This application has no explicit mapping for /error

@SpringBootApplication
public class DemoApplication {
   public static void main(String[] args) {
      SpringApplication.run(DemoApplication.class, args);
   }

   @RequestMapping("/")
   public String index(){
      return "hello spring boot!";
   }
}

报标题的错误是因为没有添加

@RestController注解，导致url路由失败，加上之后就可以了