// 控制器层
public class UserController {
    private UserService userService = new UserServiceImpl();
    
    public void login() {
        String username = "admin";
        String password = "123456";
        boolean success = userService.login(username, password);
        if (success) {
            // 登录成功
        } else {
            // 登录失败
        }
    }
}

// 服务层
public class UserServiceImpl implements UserService {
    private UserDao userDao = new UserDaoImplForMySQL();
    
    public boolean login(String username, String password) {
        User user = userDao.selectByUsernameAndPassword(username, password);
        return user != null;
    }
}

// 数据访问层
public class UserDaoImplForMySQL implements UserDao {
    public User selectByUsernameAndPassword(String username, String password) {
        // 连接MySQL数据库，根据用户名和密码查询用户信息
        return null;
    }
}