Add binding support to functional servlet and reactive stack

[mdeinum]

When using regular @Controller for both the Servlet as well as the Reactive stack there is support for data binding using @ModelAttribute and there is support for deserializing a request body with @RequestBody.

When using the functional approach in either the Servlet or Reactive stack deserializing is supported through the body methods of the respective ServerRequest interface/implementation. There is no direct support for data-binding. It currently requires, some work, to do data-binding in a method.

public ServerResponse handle(ServerRequest request) {
  MyObject instance = new MyObject();
  ExtendedServletRequestDataBinder binder = new ExtendedServletRequestDataBinder(instance, "myObject");
  binder.bind(request.servletRequest());
  return ServerResponse.ok().render("view", instance).build();.
}

Although this will work it requires more knowledge of the internal of data binding then one should need.

It would be nice if there would be a supporting bind method(s) to support binding to a type like

<T> T bind(Class<?> bindType);
<T> T bind(Class<?> bindType, String objectName);

Which would reduce the code to something like this

public ServerResponse handle(ServerRequest request) {
  MyObject instance =request.bind(MyObject.class, "myObject");
  return ServerResponse.ok().render("view", instance).build();.
}

This could use the already available WebDataBinder implementations to do the binding (at least the reactive stack has a special DataBinder the functional servlet one needs to be created).

[rstoyanchev]

Indeed, there is no binding in neither WebFlux.fn nor WebMvc.fn currently.

[poutsma]

This turned out to be more complicated than I initially considered, but it will make it into 5.3.3 I hope.

[poutsma]

In order for the functional endpoints to support constructor binding, we will need to refactor some code out of the model attribute argument resolvers. See #26721.