Inconsistent behaviour with sealed classes serialization

I faced a problem with sealed classes serialization.

To Reproduce

import kotlinx.serialization.Serializable
import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.Json

@Serializable
sealed class Project {
	abstract val name: String

	@Serializable
	class OwnedProject(override val name: String, val owner: String) : Project()
}

fun main() {
	println(Json.encodeToString(Project.OwnedProject("kotlinx.coroutines", "kotlin")))
	// prints: {"name":"kotlinx.coroutines","owner":"kotlin"}
	val project: Project = Project.OwnedProject("kotlinx.coroutines", "kotlin")
	// prints: {"type":"com.example.Project.OwnedProject","name":"kotlinx.coroutines","owner":"kotlin"}
	println(Json.encodeToString(project))
}

I thought that result in both cases should be equal, but it prints different JSON strings.

Environment

• Kotlin version: 1.4.10
• Library version : Kotlinx Serialization Plugin 1.4.10
• Kotlin platforms: JVM
• Gradle version: 6.6.1
• Other relevant context: JRE version: 1.8

[chipnesh]

Is there any way to enable a discriminator field to all class hierarchy forcefully? I mean like JsonTypeInfo annotation for Jackson mapper or something? If I remember correctly Jackson includes a discriminator field in both cases.

My case is related to response serialization in “ktor” route. Because of this problem, it looks like this:

	post("/login") {
		val request = call.receive<LoginCredentials>()
		val response: TokenResponse = authService.login(request)
		call.respond(Json.encodeToString(response))
	}

I have to do manual encoding through encodeToString and have to specify the “needed” response type explicitly.

[pdvrieze]

@joffrey-bion I had been assuming that you were using your own converter. I see that there is an implementation in Spring. Looking at that code: https://github.com/spring-projects/spring-framework/blob/2d53570f4cc148250378b852e671fcb0369f754b/spring-web/src/main/java/org/springframework/http/converter/json/KotlinSerializationJsonHttpMessageConverter.java#L144-L149

it is clear that it does not have an existing mechanism to handle polymorphic types (the code determining the serializer even throws an error). You would have to create your own version (or create a patch/pull request for Spring). The function does have a type parameter in writeInternal (very often null according to references in github), but it is not generally specified. I would say that it would be worthwhile to update the class.