@Service??
public?class?OrderServiceImpl?implements?OrderService?{
????@Autowired??
????private?UserMapper userMapper;
????@Autowired??
????private?ProductMapper productMapper;
????@Autowired??
????private?OrderMapper orderMapper;
????@Autowired??
????private?KafkaTemplate kafkaTemplate;
??
????/**
?????* 购买商品，提交订单
?????*?@param?userId 用户ID
?????*?@param?productId 商品ID
?????*?@return??
?????*/??
????public?Result?submit(Long userId, Long productId)?throws?BizException?{
????????// 验证账号
????????UserDO userDO = userMapper.findById(userId);
????????if?(userDO ==?null) {
????????????throw?BizException(USER_NOT_EXISTS);
????????}
????????// 查看商品信息及打折信息
????????ProductDO productDO = productMapper.findById(productId);
????????Double delta = HttpUtils.getDiscount(productId);
????????double?actualPayment = productDO.getPrice() - delta;
????????Money money = userDO.getMoney();
????????if?(actualPayment > money.getRemain()) {
????????????// 如果商品价格 - 优惠价格 > 用户钱包，则说明不够付
????????????return?Result.fail("余额不足");
????????}
????????// 钱包够付，扣除金额
????????double?remain = money.getRemain() - actualPayment;
????????money.setRemain(remain);
????????// 更新账号钱包余额
????????userMapper.update(userDO);
????????// 生成订单信息
????????OrderDO orderDO =?new?OrderDO();
????????orderDO.setUserId(userId);
????????orderDO.setProductId(productId);
????????orderMapper.save(orderDO);
????????// 通知用户订单已生成，等待收货
????????kafkaTemplate.send("orderTopic", orderDO);
????????return?Result.ok();
????}
}

上面代码写好了，而且可以实现相关功能，但是随着业务的迭代，可能会出现很多问题：

可维护性差

1. XxMapper是基于Mybatis实现数据操作层，也就把技术细节带入业务逻辑中了，如果技术实现变了（改为使用Hibernate，或Mybatis版本升级造成用法改变等），业务代码就得改变。

2. XxDO 是和数据表绑定的，数据表结构变更等也会影响业务代码。

3. 调用第三方API，直接在业务代码中调用HttpUtils完成，未来第三方API修改了方法签名或返回值，或改为了RPC接口，那么业务代码也会随着改变。

4. 发送消息直接使用KafkaTemplate，如果技术选型变了要改为使用RocketMQ，那么业务代码还得变。

可扩展性差

1. 如果商品因为做活动又加了其他的优惠，或商品某一段时间不打折了，那么原有的代码就会重新改来改去；

2. 业务逻辑和数据存储结构是强依赖的，数据存储结构的变化对业务的影响可想而知；

可测试性差

因为直接依赖了数据库，第三方接口，中间件，所以需要所有技术实现后才能进行测试，测试成本和时间都比较大。

代码优化一

我们上面说了，数据库操作不应该直接暴露在业务逻辑中，因此把数据库操作“隔离”开。

public?interface?UserRepository?{
????User?findById(Long userId);
}

public?class?Money?{
????private?double?remain;
????public?double?getRemain()?{
????????return?remain;
????}
????public?void?setRemain(double?remain)?{
????????this.remain = remain;
????}
????/**
?????* 扣费
?????*?@param?delta
?????*?@return??
?????*/??
????public?boolean?charge(double?delta)?{
????????if?(remain < delta) {
????????????return?false;
????????}
????????this.remain -= delta;
????????return?true;
????}
}

/**
?* 防腐层/适配层
?*/??
@Service??
public?class?PayServiceImpl?implements?PayService?{
???
????@Autowired??
????private?DiscountFacade discountFacade;
??
????/**
?????* 支付
?????*?@param?money
?????*?@param?product
?????*?@return??
?????*/??
????public?boolean?pay(Money money, Product product)?{
????????// 获取优惠
????????Double delta = discountFacade.getDiscount(product.getId());
????????// 扣除费用
????????return?money.charge(product.getPrice() - delta);
????}
}

public?interface?MessageProducer<T,?R>?{
????Result<R>?send(T message);
}

@Autowired??
private?UserRepository userRepository;
@Autowired??
private?ProductRepository productRepository;
@Autowired??
private?OrderRepository orderRepository;
@Autowired??
private?MessageProducer<Order,Result> messageProducer;
@Autowired??
private?PayService payService;
??
/**
?* 购买商品，提交订单
?*?@param?userId 用户ID
?*?@param?productId 商品ID
?*?@return??
?*/??
public?Result?submit(Long userId, Long productId)?throws?BizException?{
????// 验证
????User user = userRepository.findByUserId(userId);
????if?(user ==?null) {
????????throw?BizException(USER_NOT_EXISTS);
????}
????// 支付
????Product product = productRepository.findById(productId);
????boolean?f = payService.pay(user.getMoney(), product);
????if?(!f) {
????????return?Result.fail("费用扣除失败");
????}
????// 更新账户
????userRepository.update(user);
????// 生成订单信息
????Order order = OrderFactory.create(user, product);
????orderRepository.add(order);
????// 通知用户订单已生成，等待收货
????messageProducer.send(order);
????return?Result.ok();
}