Play Framework: How to convert JSON to List[Int]

Question

Given the following JSON string...

val jsonStr = "[1, 2, 3]"

... how do I convert it into a List[Int]? The following statement returns a JsValue, which does not contain method read:

Json.parse(jsonStr)

Answer 1

Play provides several ways to decode JSON. The "simplest" is as:

scala> import play.api.libs.json._
import play.api.libs.json._

scala> val jsonStr = "[1, 2, 3]"
jsonStr: String = [1, 2, 3]

scala> val json = Json.parse(jsonStr)
json: play.api.libs.json.JsValue = [1,2,3]

scala> val xs = json.as[List[Int]]
xs: List[Int] = List(1, 2, 3)

This will throw an exception in the case that you don't actually have a list of integers, though, so it's generally a bad idea. asOpt and validate are much better:

scala> val xsMaybe = json.asOpt[List[Int]]
xsMaybe: Option[List[Int]] = Some(List(1, 2, 3))

scala> val xsResult = json.validate[List[Int]]
xsResult: play.api.libs.json.JsResult[List[Int]] = JsSuccess(List(1, 2, 3),)

Now you're forced by the type system to deal with the possibility of error, which means fewer surprises at runtime.

All of these methods take an implicit Reads[_] argument. Play provides instances for Reads[Int] and Reads[List[A: Reads]] out of the box, and you can get the same syntax for your own types by defining your own Reads instances.