Java - Spring 3.0 MVC and @ModelAttribute

Question

I need some clarification about Spring 3.0 MVC and @ModelAttribute annotated method parameter. I have a Controller which looks like this one:

RequestMapping(value = "/home")
@Controller
public class MyController {

 @RequestMapping(method = RequestMethod.GET)
 public ModelAndView foo() {

               // do something
 }

@RequestMapping(method = RequestMethod.POST)
public ModelAndView bar(
        @ModelAttribute("barCommand") SomeObject obj) {

                    // do sometihng with obj and data sent from the form
}


}

and on my home.jsp i have a form like this one which sends his data to the RequestMethod.POST method of MyController

<form:form  action="home"  commandName="barCommband">

</form:form

Now if i try to access home.jsp i get this Exception:

java.lang.IllegalStateException:
Neither BindingResult nor plain target object for bean name 'barCommand' available as  request attribute

To resolve this i found that i need to add the

@ModelAttribute("barCommand") SomeObject obj 

parameter to the Request.GET method of MyController, even if i won't use obj in that method. And for example if add another form to home.jsp with a different commandName like this:

<form:form  action="home/doSomething"  commandName="anotherCommand">

</form:form

i also have to add that parameter on the RequestMethod.GET, which will now look like:

@RequestMapping(method = RequestMethod.GET)
 public ModelAndView foo( @ModelAttribute("barCommand") SomeObject obj1,
  @ModelAttribute("anotherCommand") AnotherObj obj2) {

               // do something
 }

or i get the same exception. What i'm asking is if this is a normal Spring 3 MVC behaviour or if i'm doing something wrong. And why do i need to put all the @ModelAttribute parameters on the RequestMethod.GET method?

Thanks in advance for you help

Stefano

Answer 1

Here is the spring mvc reference. Looked through it briefly and found 2 approaches:

1. @InitBinder
2. @ModelAttribute("bean_name") with method.

You may use first to customize data binding and, thus, create command objects on-the-fly. Second allows you to annotate method with it and prepopulate model attributes with this name:

@ModelAttribute("bean_name")
public Collection<PetType> populatePetTypes() {
    return this.clinic.getPetTypes();
} 

I hope it will populate model attributes with name 'bean_name' if it is null.