I wanted to control the number of threads in ExecutionContext. So I created a instance of ThreadPoolExecutor and then created ExecutionContext from it.
And I created some Futures and attached onSuccess callbacks on them. I expected each onSuccess callback was called when each Future work finished. But I found all onSuccess callbacks were executed at the same time.
import java.util.concurrent.{ Executors, ForkJoinPool }
import scala.concurrent.{ Await, ExecutionContext, Future }
import scala.concurrent.duration.Duration
object Main extends App {
implicit val ec = ExecutionContext.fromExecutorService(Executors.newFixedThreadPool(2))
// implicit val ec = ExecutionContext.fromExecutorService(new ForkJoinPool(2))
val start = System.currentTimeMillis()
val futures = for {
i <- 1 to 10
} yield Future[Int] {
Thread.sleep(i * 1000)
i
}
futures.foreach { f =>
f.onSuccess { case i =>
println(s"${i} Success. ${System.currentTimeMillis() - start}ms elapsed.")
}
}
Await.ready(Future.sequence(futures.toList), Duration.Inf)
ec.shutdown()
}
// ThreadPoolExecutor Result
// 1 Success. 25060ms elapsed.
// 2 Success. 25064ms elapsed.
// 3 Success. 25064ms elapsed.
// 4 Success. 25064ms elapsed.
// 5 Success. 25064ms elapsed.
// 6 Success. 25064ms elapsed.
// 7 Success. 25065ms elapsed.
// 8 Success. 25065ms elapsed.
// 9 Success. 25065ms elapsed.
// 10 Success. 30063ms elapsed.
// ForkJoinPool Result
// 1 Success. 1039ms elapsed.
// 2 Success. 2036ms elapsed.
// 3 Success. 4047ms elapsed.
// 4 Success. 6041ms elapsed.
// 5 Success. 12042ms elapsed.
// 6 Success. 12043ms elapsed.
// 7 Success. 25060ms elapsed.
// 8 Success. 25060ms elapsed.
// 9 Success. 25060ms elapsed.
// 10 Success. 30050ms elapsed.
The result above was printed at the same time not respectively. But when I use ForkJoinPool instead of ThreadPoolExecutor this problem is mitigated. Did I misuse ExecutionContext and Future?
edited: I found the problem happens when the number of threads is less than the number of futures. So I've edited above code to reproduce the problem and print the execution time.
I think future callback should be called on time even if the number of threads is small...
