Get UserDetails object from Security Context in Spring MVC controller

I'm using Spring Security 3 and Spring MVC 3.05.

I would like to print username of currently logged in user,how can I fetch UserDetails in my Controller?

@RequestMapping(value="/index.html", method=RequestMethod.GET)
    public ModelAndView indexView(){
         UserDetails user = ?
                mv.addObject("username", user.getUsername());
        ModelAndView mv = new ModelAndView("index");
        return mv;
    }