2.4 策略模式

1.定义
定义了算法家族，分别封装起来，让他们之间可以相互替换，此模式让算法的变化不会影响到使用算法的用户。

2.优点
符合开闭原则，避免使用多重条件语句。

3.代码示例

public interface PromotionStrategy {
    void doPromotion();
}

public class LijianPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("立减20促销活动");
    }
}

public class ManjianPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("满200减20促销活动");
    }
}

public class FanxianPromotionStrategy implements PromotionStrategy {
    @Override
    public void doPromotion() {
        System.out.println("返现20促销活动");
    }
}

public class PromotionActivity {
    private PromotionStrategy promotionStrategy;

    public PromotionActivity(PromotionStrategy promotionStrategy) {
        this.promotionStrategy = promotionStrategy;
    }

    public void executePromotionActivity() {
        promotionStrategy.doPromotion();
    }
}

public class Test {
    public static void main(String[] args) {
        PromotionActivity promotionActivity = new PromotionActivity(new LijianPromotionStrategy());
        promotionActivity.executePromotionActivity();
    }
}

立减20促销活动

4.源码解析

public class DefaultResourceLoader implements ResourceLoader {
  @Override
  public Resource getResource(String location) {
    Assert.notNull(location, "Location must not be null");
    //1.用户自定义协议资源解决策略，返回Resource
    for (ProtocolResolver protocolResolver : this.protocolResolvers) {
      Resource resource = protocolResolver.resolve(location, this);
      if (resource != null) {
        return resource;
      }
    }

    //2.如果是以/开头，则构造ClassPathContextResource类型资源返回
    if (location.startsWith("/")) {
      return getResourceByPath(location);
    }else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
      //3.如果以classpath:开头，则构造ClassPathResource类型资源返回
      return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
    }else {
      try {
        //4.构造URL，尝试进行资源定位，返回FileUrlResource或者UrlResource，如果没有定位到，则抛出MalformedURLException异常，以ClassPathContextResource类型资源返回
        URL url = new URL(location);
        return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
      }catch (MalformedURLException ex) {
        //构造ClassPathContextResource类型资源返回
        return getResourceByPath(location);
      }
    }
  }
}