How does json4s use DefaultFormats to serialize and deserialize json?

Question

I'm trying to understand how json4s serializes and deserializes json, specifically how it uses formats. Are there any online references that show how json4s uses the DefaultFormats to serialize and deserialize json? The official json4s website doesn't shed much light on this.

Answer 1

DefaultFormats is a provided implementation of Formats trait.

You can see how it used in Extraction.decompose and Extraction.extract methods (json4s uses these methods for serialization/deserialization).

Extraction.extract uses Extraction.convert :

private[this] def convert(key: String, target: ScalaType, formats: Formats): Any = {
  val targetType = target.erasure
  targetType match {
    case tt if tt == classOf[String] => key
    case tt if tt == classOf[Symbol] => Symbol(key)
    case tt if tt == classOf[Int] => key.toInt
    case tt if tt == classOf[JavaInteger] => JavaInteger.valueOf(key.toInt)
    case tt if tt == classOf[BigInt] => key.toInt
    case tt if tt == classOf[Long] => key.toLong
    case tt if tt == classOf[JavaLong] => JavaLong.valueOf(key.toLong)
    case tt if tt == classOf[Short] => key.toShort
    case tt if tt == classOf[JavaShort] => JavaShort.valueOf(key.toShort)
    case tt if tt == classOf[Date] => formatDate(key, formats)
    case tt if tt == classOf[Timestamp] => formatTimestamp(key, formats)
    case _ =>
      val deserializer = formats.customKeyDeserializer(formats)
      val typeInfo = TypeInfo(targetType, None)
      if(deserializer.isDefinedAt((typeInfo, key))) {
        deserializer((typeInfo, key))
      } else {
        fail("Do not know how to deserialize key of type " + targetType + ". Consider implementing a CustomKeyDeserializer.")
      }
  }
}

So, json4s tries to find in Formats custom deserializer for all unknown types.

Extraction.decompose uses Extraction.decomposeObject under the hood, where json4s try to serialize all types by a custom serializer:

if (formats.customSerializer(formats).isDefinedAt(a)) {
  current addJValue formats.customSerializer(formats)(a)
}