现线程安全，涉及到修改对象状态时，则创建一个新的对象来实现，如：

//成员balance状态一旦被赋值，便不能更改
//因而它也是线程安全的
class Person(val age: Integer) {  def getAge() = age
}  object Person{  //创建新的对象来实现对象状态修改def increment(person: Person): Person{  new Person(Person.getAge() + 1)  }
}

通过不变对象实现并发编程，可以简化编程模型，使并发程序更容易现实和控制。

2.Scala Actor并发编程模型

java中的并发主要是通过线程来实现，各线程采用共享资源的机制来实现程序的并发，这里面临竞争资源的问题，虽然采用锁机制可以避免竞争资源的问题，但会存在死锁问题，要开发一套健壮的并发应用程序具有一定的难度。而scala的并发模型相比于java它更简单，它采用消息传递而非资源共享来实现程序的并发，消息传递正是通过Actor来实现的。下面的代码给出了Actor使用示例

//混入Actor特质，然后实现act方法
//如同java中的Runnable接口一样
//各线程的run方法是并发执行的
//Actor中的act方法也是并发执行的
class ActorDemo extends Actor{//实现 act()方法def act(){while(true){//receive从邮箱中获取一条消息//然后传递给它的参数//该参数是一个偏函数receive{case "actorDemo" => println("receive....ActorDemo")}      }}
}
object ActorDemo extends App{val actor=new ActorDemo//启动创建的actor actor.start()//主线程发送消息给actoractor!"actorDemo"
}

下面给的是recieve方法的部分源代码

def receive[R](f: PartialFunction[Any, R]): R = {assert(Actor.self(scheduler) == this, "receive from channel belonging to other actor")synchronized {if (shouldExit) exit() // linksdrainSendBuffer(mailbox)}var done = falsewhile (!done) {val qel = mailbox.extractFirst((m: Any, replyTo: OutputChannel[Any]) => {senders = replyTo :: sendersval matches = f.isDefinedAt(m)senders = senders.tailmatches})
................

从上述代码中不能看出，receive方法接受的参数是一个偏函数，并且是通过mailbox来实现消息的发送与接收。

在前述的class ActorDemo中，receive方法的参数为

{case "actorDemo" => println("receive....ActorDemo")
}

该代码块在执行时被转换为一个PartialFunction[Any, R]的偏函数，其中R是偏函数的返回类型，对应case 语句=> 右边的部分，在本例子中R是Unit类型，而Any对应的则对应case语句的模式部分。

前面给的是通过extends Actor的方式来创建一个Actor类，其实scala.actors.Actor中提供了一个actor工具方法，可以非常方便地直接创建Actor对象如：

import scala.actors.Actor._object ActorFromMethod extends App{//通过工具方法actor直接创建Actor对象val methodActor = actor {for (i <- 1 to 5)println("That is the question.")Thread.sleep(1000)}
}

上述代码创建的actor对象无需调用start方法，对象创建完成后会立即执行。

scala中本地线程也可用作Actor，下面的代码演示了如何在REPL命令行中将本地线程当作Actor；

scala> import scala.actors.Actor._
import scala.actors.Actor._//self引用本地线程，并发送消息
scala> self ! "hello"
//接收消息
scala> self.receive { case x:String => x }
res1: String = hello

上述代码中，如果发送的消息不是String类型的，线程将被阻塞，为避免这个问题，可以采用receiveWithin方法，

scala> self ! 123scala> self.receiveWithin(1000) { case x => x }
res6: Any = 123scala> self.receiveWithin(1000) { case x => x }
res7: Any = TIMEOUT

3. react模型

scala中的Actor也是构建在java线程基础之上的，前面在使用Actor时都是通过创建Actor对象，然后再调用act方法来启动actor。我们知道，java中线程的创建、销毁及线程间的切换是比较耗时的，因此实际中尽量避免频繁的线程创建、销毁和销毁。Scala中提供React方法，在方法执行结束后，线程仍然被保留。下面的代码演示了react方法的使用：

package cn.scala.xtwy.concurrency
import scala.actors._object NameResolver extends Actor {import java.net.{ InetAddress, UnknownHostException }def act() {react {//匹配主线程发来的("www.scala-lang.org", NameResolver)case (name: String, actor: Actor) =>//向actor发送解析后的IP地址信息//由于本例中传进来的actor就是NameResolver自身//也即自己给自己发送消息，并存入将消息存入邮箱actor ! getIp(name)//再次调用act方法，试图从邮箱中提取信息//如果邮箱中信息为空，则进入等待模式act()case "EXIT" =>println("Name resolver exiting.")// quit//匹配邮箱中的单个信息，本例中会匹配邮箱中的IP地址信息case msg =>println("Unhandled message: " + msg)act()}}def getIp(name: String): Option[InetAddress] = {try {Some(InetAddress.getByName(name))} catch {case _: UnknownHostException => None}}
}
object Main extends App{NameResolver.start()//主线程向NameResolver发送消息("www.scala-lang.org", NameResolver)NameResolver ! ("www.scala-lang.org", NameResolver)NameResolver ! ("wwwwww.scala-lang.org", NameResolver)}

