Convert CompletableFuture (Java 8) to Future (Scala)

Question

I want to change the return type of the method below to Future[Unit]

def send(data: Array[Byte]): CompletableFuture[Void] = {
  val f:CompletableFuture[Void] = client.send(data)
  f
}

I'm using Scala 2.12.1. Is there any convenient way to convert from Java 8 CompletableFuture to Scala's Future?

https://github.com/scala/scala-java8-compat/blob/master/src/main/scala/scala/compat/java8/FutureConverters.scala — maasg, Jan 05 '17 at 18:48

score 45 · Accepted Answer · answered Jan 05 '17 at 18:29

45

Yes, it's called scala-java8-compat, and here's the method you're looking for.

answered Jan 05 '17 at 18:29

Viktor Klang

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3

Could you give an example on how to use the method? I think that would be helpful – Kael Vergara Jun 20 '18 at 04:03
3

You just call it? `scala.compat.java8.FutureConverters.toScala(completionStage)` – Viktor Klang Jun 20 '18 at 12:11

score 20 · Answer 2 · answered Jun 24 '20 at 14:05

Though 2.12 was mentioned by asker (in 2017), now we have 2.13 and the conversion has become easier:

Starting Scala 2.13, the standard library includes scala.jdk.FutureConverters which provides Java to Scala CompletableFuture/Future implicit conversions:

import scala.jdk.FutureConverters._

val scalaFuture = javaFuture.asScala

Convert a Java Future to a Scala Future

score 1 · Answer 3 · answered Jul 05 '22 at 15:04

To convert a CompletableFuture to Scala's Future and get rid of the wrapping CompletionException:

package mycompany.common.concurrent

object Extensions {
  implicit class UnwrappedCompletableFuture[T](cf: CompletableFuture[T]) {

    import scala.jdk.FutureConverters._

    def toScala: Future[T] = {
      val f = cf.asScala
      f.recover {
        case ex: CompletionException if ex.getCause != null => throw ex.getCause
      }(ExecutionContext.global)
    }
  }
}

Usage:

import mycompany.common.concurrent.Extensions._

val result = fetcher.fetchAsync().toScala.recover {
  case ex: YourActualException => 
}

Convert CompletableFuture (Java 8) to Future (Scala)

3 Answers3