一、为什么需要策略模式在实际业务开发中经常会遇到一个接口有多种不同实现方式的场景。例如支付系统微信支付、支付宝支付、银行卡支付订单折扣满减、打折、VIP特价文件处理PDF导出、Excel导出、CSV导出通知发送短信、邮件、Push传统的做法是使用if-else或switch分支判断但这会导致代码臃肿、难以维护并且每次新增一种策略都需要修改原有逻辑违反开闭原则对扩展开放对修改关闭。策略模式能很好地解决这个问题它将算法族分别封装起来让它们可以互相替换使得算法的变化独立于使用算法的客户。二、策略模式核心概念策略接口定义折扣算法的公共方法。具体策略实现接口封装具体折扣计算逻辑。上下文持有策略引用负责调用算法对外提供统一入口。在 Spring Boot 中上下文通常是一个 Service具体策略通过 IoC 容器自动注册。三、业务场景与痛点在电商系统中订单折扣规则经常变化新人首单优惠、满减折扣、会员等级折扣、限时秒杀等。传统做法是在业务代码中堆砌if-elsepublic double calculatePrice(Order order) { if (order.isNewUser()) { // 新人优惠逻辑 } else if (order.getAmount() 100) { // 满减逻辑 } else if (VIP.equals(order.getUserLevel())) { // VIP折扣 } // 每次新增规则都要修改这个方法极易出错 }这种代码违背开闭原则维护成本高测试困难。策略模式可以将每种折扣算法独立封装通过 Spring 的依赖注入实现动态切换极大增强系统扩展性。四、案例实战订单折扣系统基础版策略接口 Spring List 注入步骤1定义策略接口package com.example.discount.strategy; import com.example.discount.dto.OrderDTO; public interface DiscountStrategy { /** * 计算折扣后的金额 * param order 订单信息 * return 折扣结果包含原始金额、优惠金额、最终金额 */ DiscountResult calculate(OrderDTO order); /** * 策略标识用于客户端选择 */ String getType(); }步骤2实现两种具体策略新人折扣首单立减10元package com.example.discount.strategy; import com.example.discount.dto.OrderDTO; import com.example.discount.dto.DiscountResult; import org.springframework.stereotype.Component; Component public class NewUserDiscount implements DiscountStrategy { Override public DiscountResult calculate(OrderDTO order) { double original order.getAmount(); double discount 10.0; double finalAmount Math.max(0, original - discount); return new DiscountResult(original, discount, finalAmount, 新人首单减10元); } Override public String getType() { return new_user; } }满减折扣满100减20满200减50Component public class AmountThresholdDiscount implements DiscountStrategy { Override public DiscountResult calculate(OrderDTO order) { double original order.getAmount(); double discount 0; if (original 200) { discount 50; } else if (original 100) { discount 20; } return new DiscountResult(original, discount, original - discount, 满减折扣); } Override public String getType() { return amount_threshold; } }步骤3上下文 Service - 自动注册策略package com.example.discount.service; import com.example.discount.dto.DiscountResult; import com.example.discount.dto.OrderDTO; import com.example.discount.strategy.DiscountStrategy; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Service; import javax.annotation.PostConstruct; import java.util.HashMap; import java.util.List; import java.util.Map; Service public class OrderService { Autowired private ListDiscountStrategy discountStrategies; // Spring自动注入所有策略Bean private MapString, DiscountStrategy strategyMap new HashMap(); PostConstruct public void init() { for (DiscountStrategy strategy : discountStrategies) { strategyMap.put(strategy.getType(), strategy); } } /** * 根据折扣类型计算订单最终价格 * param order 订单信息 * param discountType 折扣类型new_user / amount_threshold */ public DiscountResult applyDiscount(OrderDTO order, String discountType) { DiscountStrategy strategy strategyMap.get(discountType); if (strategy null) { throw new IllegalArgumentException(不支持的折扣类型: discountType); } return strategy.calculate(order); } }步骤4Controller 与 DTORestController RequestMapping(/order) public class OrderController { Autowired private OrderService orderService; PostMapping(/discount) public DiscountResult applyDiscount(RequestBody OrderDTO order, RequestParam String discountType) { return orderService.applyDiscount(order, discountType); } }DTOpublic class OrderDTO { private Double amount; private String userId; private Boolean newUser; // getter/setter 省略 } public class DiscountResult { private double originalAmount; private double discountAmount; private double finalAmount; private String description; // 构造方法、getter/setter 省略 }测试效果请求/order/discount?discountTypenew_user返回新人折扣请求/order/discount?discountTypeamount_threshold返回满减折扣扩展性新增“VIP折扣”时只需新建VipDiscount类实现DiscountStrategy标注Component重写getType()返回vip无需修改任何现有代码。