Scala中的后台任务

Question

我有一个缓存，我想定期检查和修剪。在 Java 中，我会执行以下操作：

new Thread(new Runnable() {
  void run() {
    while (true) { 
      Thread.sleep(1000);
      // clear the cache's old entries
    }
  }
}).start();

当然，我在将线程安全类型用作缓存时会遇到一些问题，但撇开这些不谈，我的问题很简单。 Scala 运行重复后台任务的方式是什么——您不想在应用程序的主线程中运行？

我已经使用了一些演员，我想我在这种情况下的问题是我没有任何东西可以生成一条消息，表明是时候清除缓存了。或者更确切地说，我能想象生成这些消息的唯一方法是创建一个线程来执行它......

编辑：我需要人们对答案进行投票——他们对我来说都很好

Answer 1

有很多方法可以做到这一点，但我会做一些简单的事情，如下所示。

import scala.concurrent.ops._

spawn {
  while (true) { 
    Thread.sleep(1000);
    // clear the cache's old entries
  }
}

希望这会有所帮助。

Answer 2

您可以使用Akka Scheduler，它允许您向执行该工作的（akka）演员发送重复消息。从文档中，只需使用：

import akka.actor.Scheduler

//Sends messageToBeSent to receiverActor after initialDelayBeforeSending and then after each delayBetweenMessages
Scheduler.schedule(receiverActor, messageToBeSent, initialDelayBeforeSending, delayBetweenMessages, timeUnit)

Answer 3

Futures 是一种无需显式启动新线程的简单方法

import scala.actors.Futures._

// main thread code here

future {
   // second thread code here
}

// main thread code here

Answer 4

spawn 很好，但请注意您的示例代码也适用于 Scala：

new Thread(new Runnable() {
  override def run() {
    while (true) { 
      Thread.sleep(1000);
      // clear the cache's old entries
    }
  }
}).start();

只需使用隐式转换来清理它：

implicit def funcToRunnable(f: => ()) = new Runnable() { override def run() { f() } }

new Thread{
  while(true) {
    Thread.sleep(1000);
    // blah
  }
}.start()

Answer 5

使用 Actors 而不绑定线程：

import actors.{TIMEOUT, Actor}
import actors.Actor._

private case class Ping( client: Actor, update: Int )
private case class Pulse()
case class Subscribe( actor: Actor )
case class Unsubscribe( actor: Actor )

class PulseActor extends Actor {
  def act = eventloop {
        case ping: Ping => { sleep(ping.update); ping.client ! Pulse }
  }
  def sleep(millis: Long) =
    receiveWithin(millis) {
      case TIMEOUT =>
  }
}

class ServiceActor extends Actor {

  var observers: Set[Actor] = Set.empty
  val pulseactor = new PulseActor
  val update = 2000

  def act = {
    pulseactor.start
    pulseactor ! new Ping( this, update )
    loop {
      react {
        case sub: Subscribe => observers += sub.actor
        case unsub: Unsubscribe => observers -= unsub.actor
        case Pulse => pulse
      }
    }
  }


  def pulse {  
    //cpuload = CPUprofile.getCPUload.getOrElse{ List(0.0) }  //real work
    observers foreach { observer => observer ! "CPUloadReport( cpuload )" }
    pulseactor ! Ping(this, update)
  }
}

object Exercise extends App {
  val deamon = new ServiceActor
  deamon.start
}

Answer 6

从 Scala 2.11.x 开始，恕我直言，Akka 演员似乎是最好的方法。首先创建一个任务调度库：

import akka.actor.ActorSystem
import scala.language.postfixOps
import scala.concurrent.duration._
val actorSystem = ActorSystem()
val scheduler = actorSystem.scheduler
implicit val executor = actorSystem.dispatcher

// do once after period millis
def doOnce(fn: => Unit, period:Long) = scheduler.scheduleOnce(period milliseconds)(fn) 
// do regularly every period millis starting now
def doRegularly(fn: => Unit, period:Long) = scheduler.schedule(0 seconds, period milliseconds)(fn)
// do regularly every period millis if whileFn is true, starting now 
def doWhile(fn: => Unit, whileFn: => Boolean, period:Long) {
 if (whileFn) {
    fn
    doOnce(doWhile(fn, whileFn, period), period)
 }
}

然后将其用作：

doRegularly({
  println("hello world!")
}, 1000) // repeat every 1000 millisecs

doWhile({
  println("Sleeping!")
}, whileDaylight, 1000) // repeat every 1000 millisecs whileDaylight is true