浅谈JAVA设计模式之——享元模式(Flyweight)

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：

1.一个应用程序使用了大量的对象。

2.完全由于使用大量的对象，造成很大的存储开销。

3.对象的大多数状态都可变为外部状态。

4.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。

5.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

1.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。

2.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。

3.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。

4.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 liuyazhuang
 *
 */
public interface Flyweight {
    void action(int arg);
}

ConcreteFlyweight

package com.lyz.design.flyweight;
/**
 * ConcreteFlyweight
 * @author liuyazhuang
 *
 */
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 liuyazhuang
 *
 */
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 liuyazhuang
 */
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());
    }
}