Strategy

概念
策略模式：Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. 定义一族算法类，将每个算法分别封装起来，让它们可以互相替换。策略模式可以使算法的变化独立于使用它们的客户端。
策略模式用于解耦策略的定义、创建、使用三部分。
常见的应用场景是避免冗长的 if-else 或 switch 分支判断。也能像模板方法一样，提供框架扩展点。
示例
定义
public interface Strategy {
  void algorithmInterface();
}
​
public class ConcreteStrategyA implements Strategy {
  @Override
  public void  algorithmInterface() {
    //具体的算法...
  }
}
​
public class ConcreteStrategyB implements Strategy {
  ...
}
创建
public class StrategyFactory {
  private static final Map<String, Strategy> strategies = new HashMap<>();
​
  static {
    strategies.put("A", new ConcreteStrategyA());
    strategies.put("B", new ConcreteStrategyB());
  }
​
  public static Strategy getStrategy(String type) {
    if (type == null || type.isEmpty()) {
      throw new IllegalArgumentException("type should not be empty.");
    }
    return strategies.get(type);
  }
}
对于无状态的策略类，可以使用上面的方式，事先创建好策略实例。如果是有状态的，则需要每次新创建策略实例。
使用
// 策略接口：EvictionStrategy
// 策略类：LruEvictionStrategy、FifoEvictionStrategy、LfuEvictionStrategy...
// 策略工厂：EvictionStrategyFactory
​
// 运行时动态确定，根据配置文件的配置决定使用哪种策略
public class Application {
  public static void main(String[] args) throws Exception {
    EvictionStrategy evictionStrategy = null;
    Properties props = new Properties();
    props.load(new FileInputStream("./config.properties"));
    String type = props.getProperty("eviction_type");
    evictionStrategy = EvictionStrategyFactory.getEvictionStrategy(type);
    UserCache userCache = new UserCache(evictionStrategy);
    //...
  }
}
Template Method
State
Last modified 2yr ago