从上述代码中可以看到，通过在react方法执行结束时加入act方法，方法执行完成后没有被销毁，而是继续试图从邮箱中获取信息，获取不到则等待。

4. Actor的几种状态

Actor有下列几种状态：

初始状态（New），Actor对象被创建，但还没有启动即没有执行start方法时的状态
执行状态（Runnable），正在执行时的状态
挂起状态（Suspended），在react方法中等待时的状态
时间点挂起状态（TimedSuspended），挂起状态的一种特殊形式，reactWithin方法中的等待时的状态
阻塞状态（Blocked），在receive方法中阻塞等待时的状态
时间点阻塞状态（TimedBlocked），在receiveWithin方法中阻塞等待时的状态
结束状态（Terminated），执行完成后被销毁

5. Actor深入使用解析

1 receive方法单次执行：


object Actor2{case class Speak(line : String)case class Gesture(bodyPart : String, action : String)case class NegotiateNewContract()def main(args : Array[String]) ={val badActor =actor{//这里receive方法只会匹配一次便结束receive{case NegotiateNewContract =>System.out.println("I won't do it for less than $1 million!")case Speak(line) =>System.out.println(line)case Gesture(bodyPart, action) =>System.out.println("(" + action + "s " + bodyPart + ")")case _ =>System.out.println("Huh? I'll be in my trailer.")}}//receive方法只处理下面这条语句发送的消息badActor ! NegotiateNewContract//下面其余的消息不会被处理badActor ! Speak("Do ya feel lucky, punk?")badActor ! Gesture("face", "grimaces")badActor ! Speak("Well, do ya?")}}

上述代码只会输出：
I won’t do it for less than $1 million!
即后面发送的消息如：
badActor ! Speak(“Do ya feel lucky, punk?”)
badActor ! Gesture(“face”, “grimaces”)
badActor ! Speak(“Well, do ya?”)
不会被处理。这是因为receive方法的单次执行问题。

2 能够处理多个消息的receive方法：

object Actor2{case class Speak(line : String);case class Gesture(bodyPart : String, action : String);case class NegotiateNewContract()//处理结束消息case class ThatsAWrap()def main(args : Array[String]) ={val badActor =actor{var done = false//while循环while (! done){receive{case NegotiateNewContract =>System.out.println("I won't do it for less than $1 million!")case Speak(line) =>System.out.println(line)case Gesture(bodyPart, action) =>System.out.println("(" + action + "s " + bodyPart + ")")case ThatsAWrap =>System.out.println("Great cast party, everybody! See ya!")done = truecase _ =>System.out.println("Huh? I'll be in my trailer.")}}}//下面所有的消息都能被处理badActor ! NegotiateNewContractbadActor ! Speak("Do ya feel lucky, punk?")badActor ! Gesture("face", "grimaces")badActor ! Speak("Well, do ya?")//消息发送后，receive方法执行完毕badActor ! ThatsAWrap}}

3 Actor后面实现原理仍然是线程的证据

object Actor3{case class Speak(line : String);case class Gesture(bodyPart : String, action : String);case class NegotiateNewContract;case class ThatsAWrap;def main(args : Array[String]) ={def ct ="Thread " + Thread.currentThread().getName() + ": "val badActor =actor{var done = falsewhile (! done){receive{case NegotiateNewContract =>System.out.println(ct + "I won't do it for less than $1 million!")case Speak(line) =>System.out.println(ct + line)case Gesture(bodyPart, action) =>System.out.println(ct + "(" + action + "s " + bodyPart + ")")case ThatsAWrap =>System.out.println(ct + "Great cast party, everybody! See ya!")done = truecase _ =>System.out.println(ct + "Huh? I'll be in my trailer.")}}}System.out.println(ct + "Negotiating...")badActor ! NegotiateNewContractSystem.out.println(ct + "Speaking...")badActor ! Speak("Do ya feel lucky, punk?")System.out.println(ct + "Gesturing...")badActor ! Gesture("face", "grimaces")System.out.println(ct + "Speaking again...")badActor ! Speak("Well, do ya?")System.out.println(ct + "Wrapping up")badActor ! ThatsAWrap}}

