Gson Adapter for Scala

GsonAdapter.scala

import com.google.gson._
import sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl

import java.lang.reflect.{ParameterizedType, Type}
import java.util.{ArrayList => JArrayList, LinkedHashMap => JListMap, List => JList}
import scala.collection.JavaConverters._
import scala.collection.immutable.ListMap

object GsonAdapter {

  lazy val gson = (new GsonBuilder()
    .setPrettyPrinting()
    .disableHtmlEscaping()
    .registerTypeAdapter(classOf[ListMap[_, _]], ListMapAdapter)
    .registerTypeAdapter(classOf[Option[_]], OptionAdapter)
    .registerTypeAdapter(classOf[Seq[_]], SeqAdapter)
    .create())

  def getInnerTypes(outerType: Type): Array[Type] =
    outerType match {
      case pt: ParameterizedType => pt.getActualTypeArguments()
      case _ => throw new UnsupportedOperationException(outerType.toString)
    }

  object ListMapAdapter extends JsonSerializer[ListMap[_, _]] with JsonDeserializer[ListMap[_, _]] {

    override def serialize(obj: ListMap[_, _], typeOfT: Type, context: JsonSerializationContext): JsonElement = {
      val innerTypes = getInnerTypes(typeOfT)
      val pt = ParameterizedTypeImpl.make(classOf[JListMap[_, _]], innerTypes, null)
      val jListMap = new JListMap[Any, Any]()
      obj.foreach { case (k, v) => jListMap.put(k, v) }

      context.serialize(jListMap, pt)
    }

    override def deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): ListMap[_, _] = {
      val innerTypes = getInnerTypes(typeOfT)
      val pt = ParameterizedTypeImpl.make(classOf[JListMap[_, _]], innerTypes, null)
      val jListMap: JListMap[_, _] = context.deserialize(json, pt)

      ListMap(jListMap.asScala.toSeq: _*)
    }
  }

  object OptionAdapter extends JsonSerializer[Option[_]] with JsonDeserializer[Option[_]] {

    override def serialize(obj: Option[_], typeOfT: Type, context: JsonSerializationContext): JsonElement = {
      val jList = new JArrayList[Any]()
      obj.foreach(jList.add)

      val innerTypes = getInnerTypes(typeOfT)
      val pt = ParameterizedTypeImpl.make(classOf[JList[_]], innerTypes, null)
      context.serialize(jList, pt)
    }

    override def deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): Option[_] = {
      val innerTypes = getInnerTypes(typeOfT)
      val pt = ParameterizedTypeImpl.make(classOf[JList[_]], innerTypes, null)
      val jList: JList[_] = context.deserialize(json, pt)

      jList.asScala.headOption
    }
  }

  object SeqAdapter extends JsonSerializer[Seq[_]] with JsonDeserializer[Seq[_]] {

    override def serialize(obj: Seq[_], typeOfT: Type, context: JsonSerializationContext): JsonElement = {
      val jList = new JArrayList[Any]()
      jList.addAll(obj.asJava)

      val innerTypes = getInnerTypes(typeOfT)
      val pt = ParameterizedTypeImpl.make(classOf[JList[_]], innerTypes, null)
      context.serialize(jList, pt)
    }

    override def deserialize(json: JsonElement, typeOfT: Type, context: JsonDeserializationContext): Seq[_] = {
      val innerTypes = getInnerTypes(typeOfT)
      val pt = ParameterizedTypeImpl.make(classOf[JList[_]], innerTypes, null)
      val jList: JList[_] = context.deserialize(json, pt)

      jList.asScala.toSeq
    }
  }
}