一、计算逻辑的类结构图

 在这张图里，顶层接口 RuleTemplate 定义了 calculate 方法，抽象模板类 AbstractRuleTemplate 将通用的模板计算逻辑在 calculate 方法中实现，同时它还定义了一个抽象方法 calculateNewPrice 作为子类的扩展点。各个具体的优惠计算类通过继承 AbstractRuleTemplate，并实现 calculateNewPrice 来编写自己的优惠计算方式。

二、代码实现

1、RuleTemplate.java

public interface RuleTemplate {// 计算优惠券ShoppingCart calculate(ShoppingCart settlement);
}

2、AbstractRuleTemplate.java

public ShoppingCart calculate(ShoppingCart order) {// 获取订单总价Long orderTotalAmount = getTotalPrice(order.getProducts());// 获取以shopId为维度的总价统计Map<Long, Long> sumAmount = getTotalPriceGroupByShop(order.getProducts());CouponTemplateInfo template = order.getCouponInfos().get(0).getTemplate();// 最低消费限制Long threshold = template.getRule().getDiscount().getThreshold();// 优惠金额或者打折比例Long quota = template.getRule().getDiscount().getQuota();// 如果优惠券未指定shopId，则shopTotalAmount=orderTotalAmount// 如果指定了shopId，则shopTotalAmount=对应门店下商品总价Long shopId = template.getShopId();Long shopTotalAmount = (shopId == null) ? orderTotalAmount : sumAmount.get(shopId);// 如果不符合优惠券使用标准, 则直接按原价走，不使用优惠券if (shopTotalAmount == null || shopTotalAmount < threshold) {log.debug("Totals of amount not meet");order.setCost(orderTotalAmount);order.setCouponInfos(Collections.emptyList());return order;}// 子类中实现calculateNewPrice计算新的价格Long newCost = calculateNewPrice(orderTotalAmount, shopTotalAmount, quota);if (newCost < minCost()) {newCost = minCost();}order.setCost(newCost);log.debug("original price={}, new price={}", orderTotalAmount, newCost);return order;
}

3、子类

MoneyOffTemplate.java

@Slf4j
@Component
public class MoneyOffTemplate extends AbstractRuleTemplate implements RuleTemplate {@Overrideprotected Long calculateNewPrice(Long totalAmount, Long shopAmount, Long quota) {// benefitAmount是扣减的价格// 如果当前门店的商品总价<quota，那么最多只能扣减shopAmount的钱数Long benefitAmount = shopAmount < quota ? shopAmount : quota;return totalAmount - benefitAmount;}    
}

4、工厂类

CouponTemplateFactory.java

@Component
@Slf4j
public class CouponTemplateFactory {@Autowiredprivate MoneyOffTemplate moneyOffTemplate;@Autowiredprivate DiscountTemplate discountTemplate;@Autowiredprivate RandomReductionTemplate randomReductionTemplate;@Autowiredprivate LonelyNightTemplate lonelyNightTemplate;@Autowiredprivate DummyTemplate dummyTemplate;@Autowiredprivate AntiPauTemplate antiPauTemplate;public RuleTemplate getTemplate(ShoppingCart order) {// 不使用优惠券if (CollectionUtils.isEmpty(order.getCouponInfos())) {// dummy模板直接返回原价，不进行优惠计算return dummyTemplate;}// 获取优惠券的类别// 目前每个订单只支持单张优惠券CouponTemplateInfo template = order.getCouponInfos().get(0).getTemplate();CouponType category = CouponType.convert(template.getType());switch (category) {// 订单满减券case MONEY_OFF:return moneyOffTemplate;// 随机立减券case RANDOM_DISCOUNT:return randomReductionTemplate;// 午夜下单优惠翻倍case LONELY_NIGHT_MONEY_OFF:return lonelyNightTemplate;// 打折券case DISCOUNT:return discountTemplate;case ANTI_PUA:return antiPauTemplate;// 未知类型的券模板default:return dummyTemplate;}}}

5、使用

CouponCalculationServiceImpl.java

    @Autowiredprivate CouponTemplateFactory couponProcessorFactory;// 优惠券结算// 这里通过Factory类决定使用哪个底层Rule，底层规则对上层透明@Overridepublic ShoppingCart calculateOrderPrice(@RequestBody ShoppingCart cart) {log.info("calculate order price: {}", JSON.toJSONString(cart));RuleTemplate ruleTemplate = couponProcessorFactory.getTemplate(cart);return ruleTemplate.calculate(cart);}