• Scalaz(45)- concurrency :Task-函数式多线程编程核心配件


        我们在上一节讨论了scalaz Future,我们说它是一个不完善的类型,最起码没有完整的异常处理机制,只能用在构建类库之类的内部环境。如果scalaz在Future类定义中增加异常处理工具的话,用户就会经常遇到Future[Throwable/A]这样的类型,那么在进行Monadic编程时就必须使用Monad Transformer来匹配类型,程序就会变得不必要的复杂。scalaz的解决方案就是把Future[Throwable/A]包嵌在Task类里,然后把所有Future都统一升格成Task。Task是个Monad, 这样,我们就可以统一方便地用Task来进行多线程函数式编程了。我们先看看Task的定义:scalaz.concurrent/Task.scala

    class Task[+A](val get: Future[Throwable / A]) {
    
      def flatMap[B](f: A => Task[B]): Task[B] =
        new Task(get flatMap {
          case -/(e) => Future.now(-/(e))
          case /-(a) => Task.Try(f(a)) match {
            case e @ -/(_) => Future.now(e)
            case /-(task) => task.get
          }
        })
    
      def map[B](f: A => B): Task[B] =
        new Task(get map { _ flatMap {a => Task.Try(f(a))} })
    ...

    Task实现了flatMap,所以是个Monad,我们可以在for-comprehension中使用Task。

    Task的构建方式与Future一样:

    1 val tnow = Task.now { println("run now ..."); 3+4 }
    2                                      //> run now ...
    3                                      //| tnow  : scalaz.concurrent.Task[Int] = scalaz.concurrent.Task@1a968a59
    4 val tdelay = Task.delay { println("run delay ...");  3+4 }
    5                                     //> tdelay  : scalaz.concurrent.Task[Int] = scalaz.concurrent.Task@13deb50e
    6 val tapply = Task { println("run apply ..."); 3+4 }
    7                                    //> tapply  : scalaz.concurrent.Task[Int] = scalaz.concurrent.Task@59494225

    同样,now函数是即时运算的。它就是一个lifter,能把一个普通运算直接升格为Task。

    针对Task有几种运算方法:

     1 tnow.unsafePerformSync                            //> res0: Int = 7
     2 tdelay.unsafePerformSync                          //> run delay ...
     3                                                   //| res1: Int = 7
     4 tnow.unsafePerformAsync {
     5   case /-(a) => println(s"the result is: $a")
     6   case -/(e) => println(e.getMessage)
     7 }                                                 //> the result is: 7
     8 tdelay.unsafePerformAsync {
     9   case /-(a) => println(s"the result is: $a")
    10   case -/(e) => println(e.getMessage)
    11 }                                                 //> run delay ...
    12                                                   //| the result is: 7
    13 tapply.unsafePerformAsync {
    14   case /-(a) => println(s"the result is: $a")
    15   case -/(e) => println(e.getMessage)
    16 }
    17 Thread.sleep(1000)                                //> run apply ...
    18                                                   //| the result is: 7

    从上面的例子我们可以得出:tnow已经完成了运算,因为运算结果没有"run now ..."提示了。tdelay和tapply都是存在trampoline结构里的。但tapply存在更深一层的结构里,所以我们必须拖时间来等待tapply的运算结果。tdelay存放在Future.Suspend结构里,而tapply是存放在Future.Async结构里的,所以tdelay是一种延迟运算,而tapply就是异步运算了:

     1 def delay[A](a: => A): Task[A] = suspend(now(a))
     2 def suspend[A](a: => Task[A]): Task[A] = new Task(Future.suspend(
     3     Try(a.get) match {
     4       case -/(e) => Future.now(-/(e))
     5       case /-(f) => f
     6   }))
     7 //Future.suspend:
     8  def suspend[A](f: => Future[A]): Future[A] = Suspend(() => f)
     9 
    10  def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Task[A] =
    11     new Task(Future(Try(a))(pool))
    12 //Future.apply
    13 def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Future[A] = Async { cb =>
    14     pool.submit { new Callable[Unit] { def call = cb(a).run }}
    15   }

    好了,我们再看看Task是怎样处理异常情况的:

