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Java设计模式研究之Flyweight模式
发表于：2007-07-04来源：作者：点击数：
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GOF：运用共享技术有效地支持大量细粒度的对象。 解释一下概念：也就是说在一个系统中如果有多个相同的对象，那么只共享一份就可以了，不必每个都去实例化一个对象。比如说（这里引用GOF书中的例子）一个文本系统，每个字母定一个对象，那么大小写字母一共就
GOF：运用共享技术有效地支持大量细粒度的对象。 在Flyweight模式中，由于要产生各种各样的对象，所以在Flyweight(享元)模式中常出现Factory模式。Flyweight的内部状态是用来共享的,Flyweight factory负责维护一个对象存储池（Flyweight Pool）来存放内部状态的对象。Flyweight模式是一个提高程序效率和性能的模式,会大大加快程序的运行速度.应用场合很多，下面举个例子：
先定义一个抽象的Flyweight类：
| package Flyweight; public abstract class Flyweight ... { public abstract void operation(); }//end abstract class Flyweight |
在实现一个具体类：
| package Flyweight; public class ConcreteFlyweight extends Flyweight ... { private String string; public ConcreteFlyweight(String str) ... { string = str; }//end ConcreteFlyweight(...) public void operation() ... { System.out.println("Concrete---Flyweight : " + string); }//end operation() }//end class ConcreteFlyweight |
实现一个工厂方法类：
| package Flyweight; import java.util.Hashtable; public class FlyweightFactory ... { private Hashtable flyweights = new Hashtable();//----------------------------1 public FlyweightFactory() ...{} public Flyweight getFlyWeight(Object obj) ... { Flyweight flyweight = (Flyweight) flyweights.get(obj);//----------------2 if(flyweight == null) ...{//---------------------------------------------------3 //产生新的ConcreteFlyweight flyweight = new ConcreteFlyweight((String)obj); flyweights.put(obj, flyweight);//--------------------------------------5 } return flyweight;//---------------------------------------------------------6 }//end GetFlyWeight(...) public int getFlyweightSize() ... { return flyweights.size(); } }//end class FlyweightFactory |
这个工厂方法类非常关键，这里详细解释一下：
在1处定义了一个Hashtable用来存储各个对象；在2处选出要实例化的对象，在6处将该对象返回，如果在Hashtable中没有要选择的对象，此时变量flyweight为null，产生一个新的flyweight存储在Hashtable中，并将该对象返回。
最后看看Flyweight的调用：
| package Flyweight; import java.util.Hashtable; public class FlyweightPattern ...{ FlyweightFactory factory = new FlyweightFactory(); Flyweight fly1; Flyweight fly2; Flyweight fly3; Flyweight fly4; Flyweight fly5; Flyweight fly6; /** *//** Creates a new instance of FlyweightPattern */ public FlyweightPattern() ...{ fly1 = factory.getFlyWeight("Google"); fly2 = factory.getFlyWeight("Qutr"); fly3 = factory.getFlyWeight("Google"); fly4 = factory.getFlyWeight("Google"); fly5 = factory.getFlyWeight("Google"); fly6 = factory.getFlyWeight("Google"); }//end FlyweightPattern() public void showFlyweight() ... { fly1.operation(); fly2.operation(); fly3.operation(); fly4.operation(); fly5.operation(); fly6.operation(); int objSize = factory.getFlyweightSize(); System.out.println("objSize = " + objSize); }//end showFlyweight() public static void main(String[] args) ... { System.out.println("The FlyWeight Pattern!"); FlyweightPattern fp = new FlyweightPattern(); fp.showFlyweight(); }//end main(...) }//end class FlyweightPattern |
下面是运行结果：
| Concrete---Flyweight : Google Concrete---Flyweight : Qutr Concrete---Flyweight : Google Concrete---Flyweight : Google Concrete---Flyweight : Google Concrete---Flyweight : Google objSize = 2 |
我们定义了6个对象，其中有5个是相同的，按照Flyweight模式的定义“Google”应该共享一个对象，在实际的对象数中我们可以看出实际的对象却是只有2个。
下面给出一个简易的UML图：
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