策略（Strategy）模式：分离算法，选择实现
比如对象的某个行为，在不同场景有不同实现方式，可以将这些行为的具体实现定义为一组策略，每个实现类实现一种策略，在不同场景使用不同的实现，并且可以自由切换策略，但太多的策略会导致大量代码

• 优点：
  • 不用太多if else
  • 代码优雅、简单
  • 符合开闭原则，扩展性好、便于维护
• 优点：
  • 策略如果很多的话，会造成策略类膨胀
  • 使用者必须清楚所有的策略类及其用途

如：电商活动打折，普通会员9折，黄金会员8折，钻石会员7折
1.定义一个Strategy接口，只有打折方法
2.增加3个打折类，对应9折、8折、7折
3.客户端根据会员类型，获取具体的策略算法实现类，再执行打折方法

策略模式与Spring结合

/*** 打折策略枚举*/
public enum ActivityStrategyEnum {MEMBER(1, "普通会员"),GOLD_MEMBER(2, "黄金会员"),DIAMOND_MEMBER(3, "钻石会员"),;private int code;private String name;ActivityStrategyEnum (int code, String name) {this.code = code;this.name = name;}/*** 通过code匹配对应枚举* @param code* @return*/public static ActivityStrategyEnum match(int code){ActivityStrategyEnum strategyEnum = null;for (ActivityStrategyEnum item : values()){if(item.getCode() == code){strategyEnum = item;break;}}return strategyEnum;}public int getCode() {return code;}public String getName() {return name;}
}

//打折接口
public interface IActivityStrategyService {//策略code    int getCode();/*** 打折* @param money* @return 应付金额*/BigDecimal calculate(BigDecimal money);
}@Service("memberService ")
public class MemberService implements IActivityStrategyService {@Overridepublic int getCode() {return ActivityStrategyEnum.MEMBER.getCode();}@Overridepublic BigDecimal calculate(BigDecimal money) {//普通会员9折return money.multiply(new BigDecimal(0.9)).setScale(2, RoundingMode.HALF_UP);}
}@Service("goldMemberService")
public class GoldMemberService implements IActivityStrategyService {@Overridepublic int getCode() {return ActivityStrategyEnum.GOLD_MEMBER.getCode();}@Overridepublic BigDecimal calculate(BigDecimal money) {//黄金会员8折return money.multiply(new BigDecimal(0.8)).setScale(2, RoundingMode.HALF_UP);}
}@Service("diamondMemberService")
public class DiamondMemberService implements IActivityStrategyService {@Overridepublic int getCode() {return ActivityStrategyEnum.DIAMOND_MEMBER.getCode();}@Overridepublic BigDecimal calculate(BigDecimal money) {//钻石会员7折return money.multiply(new BigDecimal(0.7)).setScale(2, RoundingMode.HALF_UP);}
}

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.stereotype.Component;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;@Component
public class ActivityStrategyHandler implements InitializingBean, ApplicationContextAware {/*** 存放策略的map,可以理解为策略的注册中心*/private final Map serviceHashMap = new ConcurrentHashMap<>();/*** spring的上下文*/private ApplicationContext applicationContext;/*** 将StrategyService的类都按照定义好的规则（枚举code），放入strategyServiceMap中** @throws Exception*/@Overridepublic void afterPropertiesSet() throws Exception {Map matchBeans = applicationContext.getBeansOfType(IActivityStrategyService.class);for (IActivityStrategyService strategyService : matchBeans.values()) {serviceHashMap.put(strategyService.getCode(), strategyService);System.out.println("初始化策略模式的键值对 code =" + strategyService.getCode() + " ,value=" + strategyService);}}/*** 注入applicationContext** @param applicationContext* @throws BeansException*/@Overridepublic void setApplicationContext(ApplicationContext applicationContext) throws BeansException {this.applicationContext = applicationContext;}/*** 通过枚举code获取对应对更新服务* @param strategyEnum* @return*/public IActivityStrategyService getActivityService(ActivityStrategyEnum strategyEnum){return serviceHashMap.get(strategyEnum.getCode());}
}

客户端调用

@RestController
public class IndexController {@Autowiredprivate ActivityStrategyHandler activityHandler;@GetMapping("/acitivity")public BigDecimal acitivity(int code) throws JsonProcessingException {//会员类型codeActivityStrategyEnum strategyEnum = ActivityStrategyEnum.match(code);IActivityStrategyService activityService = activityHandler.getActivityService(strategyEnum);//获取商品金额BigDecimal amount = new BigDecimal(100);//调用具体的打折策略算法BigDecimal price = activityService.calculate(amount);return price ;}
}