Spray Json Marshalling Mutable Objects

I'm having difficulty marshalling mutable objects in my case classes in my Application. I'm using the spray libraries and I have made the necessary imports

    import spray.json._
    import DefaultJsonProtocol._
    import spray.httpx.SprayJsonSupport._

But I get the following error when I try to provide the companion object for my case class.

    case class CasePage(pageId:String,userList:ListBuffer[String],commentList:ListBuffer[String],picList:ListBuffer[String],likeList:ListBuffer[String])

    object CasePage extends DefaultJsonProtocol {
            implicit val impUser = jsonFormat5(CasePage.apply)
            }

    could not find implicit value for evidence parameter of type CasePage.JF[scala.collection.mutable.ListBuffer[String]]

The other case classes without the mutable objects work fine. Just having trouble with scala.collection.mutable class objects. What am I missing?

Thank you

Solution

You need a RootJsonFormat instance for ListBuffer. But note that using collection.mutable in case classes is not idiomatic Scala.

package com.example

import spray.json._
import DefaultJsonProtocol._

import scala.collection.mutable.ListBuffer

object SO33943345 {
  case class CasePage(pageId: String,
    userList: ListBuffer[String],
    commentList: ListBuffer[String],
    picList: ListBuffer[String],
    likeList: ListBuffer[String])

  implicit def listBufferFormat[T :JsonFormat] = new RootJsonFormat[ListBuffer[T]] {
    def write(listBuffer: ListBuffer[T]) = JsArray(listBuffer.map(_.toJson).toVector)
    def read(value: JsValue): ListBuffer[T] = value match {
      case JsArray(elements) => elements.map(_.convertTo[T])(collection.breakOut)
      case x => deserializationError("Expected ListBuffer as JsArray, but got " + x)
    }
  }

  object CasePage extends DefaultJsonProtocol {
    implicit val impUser = jsonFormat5(CasePage.apply)
  }

  def main(args: Array[String]): Unit = {
    val cp = CasePage("1",
      ListBuffer("User1", "User2"),
      ListBuffer("Comment1", "Comment2"),
      ListBuffer("Pic1", "Pic2"),
      ListBuffer("Like1", "Like2"))

    println(cp.toJson.prettyPrint)
  }
}