ObjectJson.scala

/**
 * Created by jasonmartens on 10/13/15.
 */
object Main extends App {

  import spray.json._

  import scala.reflect.ClassTag

  trait TestingTrait
  case object TestingTypeA extends TestingTrait
  case object TestingTypeB extends TestingTrait
  case class Wrapper(data: TestingTrait)

  object SimpleClassNameExtractor {
    def extractSimpleClassName(input: String) = input.split("\\.").last.split("\\$").last
    def apply[T](input: Class[T]): String = extractSimpleClassName(input.getName)
  }

  /**
   * A custom version of the Spray DefaultJsonProtocol with an additional
   * formatter for ADT's of only case objects.
   */
  trait CaseObjectSerializationSupport extends DefaultJsonProtocol {

    def caseObjectJsonFormat[T: ClassTag](objects: T*)(implicit tag: ClassTag[T]) = new RootJsonFormat[T] {
      /** A mapping from object names to the objects */
      private val mapping = objects.map(obj ⇒ key(obj) -> obj).toMap

      override def read(json: JsValue): T = (json match {
        case JsString(value) ⇒ mapping.get(value)
        case _               ⇒ None
      }).getOrElse(deserializationError(s"Unknown json value found when converting to $tag: $json"))

      /** The toString value of a case object is its name */
      override def write(value: T): JsValue = JsString(key(value))

      private def key(input: T): String = SimpleClassNameExtractor(input.getClass)
    }

  }

  object CaseObjectSerializationSupport extends CaseObjectSerializationSupport {
    implicit val wrapperFormat = jsonFormat1(Wrapper)
  }

  import CaseObjectSerializationSupport._

  val jsResult = """{"data": "TestingA"}""".toJson.convertTo[Wrapper]
  println(s"Result: $jsResult")
}

[error] Main.scala:46: could not find implicit value for evidence parameter of type Main.CaseObjectSerializationSupport.JF[Main.TestingTrait]
[error] implicit val wrapperFormat = jsonFormat1(Wrapper)
[error] ^
[error] Main.scala:51: Cannot find JsonReader or JsonFormat type class for Main.Wrapper
[error] val jsResult = """{"data": "TestingA"}""".toJson.convertTo[Wrapper]
[error] ^
[error] two errors found
error Compilation failed