Scala教程之:Future和Promise

文章目录

定义返回Future的方法
阻塞方式获取Future的值
非阻塞方式获取Future的值
Future链
flatmap VS map
Future.sequence() VS Future.traverse()
Future.foldLeft VS Future reduceLeft
Future firstCompletedOf
Future zip VS zipWith
Future andThen
自定义threadpool
recover() recoverWith() and fallbackTo（）
promise

在scala中可以方便的实现异步操作，这里是通过Future来实现的，和java中的Future很相似，但是功能更加强大。

定义返回Future的方法

下面我们看下如何定义一个返回Future的方法：

println("Step 1: Define a method which returns a Future")
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
def donutStock(donut: String): Future[Int] = Future {// assume some long running database operationprintln("checking donut stock")10
}

注意这里需要引入scala.concurrent.ExecutionContext.Implicits.global，它会提供一个默认的线程池来异步执行Future。

阻塞方式获取Future的值

  println("\nStep 2: Call method which returns a Future")import scala.concurrent.Awaitimport scala.concurrent.duration._val vanillaDonutStock = Await.result(donutStock("vanilla donut"), 5 seconds)println(s"Stock of vanilla donut = $vanillaDonutStock")

donutStock() 是异步执行的，我们可以使用Await.result() 来阻塞主线程来等待donutStock()的执行结果。

下面是其输出：


Step 2: Call method which returns a Future
checking donut stock
Stock of vanilla donut = 10

非阻塞方式获取Future的值

我们可以使用Future.onComplete() 回调来实现非阻塞的通知：

println("\nStep 2: Non blocking future result")
import scala.util.{Failure, Success}
donutStock("vanilla donut").onComplete {case Success(stock) => println(s"Stock for vanilla donut = $stock")case Failure(e) => println(s"Failed to find vanilla donut stock, exception = $e")
}
Thread.sleep(3000)

Future.onComplete() 有两种可能情况，Success 或者 Failure，需要引入： import scala.util.{Failure, Success}。

Future链

有时候我们需要在获得一个Future之后再继续对其进行操作，有点类似于java中的管道，下面看一个例子：

println("\nStep 2: Define another method which returns a Future")
def buyDonuts(quantity: Int): Future[Boolean] = Future {println(s"buying $quantity donuts")true
}

上面我们又定义了一个方法，用来接收donutStock（）的返回值，然后再返回一个Future[Boolean] 。

我们看下使用flatmap该怎么链接他们：

println("\nStep 3: Chaining Futures using flatMap")
val buyingDonuts: Future[Boolean] = donutStock("plain donut").flatMap(qty => buyDonuts(qty))
import scala.concurrent.Await
import scala.concurrent.duration._
val isSuccess = Await.result(buyingDonuts, 5 seconds)
println(s"Buying vanilla donut was successful = $isSuccess")

同样的，我们还可以使用for语句来进行链接：


println("\nStep 3: Chaining Futures using for comprehension")
for {stock     <- donutStock("vanilla donut")isSuccess <- buyDonuts(stock)
} yield println(s"Buying vanilla donut was successful = $isSuccess")Thread.sleep(3000)

flatmap VS map

map就是对集合中的元素进行重映射，而flatmap则会将返回的值拆散然后重新组合。下面举个直观的例子：

val buyingDonuts: Future[Boolean] = donutStock("plain donut").flatMap(qty => buyDonuts(qty))

flatMap返回的值是Future[Boolean]。

val buyingDonuts: Future[Future[Boolean]] = donutStock("plain donut").Map(qty => buyDonuts(qty))

map返回的值是Future[Future[Boolean]]。

Future.sequence() VS Future.traverse()

如果我们有很多个Future，然后想让他们并行执行，则可以使用 Future.sequence() 。

println(s"\nStep 2: Create a List of future operations")
val futureOperations = List(donutStock("vanilla donut"),donutStock("plain donut"),donutStock("chocolate donut")
)println(s"\nStep 5: Call Future.sequence to run the future operations in parallel")
val futureSequenceResults = Future.sequence(futureOperations)
futureSequenceResults.onComplete {case Success(results) => println(s"Results $results")case Failure(e)       => println(s"Error processing future operations, error = ${e.getMessage}")
}

