[spray-json] How to Define JSON Format for scala.concurrent.duration.FiniteDuration?

[kra...@gmail.com]

Greetings, I am trying to serialize HttpServer.Stats as JSON using spray-json. The problem is that FiniteDuration has a companion object so I read up on how to create a format for it and came up with the following:

After some research I tried the following:

object MyJsonProtocol extends DefaultJsonProtocol {
  implicit val formatFiniteDuration = jsonFormat(FiniteDuration.apply, "length", "unit")
  implicit val formatHttpServerStats = jsonFormat9(HttpServer.Stats)
}

Problem is the first format gives an ambiguous reference to apply (since there are two) and the following doesnt work either:

  implicit val formatFiniteDuration = jsonFormat(FiniteDuration.apply(_:Long, _:String), "length", "unit")

Thats me trying to figure out a way to clarify the ambiguity but then I get:

/ecplat/src/main/scala/com/kraythensoft/ecplat/Actors.scala:21: inferred type arguments [Long,String,scala.concurrent.duration.FiniteDuration] do not conform to method jsonFormat's type parameter bounds [A,B,T <: Product]
  implicit val formatFiniteDuration = jsonFormat(FiniteDuration.apply(_:Long, _:String), "length", "unit")

Perhaps i am doing something wrong in the disambiguation?

Is there a better way to do this that I am not seeing?

Also inside FiniteDuration there is a reference to scala.concurrent.TimeUnit which contains some scala constructs I dont understand. How would I serialize them?

  type TimeUnit          = java.util.concurrent.TimeUnit
  final val DAYS         = java.util.concurrent.TimeUnit.DAYS
  final val HOURS        = java.util.concurrent.TimeUnit.HOURS
  // and so on ...

I gather the type line is a bit like a c++ typedef and the other lines are creating package objects that reflect constants in the backing Java class. No idea how to yank them into the serialization.

Thanks in advance

[Mathias]

The problem is that FiniteDuration (in Scala 2.10) is not a case class.
Therefore you cannot use the `jsonFormat` helper to construct a JsonFormat for it.
You'll have to resort to the "manual" approach…

Cheers,
Mathias

---
mat...@spray.io
http://spray.io

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[kra...@gmail.com]

Thanks for the information, if altogether disappointing. Does this mean i have to render the stats to a map or something first and then output the map or can I define a spray-json formatter manually that will pick it up? If so, how do I define it?

-- Robert

[Mathias]

Robert,

this section of the spray-json docs should show you how to do it:
https://github.com/spray/spray-json#providing-jsonformats-for-other-types

Cheers,
Mathias

---
mat...@spray.io
http://spray.io

[kra...@gmail.com]

Thanks a bunch man.

[T.J. Corrigan]

In case someone else comes across this here is the code you will need:

  implicit object finiteDurationFormat extends RootJsonFormat[FiniteDuration] {

    def write(fd: FiniteDuration) = JsObject(

      "length" -> JsNumber(fd.length),

      "unit"   -> JsString(fd.unit.name())

    )

      

    def read(value: JsValue) = {

      value.asJsObject.getFields("length", "unit") match {

        case Seq(JsNumber(length), JsString(unit)) =>

          FiniteDuration(length.toLong, unit)

        case _ => throw new DeserializationException("FiniteDuration expected")

      }

    }

  }

  implicit val statsFormat: RootJsonFormat[Stats] = jsonFormat8(Stats.apply)

Note the ordering of the implicits matters.