享元模式应用场景

在面向对象程序设计过程中，有时会面临创建大量相同或相似的对象实例的问题。创建那么多的对象将会耗费很多系统资源，它是系统提升的一个瓶颈。

例如，围棋和五子棋中的黑白棋子，图像中坐标或颜色，在局域网中的路由器，交换机和集线器等。这些对象有很多相似的地方，如果能够把她们的相同部分提取出来，则能节省大量的系统资源。

享元模式代码示例

抽象享元

package com.demo.test.flyweight.iml;

import com.demo.test.flyweight.UnsharedConcreteFlyweight;

public interface Flyweight {
    public void operation(UnsharedConcreteFlyweight state);
}

具体享元A

package com.demo.test.flyweight;

import com.demo.test.flyweight.iml.Flyweight;

public class ConcreteFlyweightA implements Flyweight {

    private String key;

    public ConcreteFlyweightA(String key){
        this.key = key;
    }
    @Override
    public void operation(UnsharedConcreteFlyweight state) {
        System.out.println("ConcreteFlyweightA：具体享元"+key+"被调用");
        System.out.println("非享元信息："+state.getInfo());
    }
}

具体享元B

package com.demo.test.flyweight;

import com.demo.test.flyweight.iml.Flyweight;

public class ConcreteFlyweightB implements Flyweight {
    private String key;

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

    @Override
    public void operation(UnsharedConcreteFlyweight state) {
        System.out.println("ConcreteFlyweightB：具体享元"+key+"被调用");
        System.out.println("非享元信息："+state.getInfo());
    }
}

非享元角色

package com.demo.test.flyweight;

public class UnsharedConcreteFlyweight {
    private String info;

    public UnsharedConcreteFlyweight(String info){
        this.info = info;
    }

    public String getInfo() {
        return info;
    }

    public void setInfo(String info) {
        this.info = info;
    }
}

享元工厂角色

package com.demo.test.flyweight;

import com.demo.test.flyweight.iml.Flyweight;

import java.util.HashMap;

public class FlyweightFactory {

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

    public Flyweight getFlyweight(String key){
        Flyweight flyweight = (Flyweight) flyweights.get(key);

        if(flyweight != null){
            System.out.println("具体享元"+key+"已经存在，被成功获取");
        }else{

            if(key.equals("a")){
                flyweight = new ConcreteFlyweightA(key);
            }else if(key.equals("b")){
                flyweight = new ConcreteFlyweightB(key);
            }else{
                return null;
            }

            flyweights.put(key,flyweight);
        }
        System.out.println("实例数量："+flyweights.size());
        return flyweight;
    }
}

客户端

package com.demo.test.flyweight;

import com.demo.test.flyweight.iml.Flyweight;

public class Client {
    public static void main(String[] agr){
        FlyweightFactory flyweightFactory = new FlyweightFactory();
        Flyweight f1 = flyweightFactory.getFlyweight("a");
        Flyweight f2 = flyweightFactory.getFlyweight("b");
        Flyweight f3 = flyweightFactory.getFlyweight("a");
        Flyweight f4 = flyweightFactory.getFlyweight("a");
        Flyweight f5 = flyweightFactory.getFlyweight("a");
        f1.operation(new UnsharedConcreteFlyweight("非享元信息a"));
        f2.operation(new UnsharedConcreteFlyweight("非享元信息b"));
        f3.operation(new UnsharedConcreteFlyweight("非享元信息3"));
        f4.operation(new UnsharedConcreteFlyweight("非享元信息4"));
        f5.operation(new UnsharedConcreteFlyweight("非享元信息5"));
    }
}

小结

个人认识享元和单例有一些相似的地方，享元模式对重复或者相似很高的实例有很好的复用功能，从示例的执行结果来看，对象只实例话了两次。