Future.traverse() 和Future.sequence() 类似，唯一不同的是，Future.traverse()可以对要执行的Future进行操作，如下所示：

println(s"\nStep 3: Call Future.traverse to convert all Option of Int into Int")
val futureTraverseResult = Future.traverse(futureOperations){ futureSomeQty =>futureSomeQty.map(someQty => someQty.getOrElse(0))
}
futureTraverseResult.onComplete {case Success(results) => println(s"Results $results")case Failure(e)       => println(s"Error processing future operations, error = ${e.getMessage}")
}

Future.foldLeft VS Future reduceLeft

foldLeft 和 reduceLeft 都是用来从左到右做集合操作的，区别在于foldLeft可以提供默认值。看下下面的例子：

println(s"\nStep 3: Call Future.foldLeft to fold over futures results from left to right")
val futureFoldLeft = Future.foldLeft(futureOperations)(0){ case (acc, someQty) =>acc + someQty.getOrElse(0)
}
futureFoldLeft.onComplete {case Success(results) => println(s"Results $results")case Failure(e)       => println(s"Error processing future operations, error = ${e.getMessage}")
}

输出结果：


Step 3: Call Future.foldLeft to fold over futures results from left to right
Results 20

println(s"\nStep 3: Call Future.reduceLeft to fold over futures results from left to right")
val futureFoldLeft = Future.reduceLeft(futureOperations){ case (acc, someQty) =>acc.map(qty => qty + someQty.getOrElse(0))
}
futureFoldLeft.onComplete {case Success(results) => println(s"Results $results")case Failure(e)       => println(s"Error processing future operations, error = ${e.getMessage}")
}

输出结果：

Step 3: Call Future.reduceLeft to fold over futures results from left to right
Results Some(20)

Future firstCompletedOf

firstCompletedOf在处理多个Future请求时，会返回第一个处理完成的future结果。

println(s"\nStep 3: Call Future.firstCompletedOf to get the results of the first future that completes")
val futureFirstCompletedResult = Future.firstCompletedOf(futureOperations)
futureFirstCompletedResult.onComplete {case Success(results) => println(s"Results $results")case Failure(e)       => println(s"Error processing future operations, error = ${e.getMessage}")
}

Future zip VS zipWith

zip用来将两个future结果组合成一个tuple. zipWith则可以自定义Function来处理future返回的结果。

println(s"\nStep 3: Zip the values of the first future with the second future")
val donutStockAndPriceOperation = donutStock("vanilla donut") zip donutPrice()
donutStockAndPriceOperation.onComplete {case Success(results) => println(s"Results $results")case Failure(e)       => println(s"Error processing future operations, error = ${e.getMessage}")
}

输出值：


Step 3: Zip the values of the first future with the second future
checking donut stock
Results (Some(10),3.25)

使用zipwith的例子：

println(s"\nStep 4: Call Future.zipWith and pass-through function qtyAndPriceF")
val donutAndPriceOperation = donutStock("vanilla donut").zipWith(donutPrice())(qtyAndPriceF)
donutAndPriceOperation.onComplete {case Success(result) => println(s"Result $result")case Failure(e)      => println(s"Error processing future operations, error = ${e.getMessage}")
}

输出结果：

Step 4: Call Future.zipWith and pass-through function qtyAndPriceF
checking donut stock
Result (10,3.25)

Future andThen

andThen后面可以跟一个自定义的PartialFunction，来处理Future返回的结果，如下所示：

println(s"\nStep 2: Call Future.andThen with a PartialFunction")
val donutStockOperation = donutStock("vanilla donut")
donutStockOperation.andThen { case stockQty => println(s"Donut stock qty = $stockQty")}

输出结果：

Step 2: Call Future.andThen with a PartialFunction
checking donut stock
Donut stock qty = Success(10)

