作用

优点：定义算法家族，根据不同的策略封装不同的算法，达到松耦合目的，也提高算法的保密性，用户只需要调用相应的api即可

同时可以替代代码中出现的大量if-else代码块

假设要实现一个促销活动，促销活动有不同的方式，比如满减，返现

本次demo实现类似的逻辑

可以根据应用层的操作实现不同的策略

策略接口

提供一个促销方法供各个策略实现类实现

public interface PromotionStrategy {
    /**
     * 促销方法
     *
     * @return void
     * @author YangYudi
     * @date 2020/12/4 14:30
     */
    void doPromotion();
}

策略类

每新增一个策略就创建一个实现策略接口的类

返现促销策略类

public class CashBackPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("返现促销");
    }
}

满减促销策略类

public class FullReductionPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("满减促销");
    }
}

促销活动类

用来调用促销策略，可以对促销策略进行封装,此处写的比较简单

public class PromotionActivity {
    /**
     * 促销策略
     */
    private PromotionStrategy promotionStrategy;
    public PromotionActivity(PromotionStrategy promotionStrategy) {
        this.promotionStrategy = promotionStrategy;
    }
    /**
     * 执行促销活动
     */
    public void executePromotion() {
        promotionStrategy.doPromotion();
    }
}

应用层调用

根据不同的场景实现不同的策略

    public static void main(String[] args) {
        String promotion = "618";
        //促销活动类
        PromotionActivity promotionActivity = null;
        //618执行满减策略
        if ("618".equals(promotion)) {
            promotionActivity = new PromotionActivity(new FullReductionPromotionStrategy());
        }
        //双十一执行返现策略
        if ("1111".equals(promotion)) {
            promotionActivity = new PromotionActivity(new CashBackPromotionStrategy());
        }
        promotionActivity.executePromotion();
    }

但是这里有一个问题

这样写并不能替换大量的if-else代码块

使用枚举类优化

根据不同的code得到不同的策略，向外提供一个接口获得策略

@Getter
@AllArgsConstructor
public enum PromotionStrategyEnum {
    /**
     * 根据不同的code查询不同策略
     */
    CASH_BACK(1111, new CashBackPromotionStrategy()),
    FULL(618, new FullReductionPromotionStrategy()),
    EMPTY(null, new EmptyPromotionStrategy());
    private Integer code;
    private PromotionStrategy promotionStrategy;
    public static PromotionStrategy getPromotionStrategy(Integer code) {
        if (ObjectUtils.isEmpty(code)) {
            return EMPTY.getPromotionStrategy();
        }
        for (PromotionStrategyEnum strategyEnum : PromotionStrategyEnum.values()) {
            if (strategyEnum.getCode().equals(code)) {
                return strategyEnum.getPromotionStrategy();
            }
        }
        return null;
    }
}

服务调用也就简单了

    public static void main(String[] args) {
        Integer promotion = 618;
        PromotionStrategy promotionStrategy = PromotionStrategyEnum.getPromotionStrategy(promotion);
        PromotionActivity promotionActivity = new PromotionActivity(promotionStrategy);
        promotionActivity.executePromotion();
    }

策略工厂模式整合

创建一个策略工厂类来获取对应策略，和枚举类的方式相似

策略工厂类

使用一个HashMap来保存策略和策略key的键值对，通过key来获取对应的策略

public class PromotionStrategyFactory {
    /**
     * 用于保存策略
     */
    private static final Map<String, PromotionStrategy> PROMOTION_STRATEGY_MAP = new ConcurrentHashMap<>();
    /**
     * 空策略
     */
    private static final PromotionStrategy EMPTY_PROMOTION_STRATEGY = new EmptyPromotionStrategy();
    static {
        //在此添加策略
        PROMOTION_STRATEGY_MAP.put(PromotionKey.FULL_REDUCTION, new FullReductionPromotionStrategy());
        PROMOTION_STRATEGY_MAP.put(PromotionKey.CASH_BACK, new CashBackPromotionStrategy());
    }
    private PromotionStrategyFactory() {
    }
    public static PromotionStrategy getPromotionStrategy(String promotionKey) {
        PromotionStrategy promotionStrategy = PROMOTION_STRATEGY_MAP.get(promotionKey);
        //如果没有就执行一个空策略
        return promotionStrategy == null ? EMPTY_PROMOTION_STRATEGY : promotionStrategy;
    }
    /**
     * 策略key 可通过应用层传入的值来获取对应的策略
     */
    private interface PromotionKey {
        /**
         * 策略值
         */
        String FULL_REDUCTION = "fullReduction";
        String CASH_BACK = "cashBack";
    }
}

如果需要设计新的策略，就需要新建策略类，并在策略工厂类里添加即可

空策略

策略工厂类可以通过应用层传入的值来获取对应的策略

如果传入的值没有就返回一个空策略

public class EmptyPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("无促销");
    }
}

应用层调用

public static void main(String[] args) {
    PromotionActivity promotionActivity = null;
    String promotionKey = "fullRedxxxxuction";
    //根据传入的值来获取策略 避免了大量的if-else语句
    PromotionStrategy promotionStrategy = PromotionStrategyFactory.getPromotionStrategy(promotionKey);
    //促销活动类
    promotionActivity = new PromotionActivity(promotionStrategy);
    promotionActivity.executePromotion();
}