JSON creation in Scala - json

Question

I am building HTTP service with scala-play. I want to serve the JSON response.
To serve JSON response, I created case classes and created instance of Writes[T] class which is similar to following
case class Point(X: Double, Y: Double)
implicit val pointWrites = new Writes[Point] {
def writes(point: Point) = Json.obj(
"X" -> point.X,
"Y" -> point.Y
)
}
I can server response like: val json = Json.toJson(Point(10, 20))
I followed above approach from play documentation.
Why do I need to define class and create Writes[T] instance for serve JSON response.
If I have to create JSON for dozens of classes, do I have to define all classes and create instances for Writes[T]?
I can easily convert any class data to JSON with following method:
def getJSON(data: Map[String, String]): JsObject = {
JsObject(data.map { el => el._1 -> JsString(el._2) })
}
All I need do is create Map[String, String] for any class to serve JSON.
Why play documentation recommends to define case class and create instance of Writes[T] to create JSON representation?
Is there any other way to serve create JSON in scala? I want to keep response time minimal so that JSON creation must be lightening fast.

Answer 1

You don't need to create the Writes instance. Play already have a default formatter for all the regular types (Integer/String/Boolean..)
You just need to write the case class and add an implicit formatter in the companion object.
See json.scala format[A] description from play:
/**
* Creates a Format[T] by resolving case class fields & required implicits at COMPILE-time
*
* If any missing implicit is discovered, compiler will break with corresponding error.
* {{{
* import play.api.libs.json.Json
*
* case class User(name: String, age: Int)
*
* implicit val userWrites = Json.format[User]
* // macro-compiler replaces Json.format[User] by injecting into compile chain
* // the exact code you would write yourself. This is strictly equivalent to:
* implicit val userWrites = (
* (__ \ 'name).format[String] and
* (__ \ 'age).format[Int]
* )(User.apply, unlift(User.unapply))
* }}}
*/
def format[A] = macro JsMacroImpl.formatImpl[A]
Back to your example, you only need:
case class Point(X: Double, Y: Double)
object Point{
implicit val fmtJson = Json.format[Point]
}
to get the json text just do:
val json = Point.fmtJson.writes(Point(10, 20)).toString()
will result: json = {"X":10,"Y":20}