组合类中的多态lift-json反序列化 [英] Polymorphic lift-json deserialization in a composed class

问题描述

我正在尝试使用Lift-Json将json对象自动反序列化为scala类，并且内部使用一个坐标类来存储GeoJson信息.

I am trying to automatically deserialize json object to a scala class using Lift-Json with a coordinate class inside used to store GeoJson information.

case class Request(name:String, geometry:Geometry)

sealed abstract class Geometry

case class Point(coordinates:(Double,Double)) extends Geometry
case class LineString(coordinates:List[Point]) extends Geometry
case class Polygon(coordinates:List[LineString]) extends Geometry

我想反序列化这样的json字符串:

I want to deserialize a json string like this:

{
name:"test",
geometry:{
   "type": "LineString",
   "coordinates": [ [100.0, 0.0], [101.0, 1.0] ]
  }
}

进入Request案例类，并在Geometry字段中使用正确的LineString运行时类.我想我应该使用TypeHint，但是如何?这是正确的方法还是我应该创建三个不同的请求(RequestPoint，RequestLineString和RequestPolygon)? 这将是反序列化的Scala代码:

into a Request case class with the right LineString runtime class in the Geometry field. I guess I should use a TypeHint but how?. Is this the correct approach or should I create three different Request (RequestPoint,RequestLineString and RequestPolygon)? This would be the Scala code to deserialize:

val json = parse(message)
json.extract[Request]

推荐答案

是的，您需要对诸如Geometry之类的求和类型使用类型提示.这是一个示例:

Yes, you need to use type hints for sum types like Geometry. Here's one example:

implicit val formats = DefaultFormats.withHints(ShortTypeHints(List(classOf[Point], classOf[LineString], classOf[Polygon])))

val r = Request("test", LineString(List(Point(100.0, 0.0), Point(101.0, 1.0))))

Serialization.write(r)

{
"name":"test",
"geometry":{
  "jsonClass":"LineString",
  "coordinates":[{"jsonClass":"Point","coordinates":{"_1$mcD$sp":100.0,"_2$mcD$sp":0.0}},{"jsonClass":"Point","coordinates":{"_1$mcD$sp":101.0,"_2$mcD$sp":1.0}}]}
}

并不是您想要的.由于要更改Point的默认序列化方案，因此需要为该类型提供一个自定义序列化器.

Not quite what you wanted. Since you want to change the default serialization scheme for Points, you need to provide a custom serializer for that type.

class PointSerializer extends Serializer[Point] {
  private val Class = classOf[Point]

  def deserialize(implicit format: Formats) = {
    case (TypeInfo(Class, _), json) => json match {
      case JArray(JDouble(x) :: JDouble(y) :: Nil) => Point(x, y)
      case x => throw new MappingException("Can't convert " + x + " to Point")
    }
  }

  def serialize(implicit format: Formats) = {
    case p: Point => JArray(JDouble(p.coordinates._1) :: JDouble(p.coordinates._2) :: Nil)
  }
}

// Configure it
implicit val formats = DefaultFormats.withHints(ShortTypeHints(List(classOf[Point], classOf[LineString], classOf[Polygon]))) + new PointSerializer

Serialization.write(r)

{
  "name":"test",
  "geometry":{
    "jsonClass":"LineString",
    "coordinates":[[100.0,0.0],[101.0,1.0]]
  }
}

更好，但如果您需要将名为"jsonClass"的默认字段更改为类型"，则还需要进行其他配置:

Better, but you need one more configuration if you need to change the default field named as 'jsonClass' to 'type':

implicit val formats = new DefaultFormats { 
  override val typeHintFieldName = "type" 
  override val typeHints = ShortTypeHints(List(classOf[Point], classOf[LineString], classOf[Polygon]))
} + new PointSerializer

Serialization.write(r)

{
  "name":"test",
  "geometry":{
    "type":"LineString",
    "coordinates":[[100.0,0.0],[101.0,1.0]]
  }
}

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