How to serialize/deserialize Kotlin sealed class?

You are probably right about the creating a custom serializer.

I have tried to serialize and de-serialize your class using the Jackson library and Kotlin.

These are the Maven dependencies for Jackson:

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.8.8</version>
</dependency>

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.8.8</version>
</dependency>

You can serialize the sealed class to JSON using this library with no extra custom serializers, but de-serialization requires a custom de-serializer.

Below is the toy code I have used to serialize and de-serialize your sealed class:

import com.fasterxml.jackson.core.JsonParser
import com.fasterxml.jackson.databind.DeserializationContext
import com.fasterxml.jackson.databind.JsonDeserializer
import com.fasterxml.jackson.databind.JsonNode
import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.databind.module.SimpleModule

sealed class ViewModel {
    data class Loaded(val value: String) : ViewModel()
    object Loading : ViewModel()
}

// Custom serializer
class ViewModelDeserializer : JsonDeserializer<ViewModel>() {
    override fun deserialize(jp: JsonParser?, p1: DeserializationContext?): ViewModel {
        val node: JsonNode? = jp?.getCodec()?.readTree(jp)
        val value = node?.get("value")
        return if (value != null) ViewModel.Loaded(value.asText()) else ViewModel.Loading
    }
}

fun main(args: Array<String>) {
    val m = createCustomMapper()
    val ser1 = m.writeValueAsString(ViewModel.Loading)
    println(ser1)
    val ser2 = m.writeValueAsString(ViewModel.Loaded("test"))
    println(ser2)
    val deserialized1 = m.readValue(ser1, ViewModel::class.java)
    val deserialized2 = m.readValue(ser2, ViewModel::class.java)
    println(deserialized1)
    println(deserialized2)
}

// Using mapper with custom serializer
private fun createCustomMapper(): ObjectMapper {
    val m = ObjectMapper()
    val sm = SimpleModule()
    sm.addDeserializer(ViewModel::class.java, ViewModelDeserializer())
    m.registerModule(sm)
    return m
}

If you run this code this is the output:

{}
{"value":"test"}
ViewModel$Loading@1753acfe
Loaded(value=test)

I had a similar problem recently (although using Jackson, not Genson.)

Assuming I have the following:

sealed class Parent(val name: String)

object ChildOne : Parent("ValOne")
object ChildTwo : Parent("ValTwo")

Then adding a JsonCreator function to the sealed class:

sealed class Parent(val name: String) {

    private companion object {
        @JsonCreator
        @JvmStatic
        fun findBySimpleClassName(simpleName: String): Parent? {
            return Parent::class.sealedSubclasses.first {
                it.simpleName == simpleName
            }.objectInstance
        }
    }
}

Now you can deserialize using ChildOne or ChildTwo as key in your json property.