CODEWITHSUNDEEP
jsonscalasprayspray-json
tmandry
tmandry

Reputation: 1375

Recursive data types and custom serializers in spray-json

I have a recursive data structure that I want to write a custom spray-json serializer for.

case class Counts(var count: Int, var properties: mutable.Map[String, Counts])

object MyJsonProtocol extends DefaultJsonProtocol {
  import DefaultJsonProtocol._

  implicit object CountsJsonFormat extends RootJsonFormat[Counts] {
    def read(json: JsValue) = ???
    def write(c: Counts) = {
      // Flatten count and properties into the same object.
      val properties = c.properties.toJson.asJsObject
      val fields = properties.fields + ("count" -> JsNumber(c.count))
      JsObject(fields.toSeq: _*)
    }
  }
}

I've seen the documentation for how to do this for a case class if you use the builtin serialization logic, but I have no idea how to apply that to a custom serializer. I get this compiler error:

Cannot find JsonWriter or JsonFormat type class for scala.collection.mutable.Map[String,co.asku.acuity.EventCounter.Counts]
val properties = c.properties.toJson.asJsObject
                              ^

Upvotes: 2

Views: 1001

Answers (3)

Reinstate Monica
Reinstate Monica

Reputation: 2490

This code provides serialization and deserialization support for mutable maps (and can be modified trivially for other mutable collections):

import spray.json._
import spray.json.DefaultJsonProtocol._
import scala.collection.mutable
...
implicit def mutableMapFormat[K : JsonFormat, V : JsonFormat] = new RootJsonFormat[mutable.Map[K, V]] {
    def write(m : mutable.Map[K, V]) = mapFormat[K, V].write(m.toMap)
    def read(value : JsValue) = mutable.Map.empty[K, V] ++ mapFormat[K, V].read(value)
}

Upvotes: 0

tmandry
tmandry

Reputation: 1375

To add to edi's answer, using toMap would have worked in the example I posted, to convert the mutable map to immutable.

However, I actually ran into a more complex usecase using nested mutable maps, so I just added a format to serialize them like this:

object JsonProtocol extends DefaultJsonProtocol {
  import DefaultJsonProtocol._

  implicit def mutableMapFormat[K :JsonFormat, V :JsonFormat] = new RootJsonFormat[mutable.Map[K, V]] {
    def read(value: JsValue) = ???
    def write(m: mutable.Map[K, V]) = m.toMap.toJson
  }

  implicit object CountsJsonFormat extends RootJsonFormat[Counts] {
    // ...
  }
}

Upvotes: 0

edi
edi

Reputation: 3122

spray-json formats can't handle mutable Maps by default (see this disussion that has happened a while ago on the mailing list). Change the type of properties to be an immutable Map (which I think is better anyways) and your format will work as expected.

Upvotes: 1

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