JSON4S 无法反序列化对象

Question

I have one class which extends a trait as shown below:

case class Phone(number: String)

trait Person {
  def name: String
  def phones: Seq[Phone]
}

case class Employee(name: String, phones: Seq[Phone] = Seq.empty) extends Person

As shown above, Employee class extends Person trait.

I am trying to serialize and then deserialize one object of type Employee as shown below:

implicit val formats = DefaultFormats

val emp: Person = Employee("foo")
val c = write(emp)
val e2 = parse(c).extract[Person]

Object emp gets serialized properly

emp: Person = Employee(foo,List())
c: String = {"name":"foo","phones":[]}

but parse(c).extract[Person] method fails with following exception:

org.json4s.package$MappingException: No constructor for type Person, 
JObject(List((name,JString(foo)), (phones,JArray(List()))))

I tried to add FieldSerializer like below but got same exception.

implicit val formats = DefaultFormats + FieldSerializer[Employee with Person]()

So I started writing custom serializer which looks like below:

case object PersonSerializer extends CustomSerializer[Person](formats => (
  {
    case JObject(
      List(
        JField("name", JString(name)),
        JField("phones", JArray(List(phones)) )
      )
    ) => Employee(name, phones)
  },
  {
    case Employee(name, phones) => JObject(JField("name", JString(name)))
  }
))

but this serializer fails to compile with following error:

type mismatch;
found   : org.json4s.JsonAST.JValue
required: Seq[Phone]
) => Employee(name, phones)

So can you please help me with either writing custom serializer or to convert JValue to Seq[Phone] ?

Answer 1

I don't think your actual problem is the one you think it is--you can't extract a Person, it's a trait! I'd extract an Employee, instead; if you need it to be a Person, just cast it to a Person after extraction.