Question

I have this class:

@Component
@Scope("session")
@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue
    @GenericGenerator(name = "incremental", strategy = "increment")
    private Long userID;

    @Column(nullable = false)
    private String username;

    @Column(nullable = false)
    private String email;

    @Column(nullable = false)
    private String password;
    // getters and setters
}

And this controller:

@Controller
@SessionAttributes("user")
@Scope("request")
public class UserCreationWizard {
    @Autowired
    private User user;

    @ModelAttribute("user")
    private User createUser() {
        return user;
    }

    @RequestMapping(value = "/new/users/page/", method = RequestMethod.GET)
    public String begin(HttpServletRequest request) {
        return "wizard"; 
    }

    @RequestMapping(value = "/new/users/page/{page}", method = RequestMethod.POST) 
    public String step(@ModelAttribute("user") User user,
                       @RequestParam("username") String username,
                       @RequestParam("email") String password,
                       @PathVariable() Integer page) {

        return "wizard" + page;
    }

    @RequestMapping(value = "/new/users/page/end", params = "submit", method = RequestMethod.POST) 
    public String end(@RequestParam("password") String password) {

        user.setPassword(password);
        user.setActive(true);
        user.setLastLoggedIn(Calendar.getInstance());

        Session s = HibernateUtils.getSessionFactory().openSession();
        Transaction t = s.beginTransaction();
        try {
            s.persist(user);
            s.flush();
            t.commit();

            s.close();
        } catch (HibernateException e) {
            t.rollback();
        }
        return "wizard";
    }
}

begin() just loads the first view (a jsp) in the user creation wizard. It has input fields for username and email. In the view, you make a POST form submission which triggers step(). In the second view (wizard+page.jsp), you have a password field and a submit input that triggers end().

1. In debug mode I noticed that in step(), where I’ve passed User as
   a ModelAttribute, I don’t need to set its fields for username and
   password. They are automatically grabbed from the RequestParams
   attributes. In end() however, where I don’t have a ModelAttribute,
   I have to set the password manually. How does Spring manage this?
2. Also if I take out the createUser() method in the Controller, the
   application fails saying it couldn’t find a session attribute for
   “user”. How is this method linked to MethodAttribute as a method
   parameter?
3. Lastly, if I take out @SessionAttributes, the application doesn’t fail but I feel like something is going wrong. Will the User user now be global to all httprequests?

My general question is: are spring beans mapped to their name? Eg. Here I have ‘user’ as a user and ‘user’ in the session, ‘password’ as a requestparam and ‘password’ as the User member variable.

Answer 1

Ok, many questions. Let’s see, all references are to the documentation for the current Spring MVC version.

1) The behaviour you see in the user attribute is explained in the section “Using @ModelAttribute on a method argument“

An @ModelAttribute on a method argument indicates the argument should
be retrieved from the model. If not present in the model, the argument
should be instantiated first and then added to the model. Once present
in the model, the argument’s fields should be populated from all
request parameters that have matching names. This is known as data
binding in Spring MVC, a very useful mechanism that saves you from
having to parse each form field individually.

How does Spring does this? Well, the source code is the ultimate answer but it’s not that hard to make a guess: Spring knows the argument is an instance of User and via reflection it can read the methods of the class, particularly its setters. In this case it finds setUsername() and setEmail() and the argument for those methods is a String so it’s compatible with the parameters from the request.

(BTW: @RequestParam("email") String password is possibly a mistake. At the very least is confusing)

2) The method createUser() is preceded by the annotation @ModelAttribute("user"). This is covered by the section “Using @ModelAttribute on a method“

An @ModelAttribute on a method indicates the purpose of that method is
to add one or more model attributes.

Therefore, this method puts an object associated with the name "user" on the model and is then available to be used as a parameter by other methods, like step(). Notice that the annotation controls the identifier used by the object in the model. If you change the code to

@ModelAttribute("strangeWeirdIdentifier")
private User createUser() { return user; }

the app will break. But it will work again if you change the step() signature to

public String step(@ModelAttribute("strangeWeirdIdentifier") User user,
                   @RequestParam("username") String username,
                   @RequestParam("email") String password,
                   @PathVariable() Integer page) {

3) The process described in 1) and 2) stores objects in the model during a request. With the class annotation @SessionAttributes("user") you extend the life of the object adding it to the current Session or something equivalent. By example you could then use the object in other Controllers in the same was as the step() method.

Finally just to be clear

• The annotation seen in your question 2 happens before the usage seen in your question 1.
• The annotation from your question 3 is probably not needed
• Spring does not map beans to their names in Java code but to the names used in the annotations. Is not uncommon to repeat the same name as in your example for the sake of clarity.

Hope this manages to be more clear than the official documentation whch, is usually too brief, I’ll give you that.