[oryan-block]

Description

Fixes #926

A custom scalar backed by a Kotlin class without visible properties lost its value on the way to the resolver:

class Token(private val value: String = "default")
data class TokenInput(val token: Token? = null)

tokenInput(input: { token: "input" }) got Token("default"), an Optional<Token> parameter got the same, and the same scalar inside an ObjectNode argument arrived as {}.

It's not the Kotlin module. Jackson only applies FAIL_ON_EMPTY_BEANS to classes without annotations. For an annotated class with no properties it builds a dummy serializer that writes {}, and every Kotlin class has @kotlin.Metadata. So the error that makes ArgumentConverter pass the value through as it is never fired, and {} was read back as a new instance. Java classes with a class annotation hit the same thing.

Now ArgumentConverter writes through a generator that fails when a bean serializer without properties closes an empty object, the same way an unannotated Java class fails. That sends the value down the existing embedded-value path, so the resolver gets the coerced instance (or a POJONode of it in an ObjectNode). Converting such a value to another class, or to a field declared as a Map or ObjectNode, still goes through {} like before, rather than failing.

Not covered: an Optional parameter of a scalar class without properties that's written with polymorphic type info (e.g. @JsonTypeInfo with a wrapper object) now fails where it used to convert, since the embedded value can't carry the type id. A plain or list parameter of the scalar skips the converter, and the same scalar inside an input object already failed on master. So it needs that exact combination. I don't think anyone does that, but it's there. A scalar like this written with a type id property or @JsonIdentityInfo still loses its value, same as on master. That's #932.

Release note: custom scalars backed by a Kotlin class without visible properties are passed to resolvers with their value, in input objects, Optional parameters and ObjectNode arguments.

Checklist

• Pull requests follows the contribution guide
• New or modified functionality is covered by tests

The new case in CustomScalarInputTest fails on master (optionalToken=default, tokenInput=default, tokenNode={}) and passes here. A second case pins that conversions to another class and to fields declared as Map/ObjectNode give the same result as master. mvn clean verify passes (430 tests). Also checked through the installed jar in a separate consumer project: the input object case returns token default on master and token input with this change. A Kotlin scalar without properties converted to another class gives the same result as master.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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