Support constructor binding for input arguments

[koenpunt]

Using spring-graphql with Kotlin works, but when a field has a data class representing an input type, the class can't be instantiated, and fails on BeanUtils.instantiateClass(ctor);.

This repo/commit demonstrates the issue by adding a Kotlin configuration to the project, and adding a test using a kotlin data class as mutation argument:

koenpunt@f85fd6e

A workaround is to not use data classes, but I do believe data classes should be supported as well.

[koenpunt]

For clarification; this used to work before 4dfb662 with the changes of this PR.

Maybe a jackson ObjectMapper would be more suitable for constructing these objects?

[bclozel]

This is indeed a limitation from the latest changes in input arguments support; as explained in 4dfb662, an object mapper based approach is not efficient and may override custom scalars.

Using a Binder works pretty well in other arrangements (Spring MVC, WebFlux, Messaging); we might refactor this in web frameworks for reusability (see spring-projects/spring-framework#25943) but I doubt this would work for the GraphQL use case. We need to adapt the existing infrastructure in web frameworks to this project to support constructor binding.

[koenpunt]

Hm that's a pity.

We need to adapt the existing infrastructure in web frameworks to this project to support constructor binding.

You mean something like this? https://github.com/spring-projects/spring-framework/blob/958eb0f964ddef1ff1440fd10c5cb850f6ee96db/spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ModelAttributeMethodArgumentResolver.java#L228-L260