用springboot写一个登录代码，要求有Controller层、entity层、mapper层、service层、imp

层、以及对应的接口文件。

Controller层：

@RestController
@RequestMapping("/user")
public class UserController {

    @Autowired
    private UserService userService;

    @PostMapping("/login")
    public String login(@RequestParam("username") String username,
                        @RequestParam("password") String password) {
        User user = userService.login(username, password);
        if (user == null) {
            return "登录失败";
        } else {
            return "登录成功";
        }
    }
}

Service层：

public interface UserService {
    User login(String username, String password);
}

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    private UserMapper userMapper;

    @Override
    public User login(String username, String password) {
        return userMapper.login(username, password);
    }
}

Mapper层：

@Mapper
public interface UserMapper {
    User login(@Param("username") String username, @Param("password") String password);
}

Entity层：

public class User {
    private Long id;
    private String username;
    private String password;

    // getters and setters
}

Imp层：

无需实现任何接口。