Wrapping Scala (Akka) Futures with Grails Promises. A very common use case. :)

grails_akka.groovy

import grails.async.Promise
import grails.async.Promises
import scala.concurrent.Future
import akka.dispatch.Futures
import akka.dispatch.OnComplete
import java.util.concurrent.Callable

def actorSystem = ctx.getBean('actorSystem')
def dispatcher = actorSystem.dispatcher()

// create Grails Promise
Promise p = Promises.createPromise()
p.onComplete {
    println "Grails Promise completed successfully"
}
p.onError {
    println "Grails Promise errored out"
}

// create Scala Future
Future f = Futures.future(new Callable<String>() {
    public String call() {
        'Scala Future Value'
    }
}, dispatcher)

// when we complete the Scala Future, tell it to complete the Grails Promise too
f.onComplete([onComplete: { throwable, result -> 
                p.accept('Grails Promise Value')
                println "Scala Future = $result" 
            }] as OnComplete, 
            dispatcher)

println "Grails Promise = ${p.get()}"