How to serialize sealed abstract class with Json4s in Scala? - scala

Question

How do i serialize a sealed abstract class with Json4s in Scala?
The following classes are defined:
sealed abstract class Person extends Product with Serializable
case class Spouse(name: String, age: Int) extends Person
case class Customer(name: String, age: Int, spouse: Spouse) extends Person
I create an object of type Customer:
val customer: Customer = Customer("Joe", 35, Spouse("Marilyn", 33))
Then I serialize to JSON:
implicit val formats = DefaultFormats
val serialized = write(customer)
That works fine. But then I try to deserialize:
val parsedObj = Serialization.read[Person](serialized)
Here I keep getting error:
org.json4s.package$MappingException: Parsed JSON values do not match with class constructor
I spent a lot of time trying to make this work...

Answer 1

After a lot of googling around and a close read of the Json4s documentation i found out that I need a custom Serializer to make it work.
However, it took me a while to figure out that I need to set the formats to actually use the custom Serializer.
Here is the code that works for me.
Simple Example:
import org.json4s._
import org.json4s.native.JsonMethods._
import org.json4s.native.Serialization.{read, write}
import org.json4s.native.Serialization
sealed abstract class Person extends Product with Serializable
case class Spouse(name: String, age: Int) extends Person
case class Customer(name: String, age: Int, spouse: Spouse) extends Person
val customer: Customer = Customer("Joe", 35, Spouse("Marilyn", 33))
implicit val formats = Serialization.formats(NoTypeHints) + PersonSerializer
val serialized = write(customer)
val parsedObj = Serialization.read[Person](serialized)
Custom Serializer:
object PersonSerializer extends Serializer[Person] {
private val PersonClass = classOf[Person]
def deserialize(implicit format: Formats)
: PartialFunction[(TypeInfo, JValue), Person] = {
case (TypeInfo(PersonClass, _), json) =>
json match {
case JObject(List(
JField("name", JString(d)),
JField("age", JInt(f)),
("spouse", JObject(List(JField("name", JString(g)), JField("age", JInt(h)))))
)) => Customer(d, f.toInt, Spouse(g, h.toInt))
case JObject(List(
JField("name", JString(d)),
JField("age", JInt(f))
)) => Spouse(d, f.toInt)
}
}
def serialize(implicit format: Formats): PartialFunction[Any, JValue] = {
case x: Customer =>
JObject(List(
JField("name", JString(x.name)),
JField("age", JInt(x.age)),
("spouse", JObject(List(JField("name", JString(x.spouse.name)), JField("age", JInt(x.spouse.age)))))
))
case x: Spouse =>
JObject(List(
JField("name", JString(x.name)),
JField("age", JInt(x.age))
))
}
}
Output
scala> val serialized = write(customer)
serialized: String = {"name":"Joe","age":35,"spouse":{"name":"Marilyn","age":33}}
scala> val parsedObj = Serialization.readPerson
parsedObj: Person = Customer(Joe,35,Spouse(Marilyn,33))