How to find the base URL of the current webapp in Spring?

I'd like to find the absolute URL of the webapp in Spring, from the Controller. I'm aware of JSTL c:url, but I need this info from inside the Controller.

@Controller
public class AuthorizeController {

    @Autowired
    private Authorizer auth;

    @RequestMapping("/auth")
    public String sendToAuthorization() {        
        String baseUrl = "http://localhost:8080/tasks/";
        return "redirect:" +  auth.getAuthorizationUrl(baseUrl);
    }

}

As you can see the baseUrl is hardcoded, and I could provide it to the Authorizer class via Spring configuration, but I am sure that it's possible to get this information from Spring within the Controller. I tried to google "spring mvc url" and could not find a way to solve this problem.