possibility to not require JsonTypeInfo annotation for sealed classes?

[pjfanning]

Scala, Kotlin and Java 17 have sealed classes. It is possible to write introspectors that find the subtypes.
It would be great if jackson-databind could infer a default JsonTypeInfo setting for sealed traits/interfaces/abstract-classes

A default like this, perhaps:

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")

• Sealed trait support pjfanning/jackson-scala-reflect-extensions#9
• Add streamlined sealed class polymorphic de/serialization #3549
• https://kotlinlang.org/docs/sealed-classes.html

[cowtowncoder]

Yes, I agree. See #3549 for related work.

I do think this probably needs to be somehow opt-in since there are probably some security concerns (I can't think of specifics but this tingles my spider sense wrt past experiences).
Just a simple MapperFeature or something.

[pjfanning]

@cowtowncoder a MapperFeature to enable a default JsonTypeInfo for sealed classes would be fine. Once enabled, any class that has subtypes would automatically support be deserialized as if it has the default JsonTypeInfo annotation on it.

[cowtowncoder]

Right. Now that I think about this, however, I realize there is the big question of what @JsonTypeInfo defaults are. Or rather how to allow configuration -- a default like shown above is fine but there probably should be a way to customize it. Unfortunately we do not have JsonTypeInfo.Value type (unlike for some of annotations), although I guess one could be added.
Specifying a class/interface with annotation could work I suppose.

Another thing to add to AnnotationIntrospector -- or, perhaps, this would be the main thing that exposes everything? -- would be a method to call to ask if given type is a "sealed type".
This is where we have some options; one would be "yes/no/don't know" result, after which introspector (or something) would need to figure out @JsonTypeInfo parameterization.
Another that it's this method that would actually return the parameterization.

Yet another is of course to instead bolt this in via existing handling of @JsonTypeInfo... but it seems cleaner to separate this out.

Regardless, modules (Kotlin, Scala, Java-17?) would provide their AnnotationIntrospectors to detect sealed types they know of and handle.

Does this sound good?

/cc @sigpwned apologies for your sealed class PR falling through the cracks -- but I think it might be possible to join forces here. The concept of sealed class is indeed a common shared concern.