Allow use of form-backing object for client requests

[aykborstelmann]

Affects: 6.+

---

I really appreciate the HttpServiceProxyFactory, it is a great feature to easily create a client matching a interface.
However, for large APIs, we usually like to encapsulate multiple parameters in convenience classes, which spring web supports with databinding.

public interface EndpointApi {
  @GetExchange("/endpoint")
  EndpointResponse getProcedures(
      EndpointFilterOptions filterOptions,
      EndpointSortOptions sortOptions,
      EndpointPaginationOptions paginationOptions);
}

@ParameterObject
public record GetProceduresFilterOptions(
        @BindParam("customName")
        UUID filterOne,
        String stringFilter,
        Boolean booleanFilter,
        Set<Types> setFilter) {}
}

However, when creating a client for EndpointApi and using it, no HttpServiceArgumentResolver exists per default to handle such parameters.

I would appreciate this feature out of the box, while I am are that, HttpServiceProxyFactory provides a way to register custom argumentResolvers.

[rstoyanchev]

@aykborstelmann, thanks for the suggestion. Note that there is currently no such mechanism if using RestClient or WebClient directly. In other words, no message converter that would take an object, turn it into form data, e.g. for POST, or query params for GET. It's not an argument against, but just mentioning.

While I understand the benefit, the main challenge is that in the general case an object can be of arbitrary depth and structure, and there will clearly need to be some limits around what is possible. It also increases the possibility of using objects that were not originally built for this, and end up sending information that wasn't meant to be sent. Even in your example record, the last property is a Set which cannot be expressed with data binding property paths since it is not an indexed collection.

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[aykborstelmann]

@rstoyanchev yes I'm aware of the difficulties involved.
In theory this must be the inverse of the ModelAttributeMethodProcessor.
I can imagine there might be even cases that are not invertible.
Still I am convinced this feature would be great to encapsulate data in APIs.

[rstoyanchev]

I've updated the title as this applies more broadly to client scenarios including RestClient, WebClient, and testing too, see #29728.