org.springframework.web.bind.annotation重定向的问题

  @RequestMapping(value="/redir/authcode")
    public ModelAndView getAuthCode(){
        String authUrl="https://openauth.alipay.com/oauth2/publicAppAuthorize.htm?app_id="+contant.APPID+"&scope=auth_user&redirect_uri="+contant.ENCODED_URL+"";
        return new ModelAndView(new RedirectView(authUrl));
    }
//重定向到外网地址

@Controller
@RequestMapping("/redir")
public class RedirController {

    @ResponseBody
    @RequestMapping(value="pay")
    public String redirect(){
        return "redirect:/pay.html";
    }
}
//这种方法无法跳转

@Controller
@RequestMapping("/redir")
public class RedirController {

    @ResponseBody
    @RequestMapping(value="pay")
    public ModelAndView redirect(){

        return new ModelAndView("redirect:/pay.html");
    }
}
//正确方法

@Controller
public class RedirController {

    @RequestMapping(value="/redir/pay")
    public String redirect(){
        return "redirect:/pay.html";
    }
}
//正确跳转方式