五、进阶优化自定义注解 自动注册基础版需要每个策略手动实现getType()容易出错。我们可以使用自定义注解声明策略标识通过ApplicationContextAware自动扫描注册。5.1 自定义注解package com.example.discount.annotation; import org.springframework.stereotype.Component; import java.lang.annotation.*; Target(ElementType.TYPE) Retention(RetentionPolicy.RUNTIME) Component // 组合Component使Spring能扫描到 public interface DiscountType { String value(); // 策略标识如 vip }5.2 修改策略类移除 getTypeDiscountType(new_user) public class NewUserDiscount implements DiscountStrategy { Override public DiscountResult calculate(OrderDTO order) { // ... 同前不再需要 getType() } }5.3 策略工厂自动注册package com.example.discount.factory; import com.example.discount.annotation.DiscountType; import com.example.discount.strategy.DiscountStrategy; import org.springframework.beans.BeansException; import org.springframework.context.ApplicationContext; import org.springframework.context.ApplicationContextAware; import org.springframework.stereotype.Component; import java.util.HashMap; import java.util.Map; Component public class DiscountStrategyFactory implements ApplicationContextAware { private final MapString, DiscountStrategy strategyMap new HashMap(); Override public void setApplicationContext(ApplicationContext context) throws BeansException { // 获取所有标注了 DiscountType 的 Bean MapString, Object beans context.getBeansWithAnnotation(DiscountType.class); for (Object bean : beans.values()) { if (bean instanceof DiscountStrategy) { DiscountType annotation bean.getClass().getAnnotation(DiscountType.class); strategyMap.put(annotation.value(), (DiscountStrategy) bean); } } } public DiscountStrategy getStrategy(String type) { DiscountStrategy strategy strategyMap.get(type); if (strategy null) { throw new IllegalArgumentException(未找到折扣策略: type); } return strategy; } }5.4 修改 OrderServiceService public class OrderService { Autowired private DiscountStrategyFactory factory; public DiscountResult applyDiscount(OrderDTO order, String discountType) { DiscountStrategy strategy factory.getStrategy(discountType); return strategy.calculate(order); } }优势新增策略只需编写类并添加DiscountType(xxx)零侵入完全符合开闭原则。六、动态策略选择结合枚举与前端配置很多时候折扣规则不是由前端直接传入字符串而是需要根据订单属性自动匹配。比如NewUserDiscount只对newUsertrue的订单生效。这时可以添加一个策略匹配器。6.1 增加策略的匹配条件在策略接口中添加一个supports方法public interface DiscountStrategy { DiscountResult calculate(OrderDTO order); boolean supports(OrderDTO order); // 判断是否适用于此订单 }6.2 实现匹配逻辑DiscountType(new_user) public class NewUserDiscount implements DiscountStrategy { Override public DiscountResult calculate(OrderDTO order) { ... } Override public boolean supports(OrderDTO order) { return Boolean.TRUE.equals(order.getNewUser()); } }6.3 自动匹配最佳策略Service public class OrderService { Autowired private ListDiscountStrategy strategies; // 注入所有策略 public DiscountResult applyBestDiscount(OrderDTO order) { for (DiscountStrategy strategy : strategies) { if (strategy.supports(order)) { return strategy.calculate(order); } } // 无任何策略匹配时返回无折扣 return new DiscountResult(order.getAmount(), 0, order.getAmount(), 无折扣); } }此时客户端无需传递discountType系统自动选择第一个匹配的规则。可以通过Order注解控制策略执行顺序。七、与 Spring 条件注解结合某些折扣策略只在特定环境启用如灰度发布、配置开关。使用ConditionalOnPropertyComponent ConditionalOnProperty(name discount.vip.enabled, havingValue true) DiscountType(vip) public class VipDiscount implements DiscountStrategy { // ... }在application.yml中配置discount.vip.enabled: true才加载 VIP 折扣否则忽略。八、总结与最佳实践方式适用场景优点缺点List注入 PostConstruct策略数量少标识稳定简单直观每个策略需实现getType自定义注解 工厂策略数量多团队协作零侵入高扩展稍微复杂需要工厂类supports自动匹配规则自动选择客户端无需传参无法处理多规则冲突条件注解根据配置动态装载灵活控制策略生效范围增加配置管理关键原则策略类应设计为无状态不保存实例变量保证线程安全。如果策略需要参数统一封装为 Context 对象传给策略方法。策略模式与工厂模式往往搭配使用工厂负责创建/获取策略上下文负责调用。避免“策略爆炸”当策略数量非常多时考虑使用责任链模式或规则引擎如 EasyRules、Drools。