@ModelAttribute annotation, when to use it?

Question

Lets say we have an entity Person, a controller PersonController and an edit.jsp page (creating a new or editing an existing person)

Controller

@RequestMapping(value = "/edit", method = RequestMethod.POST) public String editPerson(@RequestParam("fname") String fname, Model model) {     if(fname == null || fname.length() == 0){         model.addAttribute("personToEditOrCreate", new Person());     }     else{         Person p = personService.getPersonByFirstName(fname);         model.addAttribute("personToEditOrCreate", p);     }      return "persons/edit"; }  @RequestMapping(value = "/save", method = RequestMethod.POST) public String savePerson(Person person, BindingResult result) {      personService.savePerson(person);     return "redirect:/home"; } 

edit.jsp

<form:form method="post" modelAttribute="personToEditOrCreate" action="save">     <form:hidden path="id"/>      <table>         <tr>             <td><form:label path="firstName">First Name</form:label></td>             <td><form:input path="firstName" /></td>         </tr>         <tr>             <td><form:label path="lastName">Last Name</form:label></td>             <td><form:input path="lastName" /></td>         </tr>         <tr>             <td><form:label path="money">Money</form:label></td>             <td><form:input path="money" /></td>         </tr>         <tr>             <td colspan="2">                 <input type="submit" value="Add/Edit Person"/>             </td>         </tr>     </table>   </form:form> 

Im trying the code above (without using the @ModelAttribute annotation in the savePerson method, and it works correct. Why and when do i need to add the annotation to the person object:

@RequestMapping(value = "/save", method = RequestMethod.POST) public String savePerson(@ModelAttribute("personToEditOrCreate") Person person, BindingResult result) {      personService.savePerson(person);     return "redirect:/home"; } 

Answer 1

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 

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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 

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Answer 2

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.