Spray-Json: How to parse a Json Array?

Question

I'm new to the Spray-Json API and I'm trying to parse a Json response from the Docker REST API.

There is a clean example of the usage of Spray-Json to parse this Google Map Json response :

{
   "results" : [
      {
         "elevation" : 8815.7158203125,
         "location" : {
            "lat" : 27.988056,
            "lng" : 86.92527800000001
         },
         "resolution" : 152.7032318115234
      }
   ],
   "status" : "OK"
}

In the above example the outermost level is an Object. However, I need to directly parse a Json response whose outermost level is an Array composed of containers information as shown below :

[
     {
       "Id": "8dfafdbc3a40",
       "Image": "base:latest",
       "Command": "echo 1",
       "Created": 1367854155,
       "Status": "Exit 0",
       "Ports":[{"PrivatePort": 2222, "PublicPort": 3333, "Type": "tcp"}],
       "SizeRw":12288,
       "SizeRootFs":0
     },
     { ... },
     { ... }
]

Here is a code that I adapted from the Google map example :

package main

import ...

case class Container(id: String, image: String, command: String, created: Long, status: String, ports: List[Port], sizeRW: Long, sizeRootFs: Long)
case class Port(privatePort: Long, publicPort: Long, portType: String)
case class DockerApiResult[T](results: List[T])

object ContainerListJsonProtocol extends DefaultJsonProtocol {
  implicit val portFormat = jsonFormat3(Port)
  implicit val containerFormat = jsonFormat8(Container)
  implicit def dockerApiResultFormat[T :JsonFormat] = jsonFormat1(DockerApiResult.apply[T])
}

object Main extends App {

  implicit val system = ActorSystem("simple-spray-client")
  import system.dispatcher // execution context for futures below
  val log = Logging(system, getClass)

  log.info("Requesting containers info...")

  import ContainerListJsonProtocol._
  import SprayJsonSupport._
  val pipeline = sendReceive ~> unmarshal[DockerApiResult[Container]]

  val responseFuture = pipeline {
    Get("http://<ip-address>:4243/containers/json")
  }

  responseFuture onComplete {
    case Success(DockerApiResult(Container(_,_,_,_,_,_,_,_) :: _)) =>
      log.info("Id of the found image: {} ")
      shutdown()

    case Success(somethingUnexpected) =>
      log.warning("The Docker API call was successful but returned something unexpected: '{}'.", somethingUnexpected)
      shutdown()

    case Failure(error) =>
      log.error(error, "Couldn't get containers information")
      shutdown()
  }

  def shutdown(): Unit = {
    IO(Http).ask(Http.CloseAll)(1.second).await
    system.shutdown()
  }
}

And below is the exception I get (Object expected) :

spray.httpx.PipelineException: MalformedContent(Object expected,Some(spray.json.DeserializationException: Object expected))

I certainly miss something obvious but How to parse a Json Array using Spray-Json?

Also, is there a simple way to do this without having to deal with custom JsonFormat or RootJsonFormat?

Answer 1

By doing unmarshal[DockerApiResult[Container]], you're telling spray-json that you expect the format to be a json object of the form:

{ results: [...] }

since case class DockerApiResult[T](results: List[T]) is defined as an object with a single results field containing a list.

Instead you need to do:

unmarshal[List[Container]]

and then operate on the resulting list directly (or wrap it in a DockerApiResult after it has been parsed by spray-json).

If you want spray-json to unmarshal directly into a DockerApiResult, you can write a JsonFormat with something like:

implicit object DockerApiResultFormat extends RootJsonFormat[DockerApiResult] {
  def read(value: JsValue) = DockerApiResult(value.convertTo[List[Container]])
  def write(obj: DockerApiResult) = obj.results.toJson
}