23种设计模式之享元模式

享元模式

享元（Flyweight）模式的定义：运用共享技术来有效地支持大量细粒度对象的复用。它通过共享已经存在的对象来大幅度减少需要创建的对象数量、避免大量相似类的开销，从而提高系统资源的利用率。
享元模式的本质是缓存共享对象，降低内存消耗。
享元模式的主要优点是：相同对象只要保存一份，这降低了系统中对象的数量，从而降低了系统中细粒度对象给内存带来的压力。
其主要缺点是：

• 为了使对象可以共享，需要将一些不能共享的状态外部化，这将增加程序的复杂性。
• 读取享元模式的外部状态会使得运行时间稍微变长。

public class FlyWeightTest {
    public static void main(String[] args) {
        FlyWeightFactory flyWeightFactory = new FlyWeightFactory();

        FlyWeight a1 = flyWeightFactory.getFlyWeight("a");
        FlyWeight a2 = flyWeightFactory.getFlyWeight("a");
        FlyWeight a3 = flyWeightFactory.getFlyWeight("a");
        FlyWeight b1 = flyWeightFactory.getFlyWeight("b");
        FlyWeight b2 = flyWeightFactory.getFlyWeight("b");

        a1.operate(new UnFlyWeight("第1次调用a。"));
        a2.operate(new UnFlyWeight("第2次调用a。"));
        a3.operate(new UnFlyWeight("第3次调用a。"));
        b1.operate(new UnFlyWeight("第1次调用b。"));
        b2.operate(new UnFlyWeight("第2次调用b。"));

    }
}

// 享元接口
interface FlyWeight {
    void operate(UnFlyWeight unFlyWeight);
}
// 具体享元
class SpecialFlyWeight implements FlyWeight {

    private String key;

    @Override
    public void operate(UnFlyWeight unFlyWeight) {
        System.out.print("具体享元" + key + "被调用，");
        System.out.println("非享元信息是:" + unFlyWeight.getName());
    }

    public SpecialFlyWeight(String key) {
        this.key = key;
    }
}

// 享元工厂
class FlyWeightFactory {
    private Map<String, FlyWeight> flyWeightMap = new HashMap<String, FlyWeight>();

    FlyWeight getFlyWeight(String key) {
        FlyWeight flyWeight = flyWeightMap.get(key);
        if (flyWeight == null) {
            SpecialFlyWeight specialFlyWeight = new SpecialFlyWeight(key);
            flyWeightMap.put(key, specialFlyWeight);
            return specialFlyWeight;
        } else {
            System.out.println("具体享元" + key + "已经存在，被成功获取！");
        }
        return flyWeight;
    }
}

// 非享元对象
class UnFlyWeight {
    private String name;

    public UnFlyWeight(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

享元模式其实是工厂方法模式的一个改进机制，享元模式同样要求创建一个或一组对象，并且就是通过工厂方法模式生成对象的，只不过享元模式为工厂方法模式增加了缓存这一功能。
另外，在我看来，享元模式在某种程度上与多例有点像。
参考资料：
1.http://c.biancheng.net/view/1371.html
代码地址：https://gitee.com/zhoujie1/design-mode-and-principle.git