@ModelAttribute annotation, when to use it?

You don't need @ModelAttribute (parameter) just to use a Bean as a parameter

For example, these handler methods work fine with these requests:

@RequestMapping("/a")
void pathA(SomeBean someBean) {
  assertEquals("neil", someBean.getName());
}

GET /a?name=neil

@RequestMapping(value="/a", method=RequestMethod.POST)
void pathAPost(SomeBean someBean) {
  assertEquals("neil", someBean.getName());
}

POST /a
name=neil

---

Use @ModelAttribute (method) to load default data into your model on every request - for example from a database, especially when using @SessionAttributes. This can be done in a Controller or in a ControllerAdvice:

@Controller
@RequestMapping("/foos")
public class FooController {

  @ModelAttribute("foo")
  String getFoo() {
    return "bar";  // set modelMap["foo"] = "bar" on every request
  }

}

Any JSP forwarded to by FooController:

${foo} //=bar

or

@ControllerAdvice
public class MyGlobalData {

  @ModelAttribute("foo")
  String getFoo() {
    return "bar";  // set modelMap["foo"] = "bar" on every request
  }

}

Any JSP:

${foo} //=bar

---

Use @ModelAttribute (parameter) if you want to use the result of @ModelAttribute (method) as a default:

@ModelAttribute("attrib1")
SomeBean getSomeBean() {
  return new SomeBean("neil");  // set modelMap["attrib1"] = SomeBean("neil") on every request
}

@RequestMapping("/a")
void pathA(@ModelAttribute("attrib1") SomeBean someBean) {
  assertEquals("neil", someBean.getName());
}

GET /a

---

Use @ModelAttribute (parameter) to get an object stored in a flash attribute:

@RequestMapping("/a")
String pathA(RedirectAttributes redirectAttributes) {
  redirectAttributes.addFlashAttribute("attrib1", new SomeBean("from flash"));
  return "redirect:/b";
}

@RequestMapping("/b")
void pathB(@ModelAttribute("attrib1") SomeBean someBean) {
  assertEquals("from flash", someBean.getName());
}

GET /a

---

Use @ModelAttribute (parameter) to get an object stored by @SessionAttributes

@Controller
@SessionAttributes("attrib1")
public class Controller1 {

    @RequestMapping("/a")
    void pathA(Model model) {
        model.addAttribute("attrib1", new SomeBean("neil")); //this ends up in session due to @SessionAttributes on controller
    }

    @RequestMapping("/b")
    void pathB(@ModelAttribute("attrib1") SomeBean someBean) {
        assertEquals("neil", someBean.getName());
    }
}

GET /a
GET /b

---

Your question appears to be answered already:

What is @ModelAttribute in Spring MVC?

To summarize the answer and blog post: when you want your form backing object (instance of Person) to be persisted across requests.

Otherwise, without the annotation, the request mapped method will assume Person is a new object and in no way linked to your form backing object. The blog post that poster references is really awesome by the way, definitely a must-read.