How to convert java.time.LocalDate to JSValue in Play? - json

Question

I'm using Play 2.3.x. The code is writing an object into JSON.
How to convert a model, which has a field of java.time.LocalDate, to JSValue?
case class ModelA(id: Int, birthday: LocalDate)
implicit val modelAWrites: Writes[ModelA] = (
(JsPath \ "id").write[Int] and
(JsPath \ "birthday").write[LocalDate]
)(unlift(ModelA.unapply))
The compiler complains that :
No Json serializer found for type java.time.LocalDate. Try to implement an implicit Writes or Format for this type.
Thanks.

Answer 1

If representing the LocalDate as an ISO date string (e.g. "2016-07-09") is sufficient, then the formatter becomes quite simple:
implicit val localDateFormat = new Format[LocalDate] {
override def reads(json: JsValue): JsResult[LocalDate] =
json.validate[String].map(LocalDate.parse)
override def writes(o: LocalDate): JsValue = Json.toJson(o.toString)
}
Here's a free test to prove it:
package com.mypackage
import java.time.LocalDate
import org.scalatest.{Matchers, WordSpecLike}
import play.api.libs.json.{JsString, Json}
class MyJsonFormatsSpec extends WordSpecLike with Matchers {
import MyJsonFormats._
"MyJsonFormats" should {
"serialize and deserialize LocalDates" in {
Json.toJson(LocalDate.of(2016, 7, 9)) shouldEqual JsString("2016-07-09")
JsString("2016-07-09").as[LocalDate] shouldEqual LocalDate.of(2016, 7, 9)
}
}
}

Answer 2

Play is able to write most primitive data types to Json, such as Ints, Strings, and so on. It however is unable to write random types to Json, which is fair enough since otherwise the framework would have to provide a serializer for any type which seems a bit unrealistic!
So, Play is telling you it doesn't know how to serialize a type of java.time.LocalDate. You need to teach Play how to write an instance of LocalDate to Json.
See here for docs on how to do that: https://www.playframework.com/documentation/2.3.x/ScalaJsonCombinators

Answer 3

Instead using LocalDate form java.time.LocalDate use LocalDate from org.joda.time.LocalDate or write your custom implicit
implicit val dateFormat =
Format[LocalDate](Reads.jodaLocalDateReads(pattern), Writes.jodaLocalDateWrites(pattern))

Answer 4

I've had this problem too, I solved it by creating the Reads and Writes myself.
implicit val localDateReads: Reads[LocalDate] = (
(__ \ "year").read[Int] and
(__ \ "month").read[Int] and
(__ \ "day").read[Int]
) (LocalDate.of(_,_,_))
implicit val LocalDateWrites = new Writes[LocalDate] {
def writes(date: LocalDate) = Json.obj(
"day" -> date.getDayOfMonth,
"month" -> date.getMonthValue,
"year" -> date.getYear
)}
In JSON it will look like this:
"date": {
"day": 29,
"month": 8,
"year": 1993
}