一、概述
运用共享技术有效地支持大量细粒度的对象。
二、适用性
当都具备下列情况时，使用Flyweight模式：
- 一个应用程序使用了大量的对象。
- 完全由于使用大量的对象，造成很大的存储开销。
- 对象的大多数状态都可变为外部状态。
- 如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象。
- 应用程序不依赖于对象标识。由于Flyweight对象可以被共享，对于概念上明显有别的对象，标识测试将返回真值。
三、参与者
- Flyweight 描述一个接口，通过这个接口flyweight可以接受并作用于外部状态。
- ConcreteFlyweight 实现Flyweight接口，并为内部状态（如果有的话）增加存储空间。ConcreteFlyweight对象必须是可共享的。它所存储的状态必须是内部的；即，它必须独立于ConcreteFlyweight对象的场景。
- UnsharedConcreteFlyweight 并非所有的Flyweight子类都需要被共享。Flyweight接口使共享成为可能，但它并不强制共享。在Flyweight对象结构的某些层次，UnsharedConcreteFlyweight对象通常将ConcreteFlyweight对象作为子节点。
- FlyweightFactory 创建并管理flyweight对象。确保合理地共享flyweight。当用户请求一个flyweight时，FlyweightFactory对象提供一个已创建的实例或者创建一个（如果不存在的话）。
四、类图
五、示例
- Flyweight
package com.lyz.design.flyweight; /** * Flyweight * @author binghe * */ public interface Flyweight { void action(int arg); }
- ConcreteFlyweight
package com.lyz.design.flyweight; /** * ConcreteFlyweight * @author binghe * */ public class FlyweightImpl implements Flyweight { public void action(int arg) { // TODO Auto-generated method stub System.out.println("参数值: " + arg); } }
- FlyweightFactory
package com.lyz.design.flyweight; import java.util.HashMap; import java.util.Map; /** * FlyweightFactory * @author binghe * */ public class FlyweightFactory { private static Map flyweights = new HashMap(); public FlyweightFactory(String arg) { flyweights.put(arg, new FlyweightImpl()); } public static Flyweight getFlyweight(String key) { if (flyweights.get(key) == null) { flyweights.put(key, new FlyweightImpl()); } return (Flyweight) flyweights.get(key); } public static int getSize() { return flyweights.size(); } }
- Test
package com.lyz.design.flyweight; /** * Test * @author binghe */ public class Test { public static void main(String[] args) { // TODO Auto-generated method stub Flyweight fly1 = FlyweightFactory.getFlyweight("a"); fly1.action(1); Flyweight fly2 = FlyweightFactory.getFlyweight("a"); System.out.println(fly1 == fly2); Flyweight fly3 = FlyweightFactory.getFlyweight("b"); fly3.action(2); Flyweight fly4 = FlyweightFactory.getFlyweight("c"); fly4.action(3); Flyweight fly5 = FlyweightFactory.getFlyweight("d"); fly4.action(4); System.out.println(FlyweightFactory.getSize()); } }
