Understanding scala.concurrent.Await#result

[Kevin Meredith]

Given the following on Scala 2.11.8:

import scala.concurrent.Future

import scala.concurrent.duration._

import scala.concurrent.Await

import scala.concurrent.ExecutionContext.Implicits.global

scala> def fut = {

     |   Thread.sleep(5000)

     |   Future(42)

     | }

fut: scala.concurrent.Future[Int]

Why does the following not throw a java.util.TimeoutException?

scala> Await.result( fut, FiniteDuration(1, SECONDS) )

res1: Int = 42

As I understand, Await#result will throw if the time-out, i.e. its second argument, completes before the completion of the Future.

If that's true, then why does 42 return in the above example instead of an exception?

[Kevin Meredith]

Note that the following works as I had expected, per my above stated understanding:

scala> def fut = Future { Thread.sleep(5000) }

fut: scala.concurrent.Future[Unit]

scala> Await.result( fut, FiniteDuration(1, SECONDS) )

java.util.concurrent.TimeoutException: Futures timed out after [1 second]

  at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:219)

  at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:223)

  at scala.concurrent.Await$$anonfun$result$1.apply(package.scala:190)

  at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:53)

  at scala.concurrent.Await$.result(package.scala:190)

  ... 32 elided

[Konrad Malawski]

Why would it – the 42 is completed immediately.

It'd be different if you'd say Future { Thread.sleep(...); 42 }

that actually makes the future take a long time.

Your first example just takes "a long time, to get to an immediately completing future.

The timeout is on the Future, not on "getting to it". :)

-- 
Konrad `ktoso` Malawski
Akka @ Lightbend

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[Viktor Klang]

scala> def fut = {

     |   Thread.sleep(5000)

     |   Future(42)

     | }

fut: scala.concurrent.Future[Int]

scala> Await.result( fut, FiniteDuration(1, SECONDS) )

res1: Int = 42

The code above is equivalent of:

Thread.sleep(5000)

val anon = Future(42)

Await.result(anon, FiniteDuration(1, SECONDS))

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