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scalaexceptionfuturefuturetask

Why Future(Failure(new Exception)) returns Success instead of failure?

I was trying the following and thinking that I'll get a failure

val failure = Future { Failure(new Exception) }

but instead I got

Future(Success(Failure(java.lang.Exception)))

Can anyone answer why?

Solution

  • Future.failed can create a failed future, for example

    Future.failed(new Exception)

    or throw inside the future

    Future(throw new Exception)

    or call Future.fromTry

    Future.fromTry(Failure(new Exception))

    however

    Future(Failure(new Exception))

    does not represent a failed future because

    Failure(new Exception)

    is, despite possibly misleading names, just a regular value, for example,

    val x = Failure(new Exception)
val y = 42
Future(x)
Future(y)

    so Future(x) is a successful future for the same reason Future(y) is a successful future.

    You can think of Future as a kind of async try-catch, so if you are not throwing inside the try

    try {
  Failure(new Exception) // this is not a throw expression
} catch {
  case exception =>      // so exception handler does not get executed
}

    then catch handler does not get executed.