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

问题：调用的方法在一个接口类中，但我并没有注入那个被调用的类

解决：在UserEntity前加上@Autowired

@Controller

public class MainController {

// 自动装配数据库接口，不需要再写原始的Connection来操作数据库

    @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";

    }

}