How to cancel Future in Scala?

Question

Java Future has cancel method, which can interrupt the thread, which runs the Future task. For example, if I wrap an interruptible blocking call in a Java Future I can interrupt it later.

Scala Future provides no cancel method. Suppose I wrap an interruptible blocking call in a Scala Future. How can I interrupt it?

Answer 1

This is not yet a part of the Futures API, but may be added as an extension in the future.

As a workaround, you could use the firstCompletedOf to wrap 2 futures - the future you want to cancel and a future that comes from a custom Promise. You could then cancel the thus created future by failing the promise:

def cancellable[T](f: Future[T])(customCode: => Unit): (() => Unit, Future[T]) = {
  val p = Promise[T]
  val first = Future firstCompletedOf Seq(p.future, f)
  val cancellation: () => Unit = {
    () =>
      first onFailure { case e => customCode}
      p failure new Exception
  }
  (cancellation, first)
}

Now you can call this on any future to obtain a "cancellable wrapper". Example use-case:

val f = callReturningAFuture()
val (cancel, f1) = cancellable(f) {
  cancelTheCallReturningAFuture()
}

// somewhere else in code
if (condition) cancel() else println(Await.result(f1))

EDIT:

For a detailed discussion on cancellation, see Chapter 4 in the Learning concurrent programming in Scala book.