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:

package com.cgj.pattern.flyweight;

/**
 * Flyweight接口:是多有享元类的超类或接口,通过这个接口,
 *      Flyweight可以接受并作用于外部状态.
 */
public interface Flyweight {

    public abstract void operation(int extrinsicstate);
}

package com.cgj.pattern.flyweight;

/**
 * ConcreteFlyweight类实现Flyweight接口,并为内部状态增加存储空间.
 */
public class ConcreteFlyweight implements Flyweight {

    @Override
    public void operation(int extrinsicstate) {
        System.out.println("具体的Flyweight:" + extrinsicstate);
    }

}

package com.cgj.pattern.flyweight;

/**
 * UnsharedConcreteFlyweight指不需要共享的Flyweight实现类.
 *      因为实现了Flyweight接口共享成为可能,但并不强制共享.
 */
public class UnsharedConcreteFlyweight implements Flyweight {

    @Override
    public void operation(int extrinsicstate) {
        System.out.println("不共享的具体Flyweight:" + extrinsicstate);
    }

}

package com.cgj.pattern.flyweight;

import java.util.HashMap;
import java.util.Map;

/**
 * Flyweight是一个享元工厂,用来创建并管理Flyweight类对象.
 *      它主要用来确保合理的共享Flyweight,当用户请求一个Flyweight时,
 *      它创建一个Flyweight或者返回一个已经创建好的Flyweight.
 */
public class FlyweightFactory {

    // 保存Flyweight
    private Map<String, Flyweight> flyweights = new HashMap<String, Flyweight>();

    // 获取Flyweight
    public Flyweight getFlyweight(String key) {
        if (!flyweights.containsKey(key)) {
            flyweights.put(key, new ConcreteFlyweight());
        }
        return flyweights.get(key);
    }

}

package com.cgj.pattern.flyweight;

public class Test {

    public static void main(String[] args) {
        int extrinsicstate = 22;

        FlyweightFactory factory = new FlyweightFactory();

        Flyweight fx = factory.getFlyweight("X");
        fx.operation(extrinsicstate--);

        Flyweight fy = factory.getFlyweight("Y");
        fy.operation(extrinsicstate--);

        Flyweight fz = factory.getFlyweight("Z");
        fz.operation(extrinsicstate--);

        Flyweight uf = new UnsharedConcreteFlyweight();
        uf.operation(extrinsicstate);
    }

}

(本随笔参考了 程杰老师的 <<大话设计模式>>)