1. Consider single-purpose return value types like HttpEntity, Model,
View, and ModelAndView ahead of annotations like @ResponseBody and
@ModelAttribute. And reversely consider multi-purpose return value
types like Map, String, and void only after annotations like
@RB and @MA.
2. Order custom argument resolvers and return value handlers after the
built-in ones also clarifying the fact they cannot be used to override
the built-in ones in Javadoc throughout.
3. Provide hooks in RequestMappingHandlerAdapter that subclasses can use
to programmatically modify the list of argument resolvers and return
value handlers, also adding new getters so subclasses can get access
to what they need for the override.
4. Make SessionStatus available through ModelAndViewContainer and
provide an argument resolver for it.
5. Init test and javadoc improvements.
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