SortedPrettyPrinter.scala

import spray.json._

import scala.collection.immutable.{ListMap}
import scala.io.Source


trait SortedPrettyPrinter extends PrettyPrinter {
  override protected def printObject(members: Map[String, JsValue], sb: java.lang.StringBuilder, indent: Int) {
    sb.append("{\n")
    printSeq(ListMap(members.toSeq.sortBy(_._1):_*), sb.append(",\n")) { m =>
      this.printIndent(sb, indent + Indent)
      printString(m._1, sb)
      sb.append(": ")
      print(m._2, sb, indent + Indent)
    }
    sb.append('\n')
    printIndent(sb, indent)
    sb.append("}")
  }
}
object SortedPrettyPrinter extends SortedPrettyPrinter


//use like:
val contents = Map("hello", 123)
val output:String = SortedPrettyPrinter(contents.toJson)