How to extract my case class from Json using Json4s? - json

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, e.g. 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