Json - Nested polymorphism in jackson module kotlin

CodeMonkey
November 8, 2024
127 views
0 votes
2 Answers

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
}

Answers

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
    }
}

- JanItor
- November 8, 2024 at 8:37 am
- 0 votes
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
```
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Json – Nested polymorphism in jackson module kotlin

Answers