mybatis service调用dao方法空指针异常

xml:<context:component-scan base-package="cn.xd.forum.service"/>
dao层是接口
service:
@Service("userService")
public class userServiceImpl implements userService{
@Qualifier
private userDao userDao;
private Utils util;
public User login(String email, String pwd) {
String password = util.crypt(pwd);
String uid = userDao.findUidByEmail(email);
User user = userDao.getInfoByUid(uid);
if(user.getPwd().equals(password)){
return user;
}
return null;
}

(第四到第五行改为以下代码试试)
@Autowired
@Qualifier("你spring.xml里对应bean的id")
private userDao userDao;