GOF:運用共享技術有效地支持大量細粒度的對象。
解釋一下概念:也就是說在一個系統中如果有多個相同的對象,那麼只共享一份就可以了,不必每個都去實例化一個對象。比如說(這裡引用GOF書中的例子)一個文本系統,每個字母定一個對象,那麼大小寫字母一共就是52個,那麼就要定義52個對象。如果有一個1M的文本,那麼字母是何其的多,如果每個字母都定義一個對象那麼內存早就爆了。那麼如果要是每個字母都共享一個對象,那麼就大大節約了資源。
在Flyweight模式中,由於要產生各種各樣的對象,所以在Flyweight(享元)模式中常出現Factory模式。Flyweight的內部狀態是用來共享的,Flyweight factory負責維護一個對象存儲池(Flyweight Pool)來存放內部狀態的對象。Flyweight模式是一個提高程序效率和性能的模式,會大大加快程序的運行速度.應用場合很多,下面舉個例子:
先定義一個抽象的Flyweight類:
package Flyweight;
public abstract class Flyweight
...
{
public abstract void operation();
}//end abstract class Flyweight
在實現一個具體類:
package Flyweight;
public class ConcreteFlyweight extends Flyweight
...
{
private String string;
public ConcreteFlyweight(String str)
...
{
string = str;
}//end ConcreteFlyweight(...)
public void operation()
...
{
System.out.println("Concrete---Flyweight : " + string);
}//end operation()
}//end class ConcreteFlyweight
實現一個工廠方法類:
package Flyweight;
import java.util.Hashtable;
public class FlyweightFactory
...
{
private Hashtable flyweights = new Hashtable();//----------------------------1
public FlyweightFactory() ...{}
public Flyweight getFlyWeight(Object obj)
...
{
Flyweight flyweight = (Flyweight) flyweights.get(obj);//----------------2
if(flyweight == null) ...{//---------------------------------------------------3
//產生新的ConcreteFlyweight
flyweight = new ConcreteFlyweight((String)obj);
flyweights.put(obj, flyweight);//--------------------------------------5
}
return flyweight;//---------------------------------------------------------6
}//end GetFlyWeight(...)
public int getFlyweightSize()
...
{
return flyweights.size();
}
}//end class FlyweightFactory
這個工廠方法類非常關鍵,這裡詳細解釋一下:
在1處定義了一個Hashtable用來存儲各個對象;在2處選出要實例化的對象,在6處將該對象返回,如果在Hashtable中沒有要選擇的對象,此時變量flyweight為null,產生一個新的flyweight存儲在Hashtable中,並將該對象返回。
最後看看Flyweight的調用:
package Flyweight;
import java.util.Hashtable;
public class FlyweightPattern ...{
FlyweightFactory factory = new FlyweightFactory();
Flyweight fly1;
Flyweight fly2;
Flyweight fly3;
Flyweight fly4;
Flyweight fly5;
Flyweight fly6;
/** *//** Creates a new instance of FlyweightPattern */
public FlyweightPattern() ...{
fly1 = factory.getFlyWeight("Google");
fly2 = factory.getFlyWeight("Qutr");
fly3 = factory.getFlyWeight("Google");
fly4 = factory.getFlyWeight("Google");
fly5 = factory.getFlyWeight("Google");
fly6 = factory.getFlyWeight("Google");
}//end FlyweightPattern()
public void showFlyweight()
...
{
fly1.operation();
fly2.operation();
fly3.operation();
fly4.operation();
fly5.operation();
fly6.operation();
int objSize = factory.getFlyweightSize();
System.out.println("objSize = " + objSize);
}//end showFlyweight()
public static void main(String[] args)
...
{
System.out.println("The FlyWeight Pattern!");
FlyweightPattern fp = new FlyweightPattern();
fp.showFlyweight();
}//end main(...)
}//end class FlyweightPattern
下面是運行結果:
Concrete---Flyweight : Google
Concrete---Flyweight : Qutr
Concrete---Flyweight : Google
Concrete---Flyweight : Google
Concrete---Flyweight : Google
Concrete---Flyweight : Google
objSize = 2
我們定義了6個對象,其中有5個是相同的,按照Flyweight模式的定義“Google”應該共享一個對象,在實際的對象數中我們可以看出實際的對象卻是只有2個。
下面給出一個簡易的UML圖:
總結:
Flyweight(享元)模式是如此的重要,因為它能幫你在一個復雜的系統中大量的節省內存空間。在GOF的書中舉了文本處理的例子,我覺得非常恰當。那麼,在Java中String這個類型比較特殊,為什麼呢,看下面的例子:
String a = "hello";
String b = "hello";
if(a == b)
System.out.println("OK");
else
System.out.println("Error");
輸出結果是:OK。稍有經驗的人都可以看出if條件比較的是兩a和b的地址,也可以說是內存空間。那麼Sting的實現是不是使用了Flyweight模式呢,不得而知,到現在還沒有研究過。