Serializing/Deserializing case objects with Gson in Scala - scala

Question

I am using Gson to serialize and deserialize objects, and saving the results in Redis. ie object is serialized into json string then put in Redis, when the object is retreived, it is string then I use Gson.fromjson(str, className) to deserialize into object.
I am beginner with Scala so I assume my usage is incorrect.
I have the following class:
case class Status(id: String, state: State)
where State is the following:
sealed trait State {}
case object COMPLETED_SUCCESSFULLY extends State {}
case object FINISHED_POLLING extends State {}
case object CURRENTLY_DOWNLOADING extends State {}
case object FINISHED_DOWNLOADING extends State {}
case object CURRENTLY_UPLOADING extends State {}
case object FINISHED_UPLOADING extends State {}
I want to serialize Status into a json string then deserialize it back into an object.
But, when I serialize Status using Gson, I get:
"{\"id\":\"foo\",\"state\":{}}"
Why is that?
Ex:
val Status = new Status("foo", COMPLETED_SUCCESSFULLY)
I expect the serialized output to be
"{\"id\":\"foo\",\"state\":\"COMPLETED_SUCCESSFULLY\"}"

Answer 1

By default, case objects are serialized by Gson to empty json objects: {}. You have to write custom serializer to get expected behaviour:
object StateSerializer extends JsonSerializer[State] {
override def serialize(t1: State, t2: Type, jsonSerializationContext: JsonSerializationContext): JsonElement = {
val res = new JsonObject()
res.add("name", new JsonPrimitive(t1.toString))
res
}
}
val gson = new GsonBuilder().registerTypeHierarchyAdapter(classOf[State], StateSerializer)
.registerTypeHierarchyAdapter(classOf[State], StateDeserializer).setPrettyPrinting().create()
println(gson.toJson(COMPLETED_SUCCESSFULLY))
Will print:
{
"name": "COMPLETED_SUCCESSFULLY"
}
Also if you want to transform json to case object you have to implement JsonDeserializer:
object StateDeserializer extends JsonDeserializer[State] {
override def deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): State = {
val res = json match {
case o: JsonObject if o.has("name") && o.entrySet().size() == 1 =>
val name = o.get("name").getAsString
name match {
case "FINISHED_POLLING" => FINISHED_POLLING
case "FINISHED_DOWNLOADING" => FINISHED_DOWNLOADING
case "FINISHED_UPLOADING" => FINISHED_UPLOADING
case "CURRENTLY_DOWNLOADING" => CURRENTLY_DOWNLOADING
case "CURRENTLY_UPLOADING" => CURRENTLY_UPLOADING
case "COMPLETED_SUCCESSFULLY" => COMPLETED_SUCCESSFULLY
case _ => null
}
case _ => null
}
Option(res).getOrElse(throw new JsonParseException(s"$json can't be parsed to State"))
}
}
println(gson.fromJson("{\"name\": \"COMPLETED_SUCCESSFULLY\"}", classOf[State]))
Will print:
COMPLETED_SUCCESSFULLY