Catching exception in Play for Scala method

Question

This Play for Scala method (that should return Future[play.api.mvc.Result]) invokes another method run that is a future that may throw an exception. If run doesn't throw an exception, then the method should return a Json with ret equals 0. If run does throw an exception, then the method should retun a Json with ret equals 1.

Problem is that the OnComplete is throwing a compilation error:

type mismatch; found : Unit required: scala.concurrent.Future[play.api.mvc.Result]

how to fix this?

   def test (req:Int) : Future[play.api.mvc.Result] = {

       val future = run(req)  
       future.map {
          result => 
               val json = JsObject(Seq(
                          "rs" -> Json.toJson(result),
                          "ret" -> JsString("0")
                      ))
               Ok(json)
           }

       future.onComplete { // <-- compilation error
              case Success(_) => // do nothing
              case Failure(ex) => {
                  val json = JsObject(Seq(
                         "msg" -> JsString(ex.getMessage),
                         "ret" -> JsString("1")
                    ))
                  Ok(json)
              }
            }

   }

J0HN J0HN · Accepted Answer · 2017-07-06T04:22:02

In Scala result of last expression in a method becomes return value of the method. future.onComplete returns a Unit, which makes function body "return" a Unit.

What you actually want is recover

def test (req:Int) : Future[play.api.mvc.Result] = {
    run(req)
        .map(result => Ok(JsObject("rs" -> ..., "ret" -> 0)))
        .recover({ case NonFatal(ex) => 
            Ok(JsObject(Seq("msg" -> ..., "ret" -> 1))
        })
}

Also, note that map returns new future, so in your example you're attaching onComplete callback to the future returned by run, which does not include that json processing; so failure in the map call will not be captured by onComplete Failure(ex) branch. As you can see I've corrected that in the snippet above.

Catching exception in Play for Scala method

1 Answers