执行结果如下：

Thread main: Negotiating...
Thread main: Speaking...
Thread main: Gesturing...
Thread main: Speaking again...
Thread main: Wrapping up
Thread ForkJoinPool-1-worker-13: I won't do it for less than $1 million!
Thread ForkJoinPool-1-worker-13: Do ya feel lucky, punk?
Thread ForkJoinPool-1-worker-13: (grimacess face)
Thread ForkJoinPool-1-worker-13: Well, do ya?
Thread ForkJoinPool-1-worker-13: Great cast party, everybody! See ya!

从上述执行结果可以看到,Actor最终的实现仍然是线程，只不过它提供的编程模型与java中的编程模型不一样而已。

4 利用!?发送同步消息，等待返回值


import scala.actors._,Actor._object ProdConSample2{case class Message(msg : String)def main(args : Array[String]) : Unit ={val consumer =actor{var done = falsewhile (! done){receive{case msg =>System.out.println("Received message! -> " + msg)done = (msg == "DONE")reply("Already RECEIVED....."+msg)}}}System.out.println("Sending....")//获取响应值val r= consumer !? "Mares eat oats"println("replyed message"+r)System.out.println("Sending....")consumer !? "Does eat oats"System.out.println("Sending....")consumer !? "Little lambs eat ivy"System.out.println("Sending....")consumer !? "Kids eat ivy too"System.out.println("Sending....")consumer !? "DONE"      }}

代码执行结果：

Sending....
Received message! -> Mares eat oats
replyed messageAlready RECEIVED.....Mares eat oats
Sending....
Received message! -> Does eat oats
Sending....
Received message! -> Little lambs eat ivy
Sending....
Received message! -> Kids eat ivy too
Sending....
Received message! -> DONE

通过上述代码执行结果可以看到，！？因为是同步消息，发送完返回结果后才会接着发送下一条消息。

5 Spawn方法发送消息

object ProdConSampleUsingSpawn{import concurrent.ops._def main(args : Array[String]) : Unit ={// Spawn Consumerval consumer =actor{var done = falsewhile (! done){receive{case msg =>System.out.println("MESSAGE RECEIVED: " + msg)done = (msg == "DONE")reply("RECEIVED")}}}// Spawn Producerspawn  //spawn是一个定义在current.ops中的方法{val importantInfo : Array[String] = Array("Mares eat oats","Does eat oats","Little lambs eat ivy","A kid will eat ivy too","DONE");importantInfo.foreach((msg) => consumer !? msg)}}}

6 !! 发送异步消息，返回值是 Future[Any]

object ProdConSample3{case class Message(msg : String)def main(args : Array[String]) : Unit ={val consumer =actor{var done = falsewhile (! done){receive{case msg =>System.out.println("Received message! -> " + msg)done = (msg == "DONE")reply("Already RECEIVED....."+msg)}}}System.out.println("Sending....")//发送异步消息，返回val replyFuture= consumer !! "Mares eat oats"val r=replyFuture()println("replyed message*****"+r)System.out.println("Sending....")consumer !! "Does eat oats"System.out.println("Sending....")consumer !! "Little lambs eat ivy"System.out.println("Sending....")consumer !! "Kids eat ivy too"System.out.println("Sending....")consumer !! "DONE"      }}

执行结果：

Sending....
Received message! -> Mares eat oats
replyed message*****Already RECEIVED.....Mares eat oats
Sending....
Sending....
Sending....
Received message! -> Does eat oats
Sending....
Received message! -> Little lambs eat ivy
Received message! -> Kids eat ivy too
Received message! -> DONE

通过上述代码的执行结果可以看到，!!的消息发送是异步的，消息发送后无需等待结果返回便执行下一条语句，但如果要获取异步消息的返回值，如：

 val replyFuture= consumer !! "Mares eat oats"val r=replyFuture()

则执行到这两条语句的时候，程序先被阻塞，等获得结果之后再发送其它的异步消息。

7 loop方法实现react

object LoopReact extends App{val a1 = Actor.actor {//注意这里loop是一个方法，不是关键字//实现类型while循环的作用loop {react {//为整型时结束操作case x: Int=>println("a1 stop: " + x); exit()case msg: String => println("a1: " + msg)}}}a1!("我是摇摆少年梦")a1.!(23)}