     1 def eval(value: => Int) = Task { Thread.sleep(1000); value }
     2                                                   //> eval: (value: => Int)scalaz.concurrent.Task[Int]
     3 eval( 3 * 7 ).onFinish {
     4   case None => Task { println("finished calculation successfully.") }
     5   case Some(e) => Task { println(s"caught error [${e.getMessage}]") }
     6 }.unsafePerformSyncAttempt match {
     7   case -/(e) => println(s"calculation error [${e.getMessage}]")
     8   case /-(a) => println(s"the result is: $a")
     9 }                                                 //> finished calculation successfully.
    10                                                   //| the result is: 21
    11 // 异常处理
    12 eval( 3 * 7 / 0 ).onFinish {
    13   case None => Task { println("finished calculation successfully.") }
    14   case Some(e) => Task { println(s"caught error [${e.getMessage}]") }
    15 }.unsafePerformAsync {
    16   case -/(e) => println(s"calculation error [${e.getMessage}]")
    17   case /-(a) => println(s"the result is: $a")
    18 }
    19 Thread.sleep(2000)                                //> caught error [/ by zero]
    20                                                   //| calculation error [/ by zero]

    精准异常处理例子:

    1 import java.util.concurrent._
    2 val timedTask = Task {Thread.sleep(2000); 3+4}   
    3                      //> timedTask  : scalaz.concurrent.Task[Int] = scalaz.concurrent.Task@3d921e20
    4 timedTask.timed(1 second).handleWith {
    5   case e: TimeoutException => Task { println(s"calculation exceeding time limit: ${e.getMessage}") }
    6 }.unsafePerformSync           //> calculation exceeding time limit: Timed out after 1000 milliseconds
    7                               //| res2: AnyVal{def getClass(): Class[_ >: Int with Unit <: AnyVal]} = ()

    再看一些多线程编程例子:

     1 val tasks = (1 |-> 5).map(n => Task{ Thread.sleep(100); n })
     2//> tasks  : List[scalaz.concurrent.Task[Int]] = List(scalaz.concurrent.Task@61
     3//| 8b19ad, scalaz.concurrent.Task@2d3379b4, scalaz.concurrent.Task@30c15d8b, s
     4//| calaz.concurrent.Task@5e0e82ae, scalaz.concurrent.Task@6771beb3)
     5 //并行运算list of tasks
     6 Task.gatherUnordered(tasks).unsafePerformSync     //> res3: List[Int] = List(1, 2, 3, 4, 5)
     7 val sb = new StringBuffer                         //> sb  : StringBuffer = 
     8 val t1 = Task.fork { Thread.sleep(100); sb.append("a"); Task.now("a")}
     9//> t1  : scalaz.concurrent.Task[String] = scalaz.concurrent.Task@6200f9cb
    10 val t2 = Task.fork { Thread.sleep(800); sb.append("b"); Task.now("b")}
    11//> t2  : scalaz.concurrent.Task[String] = scalaz.concurrent.Task@2002fc1d
    12 val t3 = Task.fork { Thread.sleep(200); sb.append("c"); Task.now("c")}
    13//> t3  : scalaz.concurrent.Task[String] = scalaz.concurrent.Task@69453e37
    14 val t4 = Task.fork { Thread.sleep(100); sb.append("d"); Task.now("d")}
    15//> t4  : scalaz.concurrent.Task[String] = scalaz.concurrent.Task@6f4a47c7
    16 val t5 = Task.fork { Thread.sleep(400); sb.append("e"); Task.now("e")}
    17//> t5  : scalaz.concurrent.Task[String] = scalaz.concurrent.Task@ae13544
    18 val t6 = Task.fork { Thread.sleep(100); sb.append("f"); Task.now("f")}
    19//> t6  : scalaz.concurrent.Task[String] = scalaz.concurrent.Task@3d34d211
    20 val r = Nondeterminism[Task].nmap6(t1,t2,t3,t4,t5,t6)(List(_,_,_,_,_,_))
    21//> r  : scalaz.concurrent.Task[List[String]] = scalaz.concurrent.Task@394df057 
    22 r.unsafePerformSync              //> res4: List[String] = List(a, b, c, d, e, f)

    看个耗时算法的并行运算吧:

