I’m trying to deserialize a simple sealed class (w...

# serialization

[leandro]

I’m trying to deserialize a simple sealed class (with one field called __type and another called message) from my api, modeled using Retrofit and a generic response such as NetworkResponse<MyResponse, ErrorResponse>, in which the ErrorResponse is the sealed class in question.

Analysing the returned type with the debugger shows me that my generic response is

UnknownError(error=kotlinx.serialization.json.internal.JsonDecodingException: Polymorphic serializer was not found for missing class discriminator ('null')
JSON input: {"__type":"UserLambdaValidationException","message":"..."})

I’m using v1.0.0-RC, and unsure how to proceed.

[Vampire]

The default discriminator is type. If you use Json { classDiscriminator = "__type" } it will probably work properly

[leandro]

Thanks for answering! Now it does not show ('null') anymore, but right now as

(’UserLambdaValidationException
’)

. I probably wont be able to match all possible values of __type, so maybe I could use an when() expression with a custom KSerializer? I have:

@Serializer(forClass = ErrorResponse::class)
object ErrorResponseSerializer : KSerializer<ErrorResponse> {
  override val descriptor = ErrorResponse.serializer().descriptor

  override fun deserialize(decoder: Decoder): ErrorResponse {
    val surrogate = decoder.decodeSerializableValue(ErrorResponse.serializer())
    return when (surrogate.__type) {
      "UsernameExistsException" -> {
        UserExistsErrorResponse(surrogate.message, surrogate.__type)
      }
      "UserNotFoundException" -> {
        UserNotFoundErrorResponse(surrogate.message, surrogate.__type)
      }
      else -> {
        UnknownErrorResponse(surrogate.message, surrogate.__type)
      }
    }
  }
}

and that would fit my needs.

If I apply to my sealed class definition, as

@Serializable(with = ErrorResponseSerializer::class)
sealed class ErrorResponse {
  @SerialName("message") abstract val message: String
  @SerialName("__type") abstract val __type: String
}

then the app crashes with StackOverflowError. I assume I’m misusing something around

decoder.decodeSerializableValue(ErrorResponse.serializer())

but can’t figure out how to do it

[Vampire]

The same message but with ('UserLambdaValidationException')?
Afair the discriminator by default has the FQCN.
So if your class is in a package it doesn't match.
But you can use @SerialName on the class to configure the discriminator value.

[leandro]

The same message but with (‘UserLambdaValidationException’)?

Exactly 😕

but I imagine there will always be a different __type that I haven’t modeled (I don’t have access to the server side). Would a custom Kserializer be a solution? how can I deserialize and inspect/compare values?

[Vampire]

Maybe, but I have not enough experience with custom serializer to say, sorry.

[leandro]

thank you anyway, Bjorn! you already helped a lot 🙏

[Vampire]

And as thanks you rape my name? :'''-(

[Vampire]

It's Björn or Bjoern, not Bjorn

[Vampire]

One important reason for an ascii-only nickname

[leandro]

apologies Björn 🙏

[Paul Griffith]

There's a JsonContentPolymorphicSerializer stub provided builtin that seems like it would fit

[Paul Griffith]

eg this stub to choose a serializer based on a piece of the incoming JSON

Untitled

[leandro]

That’s great! It worked perfectly, thank you Paul 🙌