Json 使用 spray-json 反序列化 Scala case 对象

Question

我正在尝试使用 spray-json 为以下域模型编写自定义 JsonReader：

sealed trait OrderType
object OrderType {
  case object MARKET extends OrderType
  case object LIMIT extends OrderType
  case object STOP extends OrderType
  case object MARKET_IF_TOUCHED extends OrderType
  case object TAKE_PROFIT extends OrderType
  case object STOP_LOSS extends OrderType
  case object TRAILING_STOP_LOSS extends OrderType
}

这是我为此目的创建的自定义 JsonReader：

implicit object OrderTypeJsonReader extends JsonReader[OrderType] {
  def read(value: JsValue): OrderType = value match {
    case JsString("MARKET") => MARKET
    case JsString("LIMIT") => LIMIT
    case JsString("STOP") => STOP
    case JsString("MARKET_IF_TOUCHED") => MARKET_IF_TOUCHED
    case JsString("TAKE_PROFIT") => TAKE_PROFIT
    case JsString("STOP_LOSS") => STOP_LOSS
    case JsString("TRAILING_STOP_LOSS") => TRAILING_STOP_LOSS
    case _ => deserializationError("OrderType expected")
  }
}

鉴于json字符串和case object的名称相同，这里有什么办法可以避免代码重复吗？

Answer 1

您可以尝试用部分函数（或 Map）替换模式匹配：

val orderTypes = List(MARKET, LIMIT, STOP, MARKET_IF_TOUCHED, TAKE_PROFIT, STOP_LOSS, TRAILING_STOP_LOSS)

val string2orderType: Map[JsValue, OrderType] = 
  orderTypes.map(ot => (JsString(ot.toString), ot)).toMap

implicit object OrderTypeJsonReader extends JsonReader[OrderType] {
  def read(value: JsValue): OrderType = 
    string2orderType.getOrElse(value, deserializationError("OrderType expected"))
}

缺点是您必须手动指定所有案例对象的列表。您可以尝试使用反射来生成它。也许这个问题对有帮助。那么你可以拥有：

import scala.reflect.runtime.universe

private val tpe = universe.typeOf[OrderType]
private val clazz = tpe.typeSymbol.asClass

private def objectBy[T](name: String): T = Class.forName(OrderType.getClass.getName + name + "$").newInstance().asInstanceOf[T]

val string2orderType: Map[JsValue, OrderType] = clazz.knownDirectSubclasses.map { sc =>
  val objectName = sc.toString.stripPrefix("object ")
  (JsString(objectName), objectBy[OrderType](objectName))
}.toMap

implicit object OrderTypeJsonReader extends JsonReader[OrderType] {
  def read(value: JsValue): OrderType = string2orderType.getOrElse(value, deserializationError("OrderType expected"))
}

请参阅有关向 Spray 添加默认大小写 class 格式的讨论：https://github.com/spray/spray-json/issues/186

更新 解决评论

Is it possible to 'generify' it for any type T? I have quite a few of those sealed trait / case object enumerations and would prefer to have the boilerplate kept to minimum.

我想到了这个：

import spray.json._
import Utils._

sealed trait OrderStatus
object OrderStatus {
  case object Cancelled extends OrderStatus
  case object Delivered extends OrderStatus
  // More objects...

  implicit object OrderStatusJsonReader extends ObjectJsonReader[OrderStatus]
}

sealed trait OrderType
object OrderType {
  case object MARKET extends OrderType
  case object LIMIT extends OrderType
  // More objects...

  implicit object OrderTypeJsonReader extends ObjectJsonReader[OrderType]
}

object Utils {
  import scala.reflect.ClassTag
  import scala.reflect.runtime.universe._

  def objectBy[T: ClassTag](name: String): T = {
    val c = implicitly[ClassTag[T]]
    Class.forName(c + "$" + name + "$").newInstance().asInstanceOf[T]
  }

  def string2trait[T: TypeTag : ClassTag]: Map[JsValue, T] = {
    val clazz = typeOf[T].typeSymbol.asClass
    clazz.knownDirectSubclasses.map { sc =>
      val objectName = sc.toString.stripPrefix("object ")
      (JsString(objectName), objectBy[T](objectName))
    }.toMap
  }

  class ObjectJsonReader[T: TypeTag : ClassTag] extends JsonReader[T] {
    val string2T: Map[JsValue, T] = string2trait[T]
    def defaultValue: T = deserializationError(s"${ implicitly[ClassTag[T]].runtimeClass.getCanonicalName } expected")
    override def read(json: JsValue): T = string2T.getOrElse(json, defaultValue)
  }
}

然后你可以像这样使用它：

import OrderType._
import OrderStatus._
JsString("MARKET").convertTo[OrderType]
JsString(OrderStatus.Cancelled.toString).convertTo[OrderStatus]

I also tried code from spray-json github issue and it can be used like so:
implicit val orderTypeJsonFormat: RootJsonFormat[OrderType] = 
  caseObjectJsonFormat(MARKET, LIMIT, STOP, MARKET_IF_TOUCHED, TAKE_PROFIT, STOP_LOSS, TRAILING_STOP_LOSS)

不幸的是，这需要您明确指定所有对象。如果你想要这样，那么，我认为，我的第一个建议（不经思考）更好。（因为它没有反射:-)）

Json 使用 spray-json 反序列化 Scala case 对象

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