Return types in Spring MVC

Question

I am new to Spring MVC and trying to learn the concepts from Spring MVC Docs:

A ModelAndView object, with the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.

Can someone please help me in understanding this? What does it mean that "model implicitly enriched with command object" and also the "results of @ModelAttribute annotated reference data accessor methods"?

A Model object, with the view name implicitly determined through a RequestToViewNameTranslator and the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.

How the view name is determined in Spring here?

A Map object for exposing a model, with the view name implicitly determined through a RequestToViewNameTranslator and the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.

Same is the case here, how the view name is determined?

Answer 1

What does it mean that "model implicitly enriched with command object" and also the "results of @ModelAttribute annotated reference data accessor methods"?

Command objects are objects created by Spring from request parameters. In other words, say you had

public ModelAndView doSomething(SomeFoo foo, @ModelAttribute SomeBar bar) {
    return new ModelAndView("view-name");
}

Spring would implicitly add the objects foo and bar are referencing to the ModelAndView returned. Note that, although SomeFoo is not annotated with @ModelAttribute, Spring will still consider it a model attribute. You'll have to look into HandlerMethodArgumentResolver to understand why.

You can also annotate methods with @ModelAttribute and those will be called and the object created added to the Model on each request handled by the @Controller your method is in. That is what is meant by results of @ModelAttribute annotated reference data accessor methods.

How the view name is determined in Spring here?

It tells you exactly how it is determined: it uses RequestToViewNameTranslator, where the default is DefaultRequestToViewNameTranslator. Read the javadoc for more details.

Same is the case here, how the view name is determined?

Same as above.

---

To answer the question in your comment, Spring registers a number of HandlerMethodArgumentResolver instances to resolve the arguments to pass to your @RequestMapping annotated method. It registers them in a specific order prioritizing parameters of specific types like Model, ModelMap, ModelAndView, etc. and then annotated parameters (eg. handlers for @PathVariable, @RequestParam, etc.). Spring then iterates through this list and uses the first one it finds that can handle the parameter.

One of these HandlerMethodArgumentResolver is ServletModelAttributeMethodProcessor which handles @ModelAttribute annotated parameters. Spring registers two (2) of these. One for parameters actually annotated with @ModelAttribute and one for parameters not annotated. This way you can inject your own HandlerMethodArgumentResolver to handle parameters your way.

Registration order of HandlerMethodArgumentResolver

...
ServletModelAttributeMethodProcessor for @ModelAttribute annotated parameters
...
your own HandlerMethodArgumentResolver
...
ServletModelAttributeMethodProcessor for parameters not annotated with ModelAttribute