抛出异常在future.map scala 中不起作用？

Question

def testThrowException(number: Int): Future[Int] = {
 if (number == 0) {
     throw new Exception("number is 0")
 else {
     Future{1}
 }

对于上述函数，如果我用 testThrowException(0) 调用它，我可以看到控制台中打印的异常错误消息，但如果我执行类似的操作

def testThrowException(number: Int): Future[Int] = {
anotherFuture.map {
    if (number == 0) {
        throw new Exception("number is 0")
    } else {
        1
    }
}

我看不到控制台中打印的异常，但是如果我执行 testThrowException.onFailure，我可以看到失败消息，我在这里做错了什么吗？为什么不打印出异常

Answer 1

Future在一个单独的线程中运行，因此将 a 扔进去Future只会使该线程崩溃，同时保持主线程运行。例如，

object Hello extends App {
  println(s"Starting in main thread called ${Thread.currentThread.getName}...")
  Future(
    throw new RuntimeException(s"I crashed separate thread called ${Thread.currentThread.getName}")
  ).andThen { case Failure(e) => println(e.getMessage) }
  println("I made it!")
}

应该输出

Starting in main thread called run-main-e...
I made it!
I crashed separate thread called scala-execution-context-global-253

我们看到它scala-execution-context-global-253在主线程run-main-e保持运行的同时崩溃了单独的线程，所以I made it!打印得很好。另一方面，下面的例子抛出了一个Future

object Hello extends App {
  println(s"Starting in main thread called ${Thread.currentThread.getName}...")
  throw new RuntimeException(s"I crashed the main thread ${Thread.currentThread.getName}")
  println("I made it!")
}

哪个输出

Starting in main thread called run-main-d...
[error] (run-main-d) java.lang.RuntimeException: I crashed the main thread run-main-d

我们看到主线程run-main-d崩溃之前I made it!可以打印。

Answer 2

第一个函数testThrowException在获取 a 作为输入的情况下不会返回 Future 0。因此，程序会一直运行，直到出现异常。

然而，如下面粘贴的源代码所示，Future.map总是返回另一个 future：

  def map[S](f: T => S)(implicit executor: ExecutionContext): Future[S] = { // transform(f, identity)
    val p = Promise[S]()
    onComplete { v => p complete (v map f) }
    p.future
  }

仅定义 Future 不会打印其结果，也不会打印抛出的异常。您需要定义onSuccess、onFailure、 或onComplete来执行此操作。但是，如果 Future 的主体中存在 print 语句，那么它将执行：

def addOne(number: Int): Int = {
    if (number == 0) {
        // you can also print the exception instead of just throwing it.
        throw new Exception("number is 0")
    } else {
        println("success")
        1 + number
    }
}
Future { addOne(1) } // scala.concurrent.Future[Int] = Future(Success(2))
// the above also prints "success"
Future { addOne(0) } // scala.concurrent.Future[Int] = Future(Failure(java.lang.Exception: number is 0))
// the above does not print anything unless the exception is printed before thrown in `addOne`

您还可以使用onComplete, 来处理成功和/或失败：

// prints "got error" plus the stack trace

 - List item

Future {0}.map(addOne).onComplete {
       case Success(value) => println(s"got $value")
       case Failure(t) => println("got error: " + t.getStackTrace.mkString("\n"))
     }

请注意，使用的进口是：

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
import scala.util.{Failure, Success}