Scala教程之:Future和Promise
文章目录
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 operation
println("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.Await
import 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.Future
def donutStock(donut: String): Future[Int] = Future {
// assume some long running database operation
println("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.global
def donutStock(donut: String): Int = {
if(donut == "vanilla donut") 10
else 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.future
donutStockFuture.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.future
donutStockFuture2.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。
更多教程请参考 flydean的博客
