CODEWITHSUNDEEP
scalaasynchronousfuture
wang kai
wang kai

Reputation: 1747

Why Future(1) return diffrent result between repl and compiled prog?

Use future in repl:

scala> val a=Future{1}
a: scala.concurrent.Future[Int] = Future(<not completed>)

scala> a.value
res0: Option[scala.util.Try[Int]] = Some(Success(1))

return Some(Success(1))

Use it in IDEA:

object A extends App{
 val a=Future{1}
 println(a.value)
}

return None:

"C:\Program Files\Java\jdk1.8.0_201\bin\java.exe"...
None

Why?There is no something like Thread.Sleep,so in any situation,I think the Future will return immediately,give me Some(Successs(1))

Thanks!

Upvotes: 0

Views: 43

Answers (1)

Dima
Dima

Reputation: 40500

The Future is executed asynchronously. It is submitted to the thread pool queue, where one of the available threads picks it up eventually, and executes.

When you are running in repl, somewhere (during IO probably), the current thread looses control, the context switches, and another thread gets a chance to pick up the task from the queue and complete it.

When running it as a program, the a.value is executed immediately after a=Future, in the same thread, the asynchronous task is still sitting in the queue.

Upvotes: 1

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