未来的Scala地图

Question

In the following example, what is the expected behaviour?

import scala.concurrent.ExecutionContext.Implicits.global
val someFuture = Future { 1+1 }.map(elem => {
  println("mapping immediately")
  elem.toString
})

someFuture.onComplete {
  case scala.util.Success(value) => println(s"Got the callback = $value")
  case Failure(e) => e.printStackTrace()
}

Is the following output always guaranteed?

mapping immediately
Got the callback = 2

Answer 1

It depends.

val someFuture = Future { 1+1 }.map(elem => {
  println("mapping immediately")
  elem.toString
})

someFuture will be executed eagerly and the println will be executed.

someFuture.onComplete {
    case scala.util.Success(value) => println(s"Got the callback = $value")
    case Failure(e) => e.printStackTrace()
}

onComplete is an event listener for a Future . This is called only when this future is completed, either through an exception, or a value , to apply the provided function.

Therefore, it will always execute after the execution of the someFuture .

Assuming that the someFuture is not going to fail, the order

`mapping immediately
 Got the callback = 2`

is guaranteed.

For what ever reason, if there's an exception thrown, then the result would change. In that case Got the callback = 2 will not be printed and mapping immediately may or may not be printed.

Hope this helps.