Counting the number of threads in an ExecutionContext

Question

Suppose I want to know how many threads there are in a given ExecutionContext.
So I am writing a function like this

def count(implicit ec: ExecutionContext): Int = {
  val promise = Promise[Unit]
  val counter = new AtomicInteger(0)
  for (_ <- 0 to 1000) Future {
    counter.getAndIncrement()
    Await.ready(promise.future, Duration.Inf)
  }
  val result = counter.get()
  promise.success(())
  result
}

It does not work for ExecutionContext.global at least so I changed the implementation:

def count(implicit ec: ExecutionContext): Int = {
  val barrier = new CyclicBarrier(1000)
  var isReset = false
  for (_ <- 0 to 1000) Future { if (!isReset) barrier.await() }
  val result = barrier.getNumberWaiting
  barrier.reset()

  // make all futures complete and release all threads to allow JVM to exit
  isReset = true 

 result
}

It works but I wonder

• why the first implementation does not work;
• how to improve the "barrier" implementation (e.g. get rid of the isReset) ;
• what the best way to count threads in an ExecutionContext is.

Answer 1

Try casting to particular executor, for example

implicit val ec = scala.concurrent.ExecutionContext.Implicits.global
Future(42)
ec.asInstanceOf[java.util.concurrent.ForkJoinPool].getPoolSize // : Int = 1