添加@Autowired后LoginController仍报NullPointerException求助

我在LoginController中给private IUserService userService添加@Autowired注解后，运行代码仍抛出空指针异常：

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause java.lang.NullPointerException: null

• 未加@Autowired时，登录页处理用户输入始终报上述空指针
• 加了@Autowired后，输入无效手机号时无响应，输入有效手机号和密码时还是报空指针
• 试过结合@Component加@Qualifier注解，问题没解决

相关代码 #

LoginController.java

@Controller
@RequestMapping("/login")
@Slf4j 
public class LoginController {

    @Autowired
//    no autowired, nullpointerexception
//    autowired, cannot receive response
    private IUserService userService;

    @RequestMapping("/toLogin") //to log in page 跳转登录页
    public String toLogin(){
        return "login";
    }

    @RequestMapping("/doLogin")
    @ResponseBody
    public RespBean doLogin(LoginVo loginVo) {
        // 接受传参 receive parameter from users with LoginVo
        return userService.doLogin(loginVo);
    }
}

IUserService.java

public interface IUserService extends IService<User> {

    RespBean doLogin(LoginVo loginVo);

}

UserServiceImpl.java

@Service
public class UserServiceImpl extends ServiceImpl<UserMapper, User> implements IUserService {

    private UserMapper userMapper;

    @Override
    public RespBean doLogin(LoginVo loginVo) {
        String mobile = loginVo.getMobile();
        String password = loginVo.getPassword();

        // check whether input is empty
        if (StringUtils.isEmpty(mobile) || StringUtils.isEmpty(password)) {
            return RespBean.error(RespBeanEnum.LOGIN_ERROR);
        }
        // check whether mobile is valid
        if (!ValidatorUtil.isMobile(mobile)) {
            return RespBean.error(RespBeanEnum.MOBILE_ERROR);
        }
        // find user with mobile
        User user = userMapper.selectById(mobile);
        if (user == null) {
            return RespBean.error(RespBeanEnum.LOGIN_ERROR);
        }

        // verify whether user password is correct
        if (!MD5Util.frontPassToDBPass(password, user.getSalt()).equals(user.getPassword())) {
            return RespBean.error(RespBeanEnum.LOGIN_ERROR);

        }
        return RespBean.success();
    }
}

1. 核心空指针原因：UserServiceImpl中的UserMapper未注入 #

你的UserServiceImpl里private UserMapper userMapper;没有添加任何注入注解，调用userMapper.selectById(mobile)时必然触发空指针——这才是问题根源，和LoginController里的userService注入无关（加@Autowired后userService已经被正确注入了）。

解决：给UserMapper加上@Autowired注解：

@Service
public class UserServiceImpl extends ServiceImpl<UserMapper, User> implements IUserService {

    @Autowired
    private UserMapper userMapper;

    // 剩余代码不变
}

2. 验证userService注入有效性 #

可以在LoginController的doLogin方法开头加日志，确认userService是否真的被注入：

@RequestMapping("/doLogin")
@ResponseBody
public RespBean doLogin(LoginVo loginVo) {
    log.info("userService是否为空：{}", userService == null);
    return userService.doLogin(loginVo);
}

如果日志输出false，说明userService注入正常，空指针完全来自未注入的userMapper。

3. 无效手机号无响应问题排查 #

• 检查ValidatorUtil.isMobile方法的逻辑是否正确，确保能正确识别无效手机号
• 在无效手机号的分支加日志，确认代码是否走到了返回逻辑：

if (!ValidatorUtil.isMobile(mobile)) {
    log.info("无效手机号：{}", mobile);
    return RespBean.error(RespBeanEnum.MOBILE_ERROR);
}

• 确认RespBean类是否正确实现了JSON序列化，保证前端能接收到返回值

内容的提问来源于stack exchange，提问作者meimaymay