How to serialize object to AST using json4s?

问题:

I am writing a Customer Serializer. In that Serializer I would like to somehow say: "and this thing you already know how to serialize".

My current approach looks like that:

    import org.json4s.native.Serialization._
    import org.json4s.JsonDSL.WithBigDecimal._

    object WindowSerializer extends CustomSerializer[Window](format =>
      ( [omitted],
        {
          case Window(frame, size) =>

            ( "size" -> size ) ~
            ( "frame" -> parse(write(frame)) )
        }))

That parse(write(frame)) things is both ugly and inefficient. How to fix that?

回答1:

You can call Extraction.decompose(a: Any)(implicit formats: Formats): JValue which produces a JValue from some value using runtime reflection.

import org.json4s._
import org.json4s.jackson.JsonMethods._
import org.json4s.JsonDSL._
import java.util.UUID

case class Thing(name: String)
case class Box(id: String, thing: Thing)

class BoxSerializer extends CustomSerializer[Box](format => ({
  case jv: JValue =>
    val id = (jv \ "id").extract[String]
    val thing = (jv \ "thing").extract[Thing]
    Box(id, thing)
}, {
  case b: Box =>
    ("token" -> UUID.randomUUID().toString()) ~
      ("id" -> box.id) ~
      ("thing" -> Extraction.decompose(box.thing))
}))

implicit val formats = DefaultFormats + new BoxSerializer

val box = Box("1000", Thing("a thing"))

// decompose the value to JSON 
val json = Extraction.decompose(box)
println(pretty(json))
//  {
//    "token" : "d9bd49dc-11b4-4380-ab10-f6df005a384c",
//    "id" : "1000",
//    "thing" : {
//      "name" : "a thing"
//    }
//  }

// and read a value of type Box back from the JSON
println(json.extract[Box])
// Box(1000,Thing(a thing))