問題：調用的方法在一個接口類中，但我並沒有註入那個被調用的類

解決：在UserEntity前加上@Autowired

@Controller

public class MainController {



// 自動裝配數據庫接口，不需要再寫原始的Connection來操作數據庫

    @Autowired

    UserRepository userRepository;



    @RequestMapping(value = "/",method = RequestMethod.GET)

    public String index(){

        return "index";

    }



    @RequestMapping(value 
 
= "/admin/users",method = RequestMethod.GET)

    public String getUsers(ModelMap modelMap){

        List<UserEntity> userList = userRepository.findAll();

        modelMap.addAttribute("userList", userList);

        return "admin/users";

    }

}

error：org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException