Why new thread instead of future {...}

Question

This answer instructs how to convert java.util.concurrent.Future into scala.concurrent.Future, while managing where the blocking will occur:

import java.util.concurrent.{Future => JFuture}
import scala.concurrent.{Future => SFuture}

val jfuture: JFuture[T] = ???
val promise = Promise[T]()
new Thread(
  new Runnable {
    def run() { promise.complete(Try{ jfuture.get }) }
  }
).start
val future = promise.future

My queston is the same as a question asked in the comments:

what's wrong with future { jfuture.get }? Why you used an extra thread combined with Promise?

It was answered as follows:

it'll block thread in your thread pull. If you have a configured ExecutionContext for such futures it's fine, but default ExecutionContext contains as many threads as you have processors.

I'm not sure I understand the explanation. To reiterate:

What's wrong with future { jfuture.get }? Isn't blocking inside a future the same as manually creating a new Thread and blocking there? If not, how is it different?

Answer 1

There is almost no difference between future { jfuture.get } and future { future { jfuture.get }}.

There are as many treads in default thread pool as many you have processors.

With jfuture.get you'll get 1 thread blocked.

Let's assume you have 8 processors. Also let's suppose each jfuture.get takes 10 seconds. Now create 8 future { jfuture.get }.

val format = new java.text.SimpleDateFormat("HH:mm:ss").format(_: Date)

val startTime = new Date
(1 to 8) map {_ => future{ Thread.sleep(10000) }}
future{
  2+2
  println(s"2+2 done. Start time: ${format(startTime)}, end time: ${format(new Date)}")
}

// 2+2 done. Start time: 20:48:18, end time: 20:48:28

10 seconds is a little too long for 2+2 evaluation.

All other futures and all actors on the same execution context will be stopped for 10 seconds.

With additional execution context:

object BlockingExecution {
  val executor = ExecutionContext.fromExecutor(new ForkJoinPool(20))
}

def blockingFuture[T](f: => T) = {
  future( f )(BlockingExecution.executor)
}

val startTime = new Date
(1 to 8) map {_ => blockingFuture{ Thread.sleep(10000) }}
future{
  2+2
  println(s"2+2 done. Start time: ${format(startTime)}, end time: ${format(new Date)}")
}

// 2+2 done. Start time: 21:26:18, end time: 21:26:18

You could implement blockingFuture using new Thread(new Runnable {..., but additional execution context allows you to limit threads count.