使用UserDetailsService的Spring Security身份验证

我对Spring安全认证有一些问题。在我的应用程序中的任何地方，一切正常（CRUD操作正常），但是登录尝试失败。

这是我的代码（我在下面用注释标记了其中userDAO为null这是身份验证失败的原因）：

@Service
public class UserServiceImpl implements UserService, UserDetailsService {

    @Autowired
    UserDAO userDAO;

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        User user = userDAO.getUserByUsername(username); //userDAO == null Causing NPE
        if (user == null)
            throw new UsernameNotFoundException("Oops!");

        List<SimpleGrantedAuthority> authorities = Arrays.asList(new SimpleGrantedAuthority(user.getRole()));

        return new org.springframework.security.core.userdetails
                .User(user.getLogin(), user.getPassword(), authorities);
    }

@Override
    public List<User> getUsers() {
        return userDAO.getUsers();//userDAO !=null
    }
//rest of code skipped

我的SecurityConfig看起来像这样

@Configuration
@EnableWebMvcSecurity
@EnableGlobalMethodSecurity(prePostEnabled = true)
public class SecurityConfig extends WebSecurityConfigurerAdapter {

 UserServiceImpl userService = new UserServiceImpl();

    @Override
    protected void configure(AuthenticationManagerBuilder auth) throws Exception {
        auth.userDetailsService(userService);
    }
//rest of code skipped

我标记了我在哪里获得NPE，但我不知道如何解决该问题。整个配置都是基于Java的，您可以在这里查看更多信息，这里

编辑：getUsers（）在控制器中以这种方式调用：

@Controller
public class LoginController {
    @Autowired
    UserService userService;

 @RequestMapping(value = "/dashboard")
    public ModelAndView userDashboard(){
        ModelAndView modelAndView = new ModelAndView("Dashboard");
        List<User> userList = userService.getUsers();
        modelAndView.addObject("users", userList);
        return modelAndView;
    }

并且在这种情况下（当调用userService.getUsers（）时）userDAO不为null

试图按照Bohuslav Burghardt的建议修复它，我得到了

 method userDetailsService in class org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder cannot be applied to given types;
  required: T
  found: com.gi.service.UserService
  reason: inferred type does not conform to upper bound(s)
    inferred: com.gi.service.UserService
    upper bound(s): org.springframework.security.core.userdetails.UserDetailsService

在auth.userDetailsS​​ervice（userService）行中；