A more up-to-date example of how map key serialization in jackson does not work.

JacksonMapTest2.scala

package test

import scala.collection.{immutable => im}
import com.fasterxml.jackson.databind.annotation.JsonDeserialize
import com.fasterxml.jackson.databind.deser.std.StdKeyDeserializer
import com.fasterxml.jackson.databind.{ObjectMapper, DeserializationContext}
import com.fasterxml.jackson.module.scala.DefaultScalaModule
import com.fasterxml.jackson.databind.JsonSerializer
import com.fasterxml.jackson.databind.annotation.JsonSerialize
import com.fasterxml.jackson.databind.SerializerProvider
import com.fasterxml.jackson.databind.ser.std.StdSerializer
import com.fasterxml.jackson.core.JsonGenerator
import com.fasterxml.jackson.annotation.JsonValue


object JacksonMapTest2 {

  def main(args: Array[String]): Unit = {
    val pair = new Pair(2, 3)
    val map = im.HashMap(pair -> "Horsey")
    val thingie = new Thingie(map)
    val mapper = new ObjectMapper()
    mapper.registerModule(DefaultScalaModule)
    val str = mapper.writeValueAsString(thingie)
    println(str)
    val result = mapper.readValue(str, classOf[Thingie])
    result._1.foreach(e => println(e._1.getClass))
    println(result)
  }
}

class Pair(val arg1: Int, val arg2: Int)
  extends Product2[Int, Int]
{
  def _1 = arg1
  def _2 = arg2
  
  override def canEqual(that: Any): Boolean = that.isInstanceOf[Pair]
  
//  override
//  def toString(): String = 
//  {
//    "{\"arg1\":%d,\"arg2\":%d}".format(arg1, arg2)
//  }
}

object Pair 
{
  class PairKD 
    extends StdKeyDeserializer(classOf[Pair])
  {
    override
    def _parse(key: String , ctxt: DeserializationContext): Pair = 
    {
      val mapper = new ObjectMapper()
      mapper.registerModule(DefaultScalaModule)
      try {
        return mapper.readValue(key, classOf[Pair])
      }
      catch {
        case ex: Exception => throw ctxt.weirdKeyException(_keyClass, key, ex.getMessage);
      }
    }
  }
  
  class PairKS
    extends StdSerializer[Pair](classOf[Pair])
  {
    override
    def serialize(value: Pair, jgen: JsonGenerator, provider: SerializerProvider)
    {
      val mapper = new ObjectMapper().registerModule(DefaultScalaModule)
      jgen.writeFieldName(mapper.writeValueAsString(value));
    }
  }
}

class Thingie(
    @JsonSerialize(keyUsing=classOf[test.Pair.PairKS])
    @JsonDeserialize(keyUsing=classOf[test.Pair.PairKD])
    val arg1: im.HashMap[Pair, String])
  extends Product1[im.HashMap[Pair, String]]
{
  def _1 = arg1
  
  override def canEqual(that: Any): Boolean = that.isInstanceOf[Thingie]
}