Spring MVC - difference between HttpSession.setAttribute and model.addObject

Question

I am trying to learn Spring MVC recently. It seems that i did not understand well the functionalities of @ModelAttribute annotation and HttpSession.

@SessionAttributes({"shoppingCart", "count"})
public class ItemController {

@ModelAttribute("shoppingCart")
public List<Item> createShoppingCart() {
    return new ArrayList<Item>();
}

@ModelAttribute("count")
public Integer createCount() {
    return 0;
}

@RequestMapping(value="/addToCart/{itemId}", method=RequestMethod.GET)
public ModelAndView addToCart(@PathVariable("itemId") Item item, 
        @ModelAttribute("shoppingCart") List<Item> shoppingCart, @ModelAttribute("count") Integer count) {

    if(item != null) {
        shoppingCart.add(item);
        count = count + 1;
    }

    return new ModelAndView(new RedirectView("showAllItems")).addObject("count", count);
}

@RequestMapping(value="/deleteFromCart/{itemId}", method=RequestMethod.GET)
public ModelAndView deleteFromCart(@PathVariable("itemId") Item item, 
        HttpSession session) {

    List<Item> list = (List<Item>) session.getAttribute("shoppingCart");
    list.remove(item);
    //session.setAttribute("shoppingCart", list);

    Integer count = (Integer) session.getAttribute("count");
    count = count - 1;
    session.setAttribute("count", count);

    return new ModelAndView(new RedirectView("showAllItems"));
}

ShoppingCart and count are the session attributes.

The problem is in the deleteFromCart method. I get the count from session, reassign it and overwrite it in session. But i cant see the updated value of count on jsp. However, the updated shoppingCart object can be seen updated, although i do not overwrite the session object (since the object is the same object which is already in session).

But why is the count not updated, although i overwrite it with session.setAttribute? When i add the new count object to the model (model.addObject("count", count)) then i can see the updated value of count. But why does not session.setAttribute give the same result?

Answer 1

First of all, @SessionAttribute does not have to use the http session. It uses a SessionAttributeStore which can have anything as its backing storage. Only the default implementation uses the http session.

The reason why your code does not work as expected lies in how @SessionAttribute works.

Before a controller method is invoked, everything listed in @SessionAttributes, in your case {"warenkorb", "count"}, is read from the session and added to the model.

After the method has returned the session is updated with everything that has been added to the model within the method.

.addObject("count", count)

-> count is added to the model and afterwards to the session.

session.setAttribute("count", count)

-> count is added to the session but not to the model. It will be added to the model before the next invocation of any controller method. But as for now the model still has the old count. And the model is what gets added to the request. And if an attribute can be found in the request scope then the jsp does not care about what's in the session.

When you use @SessionAttributesand @ModelAttribute (or Spring MVC in general) then avoid using HttpSession or HttpRequest. Even HttpResponseis of limited use. Embrace the beauty of Spring MVC instead :)