享元模式-Flyweight

享元模式:运用共享技术有效地支持大量细粒度的对象.

内部状态和外部状态:

　　享元模式可以避免大量的非常相似的类的开销.在程序设计中,有时需要生成大量细粒度的类实例来表示数据.如果能发现这些实例除了几个参数外基本上都相同,有时就能大幅度减少需要实例化的类的数量.如果能把那些参数移到类实例的外部,在方法调用时将它们传递进来,就可以通过共享,大幅度地减少单个实例的数目.

何时使用享元模式:

　　如果一个应用程序使用了大量的对象,而大量的这些对象造成了存储开销时就应该考虑使用;还有就是对象的部分状态可已放到外部,那就可以用较少的共享对象来表示很多组对象.

享元模式结构图:

代码实现:

 1 package com.cgj.pattern.flyweight;
 2
 3 /**
 4  * Flyweight接口:是多有享元类的超类或接口,通过这个接口,
 5  *      Flyweight可以接受并作用于外部状态.
 6  */
 7 public interface Flyweight {
 8
 9     public abstract void operation(int extrinsicstate);
10 }

 1 package com.cgj.pattern.flyweight;
 2
 3 /**
 4  * ConcreteFlyweight类实现Flyweight接口,并为内部状态增加存储空间.
 5  */
 6 public class ConcreteFlyweight implements Flyweight {
 7
 8     @Override
 9     public void operation(int extrinsicstate) {
10         System.out.println("具体的Flyweight:" + extrinsicstate);
11     }
12
13 }

 1 package com.cgj.pattern.flyweight;
 2
 3 /**
 4  * UnsharedConcreteFlyweight指不需要共享的Flyweight实现类.
 5  *      因为实现了Flyweight接口共享成为可能,但并不强制共享.
 6  */
 7 public class UnsharedConcreteFlyweight implements Flyweight {
 8
 9     @Override
10     public void operation(int extrinsicstate) {
11         System.out.println("不共享的具体Flyweight:" + extrinsicstate);
12     }
13
14 }

 1 package com.cgj.pattern.flyweight;
 2
 3 import java.util.HashMap;
 4 import java.util.Map;
 5
 6 /**
 7  * Flyweight是一个享元工厂,用来创建并管理Flyweight类对象.
 8  *      它主要用来确保合理的共享Flyweight,当用户请求一个Flyweight时,
 9  *      它创建一个Flyweight或者返回一个已经创建好的Flyweight.
10  */
11 public class FlyweightFactory {
12
13     // 保存Flyweight
14     private Map<String, Flyweight> flyweights = new HashMap<String, Flyweight>();
15
16     // 获取Flyweight
17     public Flyweight getFlyweight(String key) {
18         if (!flyweights.containsKey(key)) {
19             flyweights.put(key, new ConcreteFlyweight());
20         }
21         return flyweights.get(key);
22     }
23
24 }

 1 package com.cgj.pattern.flyweight;
 2
 3 public class Test {
 4
 5     public static void main(String[] args) {
 6         int extrinsicstate = 22;
 7
 8         FlyweightFactory factory = new FlyweightFactory();
 9
10         Flyweight fx = factory.getFlyweight("X");
11         fx.operation(extrinsicstate--);
12
13         Flyweight fy = factory.getFlyweight("Y");
14         fy.operation(extrinsicstate--);
15
16         Flyweight fz = factory.getFlyweight("Z");
17         fz.operation(extrinsicstate--);
18
19         Flyweight uf = new UnsharedConcreteFlyweight();
20         uf.operation(extrinsicstate);
21     }
22
23 }

(本随笔参考了 程杰老师的 <<大话设计模式>>)