如何使用 Json4s 从 Json 中提取我的案例类？

Question

In an akka scala applicatoon I consume a rest endpoint. Hence, I want to map its responses to case classes, yet I also want to ease working with those case classes by transforming certain properties, eg those containing dates.

So given a Json:

{
    "id": "20180213165959sCdJr",
    "createdAt": "2018-02-13T16:59:59.570+0000",
    "finishedAt": "2018-02-13T17:00:18.118+0000"
}

I want to create such a clase class out of it:

case class FinishedRun
(
  id: String,
  createdAt: Date,
  finishedAt: Date
)

I created this construtor:

object FinishedRun {

  def apply(id: String,
            createdAt: String,
            finishedAt: String
           ): FinishedRun = {

    val getDate = (jsonValue: String) => {
      val format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ")
      format.parse(jsonValue)
    }


    new FinishedRun(id, createdAt = getDate(createdAt), finishedAt = getDate(finishedAt))
  }
}

While this works for initializing a case class from scratch, I have trouble extracting this case class with the help of the json4s libary through the parse(json).as[FinishedRun] approach.

It appears that json4s does not call the case class' constructor and hence cannot extract it, throwing:

No usable value for createdAt
Invalid date '2018-02-13T16:59:59.570+0000'
org.json4s.package$MappingException: No usable value for createdAt
Invalid date '2018-02-13T16:59:59.570+0000'
    at org.json4s.reflect.package$.fail(package.scala:95)

What am I missing to have Json4s parse the Date properly?

---

Here is my test case:

import org.json4s._
import org.json4s.native.JsonMethods._
import org.scalatest.FlatSpec


class MarshallingTest extends FlatSpec {
  implicit val formats = DefaultFormats

  it should "marshall json object with date iso strings into a case class with Date properties" in {
    val json =
      """
        |{
        |    "id": "20180213165959sCdJr",
        |    "createdAt": "2018-02-13T16:59:59.570+0000",
        |    "finishedAt": "2018-02-13T17:00:18.118+0000"
        |}
      """.stripMargin

    val expected = FinishedRun(
      id = "20180213165959sCdJr",
      createdAt = "2018-02-13T16:59:59.570+0000",
      finishedAt = "2018-02-13T17:00:18.118+0000"
    )
    val actual = parse(json).extract[FinishedRun]

    assert(actual == expected)
  }
}

Answer 1

You need to define your CustomSerializer(1) , CustomSerializer(2) . I changed the date type from Date to ZonedDateTime :

import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter

import org.json4s._
import org.json4s.JsonAST._
import org.json4s.JsonDSL._
import org.json4s.native.JsonMethods._
import org.scalatest.FlatSpec

case class FinishedRun
(
  id: String,
  createdAt: ZonedDateTime,
  finishedAt: ZonedDateTime
)

object FinishedRunSerializer {
  val dateTimeFmt = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSZ")
}

class FinishedRunSerializer extends CustomSerializer[FinishedRun](
  format => ( {
    case jObj: JObject =>
      implicit val fmt = format
      val id = (jObj \ "id").extract[String]
      val created = ZonedDateTime.parse((jObj \ "createdAt").extract[String],
        FinishedRunSerializer.dateTimeFmt)
      val finished = ZonedDateTime.parse((jObj \ "finishedAt").extract[String],
        FinishedRunSerializer.dateTimeFmt)
      FinishedRun(id, created, finished)
  }, {
    case finishedRun: FinishedRun =>
      ("id" -> finishedRun.id) ~
        ("createdAt" -> finishedRun.createdAt.format(FinishedRunSerializer.dateTimeFmt)) ~
        ("finishedAt" -> finishedRun.finishedAt.format(FinishedRunSerializer.dateTimeFmt))
  }
  ))

In your test or the place when you use it do not forget to bring FinishedRunSerializer :

implicit val formats = DefaultFormats + new FinishedRunSerializer()

Answer 2

A simple solution to your problem might be to use the Serialization trait in org.json4s . You should be able to do something like this:

val finishedRun = read[FinishedRun](json)

Please refer to this link for details and examples: https://github.com/json4s/json4s#serializing-polymorphic-lists