How can I use Gson in Scala to serialize a List?

Question

I was hoping to use Scala and Gson together. It seems to mostly work, but when I do something like this, it treats the list as an object, not an array:

case class MyType (val x:String, val y:List[SomeOtherType]) {
    def toJson() = new Gson().toJson(this)
}

And my JSON turns out something like this:

{
    "x":"whatever",
    "y": {

    }
}

Normally Gson converts lists to arrays. I'm sure this is all because Gson doesn't know about Scala's collection classes, but any ideas on what I can do to make this work? Or other suggestions using Scala-native JSON libraries?

Answer 1

You can use Java converters in a type adapter, but it's a bit finicky:

  case class GsonListAdapter() extends JsonSerializer[List[_]] with JsonDeserializer[List[_]] {
    import sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl
    import scala.collection.JavaConverters._

    @throws(classOf[JsonParseException])
    def deserialize(jsonElement: JsonElement, t: Type, jdc: JsonDeserializationContext): List[_] = {
      val p = scalaListTypeToJava(t.asInstanceOf[ParameterizedType]) // Safe casting because List is a ParameterizedType.
      val javaList: java.util.List[_ <: Any] = jdc.deserialize(jsonElement, p)
      javaList.asScala.toList
    }

    override def serialize(obj: List[_], t: Type, jdc: JsonSerializationContext): JsonElement = {
      val p = scalaListTypeToJava(t.asInstanceOf[ParameterizedType]) // Safe casting because List is a ParameterizedType.
      jdc.serialize(obj.asInstanceOf[List[Any]].asJava, p)
    }

    private def scalaListTypeToJava(t: ParameterizedType): ParameterizedType = {
      ParameterizedTypeImpl.make(classOf[java.util.List[_]], t.getActualTypeArguments, null)
    }
  }

  val gson = new GsonBuilder().registerTypeHierarchyAdapter(classOf[List[_]], new GsonListAdapter()).create()

  val l1 = List("a", "c")
  val stringListType = new TypeToken[List[String]] {}.getType
  val json1 = gson.toJson(l1, stringListType)
  println(json1) // ["a","c"]
  val newL1: List[String] = gson.fromJson(json1, stringListType)
  assert(l1 === newL1)

  val l2 = List(1, 3)
  val intListType = new TypeToken[List[Int]] {}.getType
  val json2 = gson.toJson(l2, intListType)
  println(json2) // [1,3]
  val newL2: List[Int] = gson.fromJson(json2, intListType)
  assert(l2 === newL2)