How to execute a future immediately

Say I have a future defined as follows

val fut = Future {
  println("Doing some work")
  Thread.sleep(5000)
  println("Work done")
}

I am accessing this future later in time but would like to have it execute on startup rather than when I reference fut

I read somewhere that it can be executed immediately by doing:

scala.concurrent.ExecutionContext.Implicits.global.execute(fut)

However this will not compile as execute only takes a Runnable. Is there a way I can execute this future immediately in a different thread?

Scala Futures created using Future.apply are eagerly executed.

For non-eager execution, you will have to define your own class to indirectly create Future using Promise.

import scala.concurrent.{Future, Promise}
import scala.util.{Failure, Success, Try}

class NonEagerFuture[A](computation: => A) extends Runnable {
  private val promise = Promise[A]

  override def run(): Unit = {
    Try(computation) match {
      case Success(result)    => promise.success(result)
      case Failure(exception) => promise.failure(exception)
    }
  }

  def isFutureCompleted(): Boolean =
    promise.isCompleted

  def getResult(): A =
    if (!promise.isCompleted) {
      throw new IllegalStateException("Future is not completed yet")
    } else {
      promise.future.value.get.get
    }

  def getScalaFuture(): Future[A] =
    promise.future

}

It can be used as following,

println("creating the future")

val nonEagerFuture = new NonEagerFuture({
  println("Doing some work")
  Thread.sleep(5000)
  println("Work done")
  println("returning 5 as result")
  5
})

println("running for the future")
scala.concurrent.ExecutionContext.Implicits.global.execute(nonEagerFuture)

// can also get the internal scala future and use it
// this scala future will not have the eager problem 
println("get the scala future") 
val scalaFuture = nonEagerFuture.getScalaFuture()
println("printing scalaFuture" + scalaFuture)

// or block for completion using isFutureCompleted
println("checking for the future")
while (!nonEagerFuture.isFutureCompleted()) {
  Thread.sleep(1000)
  println("Checking completed: " + nonEagerFuture.isFutureCompleted())
}

println("getting result for the future")
println("Result: " + nonEagerFuture.getResult())