Json Question posted in
Our archive of expertly curated questions and answers provides insights and solutions to common problems related to this popular data interchange format. From parsing and manipulating JSON data to integrating it with various programming languages and web services, our archive has got you covered. Start exploring today and take your JSON skills to the next level

Json – Nested polymorphism in jackson module kotlin

I’m just trying to achieve to deserialize a nested polymorphed data class but i’m not able to do so. The previous questions and answers didn’t help me much for achieving this task that looks simple. I still get this error no matter I add constructor, add more annotation etc. Very confused.

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `Dog` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

import com.fasterxml.jackson.annotation.JsonSubTypes
import com.fasterxml.jackson.annotation.JsonTypeInfo
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "type"
)
@JsonSubTypes(
    JsonSubTypes.Type(value = Dog::class, name = "dog"),
)
sealed class Animal(val name: String)

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "type"
)
@JsonSubTypes(
    JsonSubTypes.Type(value = Labrador::class, name = "labrador")
)
sealed class Dog(name: String, val breed: String) : Animal(name)

class Labrador(name: String, val color: String) : Dog(name, "Labrador")

fun main() {
    val json = """
    {
      "type": "dog",
      "name": "Buddy",
      "breed": "Labrador",
      "color": "golden"
    }
    """

    val objectMapper = jacksonObjectMapper()
    val animal: Animal = objectMapper.readValue(json)

    println(animal.javaClass) // class com.example.Dog
    println(animal.name) // Buddy
    println((animal as Dog).breed) // Labrador
    println((animal as? Labrador)?.color) // golden
}
Tags:

2

Answers


  1. 0 
    import com.fasterxml.jackson.annotation.*
import com.fasterxml.jackson.module.kotlin.*

@JsonTypeInfo(
    use = JsonTypeInfo.Id.NAME,
    include = JsonTypeInfo.As.PROPERTY,
    property = "type"
)
@JsonSubTypes(
    JsonSubTypes.Type(value = Dog::class, name = "dog"),
    JsonSubTypes.Type(value = Labrador::class, name = "labrador")
)
sealed class Animal(val name: String)

// Make Dog an interface instead of a sealed class
interface Dog 

data class Labrador(val name: String, val color: String, val breed: String = "Labrador") : Animal(name), Dog

fun main() {
    val json = """
    {
      "type": "labrador",
      "name": "Buddy",
      "breed": "Labrador",
      "color": "golden"
    }
    """

    val objectMapper = jacksonObjectMapper()
    val animal: Animal = objectMapper.readValue(json)

    println(animal.javaClass) 
    println(animal.name) // Buddy
    if (animal is Labrador) {
        println(animal.breed) // Labrador
        println(animal.color) // golden
    }
}
    Login or Signup to reply.
  2. 0

    I’m not a Jackson expert, but I think when you specify "type": "dog" Jackson tries to create an instance of class Dog, which is a sealed class. Sealed classes are always abstract, hence the exception. If you make Dog an open class instead of sealed (and add @JsonIgnoreProperties(ignoreUnknown = true) to ignore unknown property "color": "golden"), your code will should print:

    class Dog
Buddy
Labrador
null

    If you want to get an instance of Labrador you have to change type: "type": "labrador", because type can not be both dog and labrador. Now it should print:

    class Labrador
Buddy
Labrador
golden
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search