Auto-detect sealed classes (similar to `@JsonSubTypes`)

[shartte]

I am not sure whether this is actually possible given Jacksons API, but I'd like to avoid having to specify the @JsonSubTypes annotation when working with polymorphic sealed classes.

Here's an example that'll work with the current module:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
@JsonSubTypes(
        JsonSubTypes.Type(Either.A::class, name = "a"),
        JsonSubTypes.Type(Either.B::class, name = "b")
)
sealed class Either {

    data class A(var field: String = "") : Either()

    data class B(var otherField: String = "") : Either()

}

But here's how I'd like it to look:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
sealed class Either {

    @JsonTypeName("a")
    data class A(var field: String = "") : Either()

    @JsonTypeName("b")
    data class B(var otherField: String = "") : Either()

}

This should be possible given that Kotlin exposes the sealed class's subclasses as Either::class.sealedSubclasses.

And here's a test function:

fun main() {

    val mapper = ObjectMapper()
    val either = mapper.readValue<Array<Either>>("""[
        {
            "@type": "a",
            "field": "value"
        },
        {
            "@type": "b",
            "otherField": "1234"
        }
    ]""")
    println(either.contentToString())

}

[cowtowncoder]

I don't know of feasibility on Kotlin module side, but since discovery of subtypes (usually based on @JsonSubTypes) is pluggable via AnnotationIntrospector, this seems potentially doable.
In case anyone might have time & itch to try it. I can help with details of discovery process as need be, but do not know Kotlin (or its runtime) well, nor have time to drive this.

[shartte]

@cowtowncoder I've opened a PR that implements this, including a test for both implicit and explicit specification of subtypes.

[cowtowncoder]

Now that was quick!

[araujogo]

It`s too bad we have to write the subtype name twice when the property name is the same os the object name. This is what I have to do:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "value")
sealed class Plano(val displayName: String) : SelectField() {
    @JsonTypeName("Bas")
    object Bas : Plano("Básico")
    @JsonTypeName("Emp")
    object Emp : Plano("Empresarial")
    @JsonTypeName("Ext")
    object Ext : Plano("Extra")
}

Shouldn't be a way to use the exactly class name (without leading points and outer types) as in JsonTypeInfo.Id.MINIMAL_CLASS ? It would be a lot cleaner for people who are converting ENUMs to sealed (which the client API is already defined)