Kotlin - encode json of properties using function from extended class

I try to Json.encodeToString using function from extended class:

Child class

class Body(
            var variable1: String,
            var variable2: String,
            var variable3: Int,
        ): Serializable()

Parent class

@Serializable
open class Serializable {

    fun getJson(): String{
        return Json.encodeToString(this)
    }
}

Using

var body = Body("1","1",1)
var json = body.getJson()

And this get "{}". I think it takes properties from the parent class.

EDIT:

I need annotation in Serializable class because i need to put different objects to one function:

var body = Body("1","1",1)
var response = RequestServer.Request<Boolean>("route/set", body, HttpMethod.Put)

var body2 = Body2("2","2",2)
var response = RequestServer.Request<Boolean>("route/set", body2, HttpMethod.Put)

Function:

suspend inline fun <reified T> Request(route: String, body: Serializable, method: HttpMethod): BaseResponse<T> {
        var response: HttpResponse = client.request(Address + route) {
                    this.method = method
                    contentType(ContentType.Application.Json)
                    setBody(Json.encodeToString(body))
                }

        return Json{ ignoreUnknownKeys = true }.decodeFromString(response.bodyAsText())
    }

How to make it take properties from child class?

That's because the use of @Serializable in the parent class and the method getJson() tries to serialize itself.

So, what you can do is to put the annotation in the child class, instead of the parent class.

Also, for best practices, you can change your parent class name, to avoid confusion with the standard Kotlin interface.

EDIT

I get it now you are trying to build some API, right? So what I do actually when using Kotlin + Spring is to use a data class. The data class does the magic to serializing and deserializing JSON.

Check this article in Baeldung, it basically does what you need and also uses the extension function:
https://www.baeldung.com/kotlin/data-class-json-serialize-default-values

Kotlin docs about data classes:
https://kotlinlang.org/docs/data-classes.html