Difference between #ModelAttribute and org.springframework.ui.ModelMap.get()? - spring

Question

I'm trying to use an Object as the command object of a <%# taglib uri="http://www.springframework.org/tags/form" prefix="form"%>element.
In the controller's GET method, I add the Object like this:
#RequestMapping(method = RequestMethod.GET)
public String renderForm(ModelMap model, HttpServletRequest request) {
[...]
model.addAttribute("voting", voting);
[...]
}
The rendered form does show the command object correctly when defined like this:
<form:form action="vote" method="PUT" commandName="voting" name="oform">
Now when trying to access the form's command object back in the controller, on the POST method, I have two approaches. First, I declare the #ModelAttribute in the signature:
#RequestMapping(method = RequestMethod.PUT)
public String newVoting(#ModelAttribute("voting") Voting voting, HttpServletRequest request) { [...]}
Or I access the ModelMap and get the value from the underlying map:
#RequestMapping(method = RequestMethod.PUT)
public String newVoting(ModelMap model, HttpServletRequest request) {
Voting voting = (Voting) model.get("voting");
[...]
}
When doing the first, I get the object as it was submitted by the form. Doing the latter, I get the object as it was BEFORE being handled by the form.
Why does the form's submit not change the object in the ModelMap and why does the #ModelAttribute differ from whats in the actual model? I feel like the name 'ModelAttribute' should result in the same object like getting the object directly from the model.
Maybe #ModelAttribute is kind of misleading?
EDIT
Forgot to mention that the object is a #SessionAttributes
#SessionAttributes({"voting", "state"})

Answer 1

Your first approach, using the #ModelAttribute annotation does two things:
Creating the Voting object using the submitted form data;
Exposing the Voting object to the view by adding it to the model (request, or, in your case, session because of the #SessionAttributes({"voting"})).
Your second approach uses just the ModelMap, so it's getting only model attributes (from the request or, in your case, the session, depending on the #SessionAttributes annotation). This approach does not use the data from the submitted form.