Passing from @ControllerAdvice to @Controller in Spring MVC

I want to check if user exists in ControllerAdvice and treat user as @ModelAttribute if user exists. On the other hand, I also want to access user object in @Controller directly. So I add @ModelAttribute annotation on the parameter of @RequestMapping method.

I'm using @ControllerAdvice like:

@ControllerAdvice
public class UserAdvice {

    @Autowired
    private UserService userService;

    @ModelAttribute("user")
    public User user(@PathVariable("username") String username) {
        User user = userService.findByUsername(username);
        if (user != null) {
            return user;
        }
        user = userService.findById(username);
        if (user == null) {
            throw new ResourceNotFoundException("user not found");
        }

        return user;
    }
}

And UserController Like:

@RestController
@RequestMapping("/users/{username}")
public class UserController {

    public static final Logger logger = LoggerFactory.getLogger(UserCourseListController.class);

    @Autowired
    private CourseService courseService;

    @RequestMapping(value = "", method = RequestMethod.GET)
    public void getUser(@ModelAttribute("user") User user, Model model) {
        logger.info("{}", user);//user is null
        logger.info("{}", model.asMap().get("user"));// not null
    }

}

But now, the parameter user that annotated with @ModelAttribute is null while there is a "user" obj in Model Map.

Is there any mistakes I've made in this scenario? Or any misunderstanding of the concepts of @ModelAttribute and @ControllerAdvice?

Thanks very much!