Spring MVC update ModelAttribute value

Question

When modifying a ModelAttribute that is listed as a SessionAttribute, why doesent it keep its new value?

Every time I make a request to the example below, it prints out "Initial value.", which is a correct value for the first request. But after the first request, its value should be "new value".

Why doesent ModelAttribute store its value?

I have a base class. All servlets extending this:

@SessionAttributes(value = {"test_string", "something"})
public abstract class Base<T>
    {
    public abstract T request(
            @ModelAttribute("test_string") String _test_string,
            ModelAndView _mv);

    @ModelAttribute("test_string")
    private String getTest()
    {
        return "Initial value.";
    }
}

I have a specific servlet:

@Controller
public class InterfaceController extends Base<String>
{
    @PostMapping(value = "/interface")
    @ResponseBody
    @Override
    public String request(
        @ModelAttribute("test_string") String _test_string,
        ModelAndView _mv)
    {
        System.out.println(_test_string);
        _test_string = "new value";

        return "whatever content";
    }
}

score 3 · Answer 1 · answered Jan 22 '18 at 08:44

3

I'm no Spring MVC expert but your problem seems to be understanding Java pass-by-reference and String inmutability. I've made a diagram to help you understand what the problem is but you may need to research more info about it.

When you invoke sysout, you are printing the value pointed by "_test_string" (method argument), that at this point is the same that ModelAttribute "test_string".
When you assign "new value" to "_test_string" (method argument), notice that you're NOT changing the value of "test_string" (ModelAttribute)
It's what I think you have to do to overwrite the value stored in the Model.

answered Jan 22 '18 at 08:44

RubioRic

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I think it's even have nothing to do with String `immutabilty` as the value is yet to be reassigned. – VPK Jan 22 '18 at 08:55
Ok, it makes sense. So if I must return a String (because I am rendering json with Gson), how do I update the Model? If not possible, then the less elegant way is to instead of String, return the updated ModelAndView, and pass the String Json to the view. How? I tried with the MappingJackson2JsonView and it renders the whole Model to Json which is not a good idea. – Gábor DANI Jan 22 '18 at 10:45
That's a different question. As I said, I'm no Spring MVC expert, sorry. But I don't undestand your new problem. If you need to pass a simple String between your controller and your view, you can use my point (3) statement or another similar approach. You can recover attribute "test_string" in your view. If you use Json to represent all your model, I guess that you can use some library to extract the specific data you need. – RubioRic Jan 22 '18 at 11:17

Spring MVC update ModelAttribute value

1 Answers1