Custom json serialization of structured scala case classes - scala

Question

I have some working jackson scala module code for roundtripping scala case classes. Jackson worked great for flat case classes but when I made one which contains a list of other case classes the amount of code I seemed to need was a lot. Consider:
abstract class Message
case class CardDrawn(player: Long, card: Int, mType: String = "CardDrawn") extends Message
case class CardSet(cards: List[CardDrawn], mType: String = "CardSet") extends Message
To get the CardSet to roundtrip to/from json with jackson scala module I used a custom serializer/deserializer written in java:
object ScrumGameMashaller {
val mapper = new ObjectMapper()
val module = new SimpleModule("CustomSerializer")
module.addSerializer(classOf[CardSet], new CardSetSerializer)
module.addDeserializer(classOf[CardSet], new CardSetDeserializer)
val scalaModule = DefaultScalaModule
mapper.registerModule(scalaModule)
mapper.registerModule(module)
def jsonFrom(value: Any): String = {
import java.io.StringWriter
val writer = new StringWriter()
mapper.writeValue(writer, value)
writer.toString
}
private[this] def objectFrom[T: Manifest](value: String): T =
mapper.readValue(value, typeReference[T])
private[this] def typeReference[T: Manifest] = new TypeReference[T] {
override def getType = typeFromManifest(manifest[T])
}
private[this] def typeFromManifest(m: Manifest[_]): Type = {
if (m.typeArguments.isEmpty) { m.runtimeClass }
else new ParameterizedType {
def getRawType = m.runtimeClass
def getActualTypeArguments = m.typeArguments.map(typeFromManifest).toArray
def getOwnerType = null
}
}
with serializer:
public class CardSetSerializer extends JsonSerializer<CardSet> {
#Override
public void serialize(CardSet cardSet, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
jgen.writeStartObject();
jgen.writeArrayFieldStart("cards");
List<CardDrawn> cardsDrawn = cardSet.cards();
scala.collection.Iterator<CardDrawn> iter = cardsDrawn.iterator();
while(iter.hasNext()){
CardDrawn cd = iter.next();
cdSerialize(jgen,cd);
}
jgen.writeEndArray();
jgen.writeStringField("mType", "CardSet");
jgen.writeEndObject();
}
private void cdSerialize(JsonGenerator jgen, CardDrawn cd) throws IOException, JsonProcessingException {
jgen.writeStartObject();
jgen.writeNumberField("player", cd.player());
jgen.writeNumberField("card", cd.card());
jgen.writeEndObject();
}
}
and matching deserializer:
public class CardSetDeserializer extends JsonDeserializer<CardSet> {
private static class CardDrawnTuple {
Long player;
Integer card;
}
#Override
public CardSet deserialize(JsonParser jsonParser, DeserializationContext cxt) throws IOException, JsonProcessingException {
ObjectCodec oc = jsonParser.getCodec();
JsonNode root = oc.readTree(jsonParser);
JsonNode cards = root.get("cards");
Iterator<JsonNode> i = cards.elements();
List<CardDrawn> cardObjects = new ArrayList<>();
while( i.hasNext() ){
CardDrawnTuple t = new CardDrawnTuple();
ObjectNode c = (ObjectNode) i.next();
Iterator<Entry<String, JsonNode>> fields = c.fields();
while( fields.hasNext() ){
Entry<String,JsonNode> f = fields.next();
if( f.getKey().equals("player")) {
t.player = f.getValue().asLong();
} else if( f.getKey().equals("card")){
t.card = f.getValue().asInt();
} else {
System.err.println(CardSetDeserializer.class.getCanonicalName()+ " : unknown field " + f.getKey());
}
}
CardDrawn cd = new CardDrawn(t.player, t.card, "CardDrawn");
cardObjects.add(cd);
}
return new CardSet(JavaConversions.asScalaBuffer(cardObjects).toList(), "CardSet");
}
}
This seems like a lot code to deal with something fairly vanilla in the scala. Can this code be improved (what did I miss that jackson has to make this easy)? Else is there a library which will do structured case classes automatically? The jerkson examples looked easy but that seems to have been abandoned.

Answer 1

Argonaut does a great job. Mark Hibbard helped me out with getting the example below working. All that is needed is to create a codec for the types and it will implicitly add an asJson to your objects to turn them into strings. It will also add a decodeOption[YourClass] to strings to extract an object. The following:
package argonaut.example
import argonaut._, Argonaut._
abstract class Message
case class CardDrawn(player: Long, card: Int, mType: String = "CardDrawn") extends Message
case class CardSet(cards: List[CardDrawn], mType: String = "CardSet") extends Message
object CardSetExample {
implicit lazy val CodecCardSet: CodecJson[CardSet] = casecodec2(CardSet.apply, CardSet.unapply)("cards","mType")
implicit lazy val CodecCardDrawn: CodecJson[CardDrawn] = casecodec3(CardDrawn.apply, CardDrawn.unapply)("player", "card", "mType")
def main(args: Array[String]): Unit = {
val value = CardSet(List(CardDrawn(1L,2),CardDrawn(3L,4)))
println(s"Got some good json ${value.asJson}")
val jstring =
"""{
| "cards":[
| {"player":"1","card":2,"mType":"CardDrawn"},
| {"player":"3","card":4,"mType":"CardDrawn"}
| ],
| "mType":"CardSet"
| }""".stripMargin
val parsed: Option[CardSet] =
jstring.decodeOption[CardSet]
println(s"Got a good object ${parsed.get}")
}
}
outputs:
Got some good json {"cards":[{"player":"1","card":2,"mType":"CardDrawn"},{"player":"3","card":4,"mType":"CardDrawn"}],"mType":"CardSet"}
Got a good object CardSet(List(CardDrawn(1,2,CardDrawn), CardDrawn(3,4,CardDrawn)),CardSet)

Answer 2

The question is old but maybe someone could still find it helpful. Apart from Argonaut, Scala has several Json libraries. Here you can find a list of them updated to the beginning of 2016 (and it still gives you a good overall picture).
Most of them (probably all) should allow you to come up with a drier version of your custom serializer/deserailizer. My preference goes to json4s which aims to provide a single AST across multiple libraries including Jackson (a bit like slf4j does for logging libraries). In this post you can find a working example of a Json custom serializer/deserializer using Json4s and Akka Http.