Problems converting JSON objects in Scala

Question

I'm trying to make a simple example of a class serialization in Scala using json4s library, but even after extensively searching for it on the internet, unfortunately I couldn't find any satisfatory sample that would solve my problem.

Basically I have a simple class called Person and I want to extract an instance of this class from a JSON string.

case class Person(
    val name: String,
    val age: Int,
    val children: Option[List[Person]]
)

So when I do this:

val jsonStr = "{\"name\":\"Socrates\", \"age\": 70}"
println(Serialization.read[Person](jsonStr))

I get this output:

"Person(Socrates,70,None)" // works fine!

But when I have no age parameter in JSON string, I get this error:

Exception in thread "main" org.json4s.package$MappingException: No usable value for age

I know that Person class has two required parameters in its constructor, but I would like to know if there's a way to make this conversion through a parser or something similar.

Also, I've tried to make this parser, but no success.

Thanks in advance for any help.

What do you want to receive as a result of parsing the JSON when there is no "age" parameter? — Dan Getz
– Dan Getz, Commented May 23, 2015 at 1:31
Obviously you need to use Option[Int] type for age or create your custom serializer: github.com/json4s/json4s#serializing-non-supported-types — sap1ens
– sap1ens, Commented May 23, 2015 at 3:17
@dan-getz, I will receive those JSON strings from a third party application, so I cannot guarantee that all parameters will be present on their requests, so I'm just trying to figure it out the options I have to make my parser robust (I started studying Scala language just a week ago). — blzn
– blzn, Commented May 23, 2015 at 17:03
@sap1ens, as I said, I've tried to make this parser using this exact sample, but no success. Thanks anyway. — blzn
– blzn, Commented May 23, 2015 at 17:05
"What are my options?" You could leave the existing exception as is, throw a different exception, return an Option, return a special value, use Option for the age parameter, use a special age number to represent "not specified", return a Try or Either... the list goes on and on. Are you asking for help coding a parser, or deciding what your code should do in the first place? — Dan Getz
– Dan Getz, Commented May 23, 2015 at 17:56

Kulu Limpa · Accepted Answer · 2015-05-23 03:43:53Z

Assuming you do not want to make age optional by setting its type to Option, then you could write a custom serializer by extending CustomSerializer[Person]. The custom serializer takes a function from Formats to a tuple of PartialFunction[JValue, Person] (your Person deserializer) and PartialFunction[Any, JValue] (your serializer).

Assuming Person.DefaultAge is the default value you want to set for age if the age is not given, then the custom serializer could look as follows:

object PersonSerializer extends CustomSerializer[Person](formats => ( {
  case JObject(JField("name", JString(name)) :: JField("age", JInt(age)) :: Nil) => Person(name, age.toInt, None)
  case JObject(JField("name", JString(name)) :: JField("age", JInt(age)) :: JField("children", JArray(children)) :: Nil) => Person(name, age.toInt, Some(children map (child => formats.customDeserializer(formats).apply(TypeInfo(classOf[Person], None), child).asInstanceOf[Person])))
  case JObject(JField("name", JString(name)) :: Nil) => Person(name, Person.DefaultAge, None)
  case JObject(JField("name", JString(name)) :: JField("children", JArray(children)) :: Nil) => Person(name, Person.DefaultAge, Some(children map (child => formats.customDeserializer(formats).apply(TypeInfo(classOf[Person], None), child).asInstanceOf[Person])))
}, {
  case Person(name, age, None) => JObject(JField("name", JString(name)) :: JField("age", JInt(age)) :: Nil)
  case Person(name, age, Some(children)) => JObject(JField("name", JString(name)) :: JField("age", JInt(age)) :: JField("children", formats.customSerializer(formats).apply(children)) :: Nil)
}))

This could probably be simplified as there is a lot of repetition. Also, there might be a better way to recursively call serialization/deserialization.

Thank you very much, @KuluLimpa! It worked softly as I expected.

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