Please also see my own StackOverflow question on this:
In summary, wish to bind JSON request form data to an existing Command Object (so data is merged) with the JSON form POST data being merged (overwriting) those attributes of a superset of data already populated into Command object prior to Form presentation on view. The links above should provide sufficient illustration of the problem.
It seems that @RequestBody is capable of using Jackson JSON converter to populate a new Command Object (which may have nested objects) with the form data, but unable to leverage @ModelAttribute to automatically write the attribute values into an existing Command object (either stored in session or retrieved again using @ModelAttribute at the method declaration level.
This is a severe limitation for anyone wishing to use the Spring MVC framework and have full support for Form submissions with JSON request and response types.