设计模式之享元模式

享元模式：在一个系统中如果有多个相同的对象，那么只共享一份就可以了，不必每个都去实例化一个对象。

在Flyweight模式中，由于要产生各种各样的对象，所以在Flyweight(享元)模式中常出现Factory模式。Flyweight的内部状态是用来共享的,Flyweight factory负责维护一个对象存储池（Flyweight Pool）来存放内部状态的对象。Flyweight模式是一个提高程序效率和性能的模式,会大大加快程序的运行速度.应用场合很多，下面举个例子：

先定义一个享元模式的抽象类

public abstract class Flyweight{
public abstract void operation();
}

再定义一个具体的实现类

public class ConcreteFlyweight extends Flyweight{
private String string;
public ConcreteFlyweight(String str){
string = str;
}
public void operation()
{
System.out.println("Concrete---Flyweight : " + string);
}
}

再实现一个工厂类

import java.util.Hashtable;
public class FlyweightFactory{
private Hashtable flyweights = new Hashtable();
public FlyweightFactory(){}
public Flyweight getFlyWeight(Object obj){
Flyweight flyweight = (Flyweight) flyweights.get(obj);
if(flyweight == null){
flyweight = new ConcreteFlyweight((String)obj);
flyweights.put(obj, flyweight);
}
return flyweight;
}
public int getFlyweightSize(){
return flyweights.size();
}
}

客户端测试类

public class Client{
FlyweightFactory factory = new FlyweightFactory();
Flyweight fly1;
Flyweight fly2;
Flyweight fly3;
Flyweight fly4;
Flyweight fly5;
Flyweight fly6;

public Client(){
fly1 = factory.getFlyWeight("Google");
fly2 = factory.getFlyWeight("Qutr");
fly3 = factory.getFlyWeight("Google");
fly4 = factory.getFlyWeight("Google");
fly5 = factory.getFlyWeight("Google");
fly6 = factory.getFlyWeight("Google");
}
public void showFlyweight(){
fly1.operation();
fly2.operation();
fly3.operation();
fly4.operation();
fly5.operation();
fly6.operation();
int objSize = factory.getFlyweightSize();
System.out.println("objSize = " + objSize);
}
public static void main(String[] args){
System.out.println("The FlyWeight Pattern!");
Client fp = new Client();
fp.showFlyweight();
}
}

在这里定义了6个对象，但是在实际上只有两个实例化的对象，这样就可以节省大量的内存空间。

核心总结，可以共享的对象，也就是说返回的同一类型的对象其实是同一实例，当客户端要求生成一个对象时，工厂会检测是否存在此对象的实例，如果存在那么直接返回此对象实例，如果不存在就创建一个并保存起来