     1 def seqFib(n: Int): Task[Int] =  n match {
     2   case 0 | 1 => Task now  1
     3   case n => {
     4     for {
     5       x <- seqFib(n-1)
     6       y <- seqFib(n-2)
     7     } yield x + y
     8   }
     9  }                                                //> seqFib: (n: Int)scalaz.concurrent.Task[Int]
    10  //并行算法
    11  def parFib(n: Int): Task[Int] = n match {
    12     case 0 | 1 => Task now 1
    13     case n => {
    14        val ND = Nondeterminism[Task]
    15        for {
    16          pair <- ND.both(parFib(n-1), parFib(n-2))
    17          (x,y) = pair
    18        } yield x + y
    19     }
    20  }                                                //> parFib: (n: Int)scalaz.concurrent.Task[Int]
    21  def msFib(n: Int, fib: Int => Task[Int]) = for {
    22    b <- Task now { System.currentTimeMillis() }
    23    a <- fib(n)
    24    e <- Task now { System.currentTimeMillis() }
    25  } yield (a, (e-b))                               //> msFib: (n: Int, fib: Int => scalaz.concurrent.Task[Int])scalaz.concurrent.T
    26                                                   //| ask[(Int, Long)]
    27  
    28  msFib(20, parFib).unsafePerformSync              //> res3: (Int, Long) = (10946,373)
    29  msFib(20, seqFib).unsafePerformSync              //> res4: (Int, Long) = (10946,17)

    哎呀!奇怪了,为什么并行算法要慢很多呢?这个问题暂且放一放,以后再研究。当然,如果有读者能给出个解释就太感激了。
    Task的线程池是如何分配的呢?看看Task.apply和Task.fork:

     /** Create a `Task` that will evaluate `a` using the given `ExecutorService`. */
      def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Task[A] =
        new Task(Future(Try(a))(pool))
    def fork[A](a: => Task[A])(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Task[A] =
        apply(a).join
    //Future.apply
    /** Create a `Future` that will evaluate `a` using the given `ExecutorService`. */
      def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Future[A] = Async { cb =>
        pool.submit { new Callable[Unit] { def call = cb(a).run }}
     

    这两个函数都包括了一个隐式参数implicit pool: ExecutorService。默认值是Strategy.DefultExecutorService。我们可以这样指定线程池:

    1  Task {longProcess}(myExecutorService)
    2  Task.fork { Task {longProcess} }(myExecutorService)

    下面是一个动态指定线程池的例子:

     1 import java.util.concurrent.{ExecutorService,Executors}
     2 type Delegated[A] = Kleisli[Task,ExecutorService,A]
     3 def delegate: Delegated[ExecutorService] = Kleisli(e => Task.now(e))
     4                             //> delegate: => demo.ws.task.Delegated[java.util.concurrent.ExecutorService]
     5 implicit def delegateTaskToPool[A](ta: Task[A]): Delegated[A] = Kleisli(x => ta)
     6//> delegateTaskToPool: [A](ta: scalaz.concurrent.Task[A])demo.ws.task.Delegated[A]
     7 val tPrg = for {
     8   p <- delegate
     9   b <- Task("x")(p)
    10   c <- Task("y")(p)
    11 } yield c                   //> tPrg  : scalaz.Kleisli[scalaz.concurrent.Task,java.util.concurrent.Executor
    12                             //| Service,String] = Kleisli(<function1>)
    13 tPrg.run(Executors.newFixedThreadPool(3)).unsafePerformSync
    14                             //> res3: String = y

    当然,Task和scala Future之间是可以相互转换的:

     1 import scala.concurrent.{Future => sFuture}
     2 import scala.util.{Success,Failure}
     3 import scala.concurrent.ExecutionContext
     4 def futureToTask[A](fut: sFuture[A])(implicit ec: ExecutionContext): Task[A] =
     5   Task.async {
     6     cb =>
     7       fut.onComplete {
     8         case Success(a) => cb(a.right)
     9         case Failure(e) => cb(e.left)
    10       }
    11   }
    12 def taskToFuture[A](ta: Task[A]): sFuture[A] = {
    13   val prom = scala.concurrent.Promise[A]
    14   ta.unsafePerformAsync {
    15     case -/(e) => prom.failure(e)
    16     case /-(a) => prom.success(a)
    17   }
    18   prom.future
    19 }

    与Future不同的是:Task增加了异常处理机制。

     

     

     

     

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  • 原文地址:https://www.cnblogs.com/tiger-xc/p/5590188.html
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