自定义threadpool

上面的例子中，我们都是使用了scala的全局ExecutionContext: scala.concurrent.ExecutionContext.Implicits.global.
同样的，我们也可以自定义你自己的ExecutionContext。下面是一个使用java.util.concurrent.Executors的例子：

  println("Step 1: Define an ExecutionContext")val executor = Executors.newSingleThreadExecutor()implicit val ec = scala.concurrent.ExecutionContext.fromExecutor(executor)println("\nStep 2: Define a method which returns a Future")import scala.concurrent.Futuredef donutStock(donut: String): Future[Int] = Future {// assume some long running database operationprintln("checking donut stock")10}println("\nStep 3: Call method which returns a Future")val donutStockOperation = donutStock("vanilla donut")donutStockOperation.onComplete {case Success(donutStock)  => println(s"Results $donutStock")case Failure(e)           => println(s"Error processing future operations, error = ${e.getMessage}")}Thread.sleep(3000)executor.shutdownNow()

recover() recoverWith() and fallbackTo（）

这三个方法主要用来处理异常的，recover是用来从你已知的异常中恢复，如下所示：

println("\nStep 3: Call Future.recover to recover from a known exception")
donutStock("unknown donut").recover { case e: IllegalStateException if e.getMessage == "Out of stock" => 0 }.onComplete {case Success(donutStock)  => println(s"Results $donutStock")case Failure(e)           => println(s"Error processing future operations, error = ${e.getMessage}")
}

recoverWith()和recover（）类似，不同的是他的返回值是一个Future。

println("\nStep 3: Call Future.recoverWith to recover from a known exception")
donutStock("unknown donut").recoverWith { case e: IllegalStateException if e.getMessage == "Out of stock" => Future.successful(0) }.onComplete {case Success(donutStock)  => println(s"Results $donutStock")case Failure(e)           => println(s"Error processing future operations, error = ${e.getMessage}")
}

fallbackTo（）是在发生异常时，去调用指定的方法：

println("\nStep 3: Call Future.fallbackTo")
val donutStockOperation = donutStock("plain donut").fallbackTo(similarDonutStock("vanilla donut")).onComplete {case Success(donutStock)  => println(s"Results $donutStock")case Failure(e)           => println(s"Error processing future operations, error = ${e.getMessage}")
}

promise

熟悉ES6的同学可能知道，promise是JS在ES6中引入的新特性，其主要目的是将回调转变成链式调动。

当然scala的promise和ES6的promise还是不一样的，我们看下scala中promise是怎么用的：

  println("Step 1: Define a method which returns a Future")import scala.concurrent.ExecutionContext.Implicits.globaldef donutStock(donut: String): Int = {if(donut == "vanilla donut") 10else throw new IllegalStateException("Out of stock")}println(s"\nStep 2: Define a Promise of type Int")val donutStockPromise = Promise[Int]()println("\nStep 3: Define a future from Promise")val donutStockFuture = donutStockPromise.futuredonutStockFuture.onComplete {case Success(stock) => println(s"Stock for vanilla donut = $stock")case Failure(e)     => println(s"Failed to find vanilla donut stock, exception = $e")}println("\nStep 4: Use Promise.success or Promise.failure to control execution of your future")val donut = "vanilla donut"if(donut == "vanilla donut") {donutStockPromise.success(donutStock(donut))} else {donutStockPromise.failure(Try(donutStock(donut)).failed.get)}println("\nStep 5: Completing Promise using Promise.complete() method")val donutStockPromise2 = Promise[Int]()val donutStockFuture2 = donutStockPromise2.futuredonutStockFuture2.onComplete {case Success(stock) => println(s"Stock for vanilla donut = $stock")case Failure(e)     => println(s"Failed to find vanilla donut stock, exception = $e")}donutStockPromise2.complete(Try(donutStock("unknown donut")))

上面例子中我们使用了 Promise.success， Promise.failure， Promise.complete() 来控制程序的运行。

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