我的配置跟老师一样，为什么提示cannot resolve MVC view"freemarker/index"

@Controller
@RequestMapping("ftl")
public class FreemarkerController {
    @Autowired
    private Resource resource;

    @RequestMapping("/index")
    public String index(ModelMap map){
        map.addAttribute("resource", resource);
        return "freemarker/index";
    }

    @RequestMapping("center")
    public String center(){
        return "freemarker/center/center";
    }

    @RequestMapping("test")
    public String test(){
        return "test";
    }
}