Why can't I parse this JSON string with a custom Scala type (Jerkson)?

Question

I've got the following code that is meant to parse the following JSON string with Jerkson, the Scala wrapper for Jackson:

Code: val vertex = Json.parse[Ranked[UserVertex]](rawString)

Json: "originalPos":"515693049","rankScore":1.2409032357406248E8,"ranked":{"userId":"101010110","userName":"John","channel":"thirtyfive"}}

The following JSON is represented by this Scala class:

case class Ranked[T] (
  val originalPos: String,

  val rankScore: Double,

  val ranked: T
)

and T is a UserVertex class with the following fields above (userId, userName, channel). When I try to parse the JSON string I get the following error:

[error] application - Could not parse the string:
{"originalPos":"655330261","rankScore":1.2423105672295567E8,"ranked":{"userId":"655330261","userName":"Sheile Mercado-Mallari","channel":"facebook"}}
java.lang.ClassNotFoundException: T
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    at com.codahale.jerkson.util.CaseClassSigParser$.loadClass(CaseClassSigParser.scala:153)

It looks like the ClassLoader can't find the class, but it's definitely loaded. Any idea what the root cause of this is? Do I have some sort of multiple class loader issue here? Thanks!

Answer 1

In this case (a generic case class) you need to define your reader (It is a known constraint in Play)

case class UserVertex(userId:String, userName: String, channel: String)
case class Ranked[T](originalPos: String, rankScore: Double, ranked: T)

Note that you don't need val keyword in case classes as it will be automatically provided by using the case keyword.

import play.api.libs.json._
import play.api.libs.functional.syntax._

implicit def vertexReader: Reads[UserVertex] = Json.reads[UserVertex]

implicit def rankedReader[T](implicit reads: Reads[T]): Reads[Ranked[T]] = (
    (__ \ "originalPos").read[String] ~ 
    (__ \ "rankScore").read[Double] ~ 
    (__ \ "ranked").read[T]
    )(Ranked.apply[T] _)

Now you can convert the following raw json to Ranked

val rawString =
    """{
      |  "originalPos": "515693049",
      |  "rankScore": 1.2409032357406248E8,
      |  "ranked": {
      |    "userId": "101010110",
      |    "userName": "John",
      |    "channel": "thirtyfive"
      |  }
      |}""".stripMargin

with the following code

val rankedJson = Json.parse(rawString)
val ranked = Json.fromJson[Ranked[UserVertex]](rankedJson).get

Disclaimer

Tested with Play 2.3.5 and Scala 2.11.4