What is the difference between return ModelAndView and return String in Spring MVC?

Question

I want to know the different between ModelAndView and String.

@RequestMapping(value="/")
    public ModelAndView mainPage() {
        return new ModelAndView("home");
}

and the second piece of code is about returning String:

@RequestMapping(value="/")
    public String mainPage() {
        return "home";
}

Answer 1

You can return many things from a controller, and Spring will automatically try to deduce what to do from there.

When you return a ModelAndView all information that needs to be passed from the controller is embedded inside it. No suprise there.

If you return a String, it will assume that this is the name of the view to use. It will wrap this in a ModelAndView object with the string as the view and the existing model embedded as well.

Spring does a lot of 'magic' on the return types, but also on the parameter types. This allows you to write code in a style that is more intuitive for you, i.e. the following two examples are the same:

@RequestMapping(value="/")
public ModelAndView mainPage() {
    Model m = new Model();
    m.put("key", "value");
    return new ModelAndView(m,"main");
}

and

@RequestMapping(value="/")
public String mainPage(Model m) {
    m.put("key", "value");
    return "main";
}

This second variant is a little less verbose and easier to test separately. The model passed in will be an empty model (unless forwarded from some other controller).