Flyweight 享元模式

概述

运用共享技术有效地支持大量细粒度的对象。

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

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)

类图

例子

Flyweight

public interface Flyweight {

    void action(int arg);
}

ConcreteFlyweight

public class FlyweightImpl implements Flyweight {

    public void action(int arg) {
        // TODO Auto-generated method stub
        System.out.println("参数值: " + arg);
    }
}

FlyweightFactory

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 flyweights.get(key);
    }
    
    public static int getSize() {
        return flyweights.size();
    }
}

Test

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());
    }
}

结果

参数值: 1
true
参数值: 2
参数值: 3
参数值: 4
4

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