設(shè)計模式之
Flyweight(享元)
Flyweight定義:
避免大量擁有相同內(nèi)容的小類的開銷(如耗費內(nèi)存),使大家共享一個類(元類).
為什么使用?
面向?qū)ο笳Z言的原則就是一切都是對象,但是如果真正使用起來,有時對象數(shù)可能顯得很龐大,比如,字處理軟件,如果以每個文字都作為一個對象,幾千個字,對象數(shù)就是幾千,無疑耗費內(nèi)存,那么我們還是要"求同存異",找出這些對象群的共同點,設(shè)計一個元類,封裝可以被共享的類,另外,還有一些特性是取決于應(yīng)用(context),是不可共享的,這也Flyweight中兩個重要概念內(nèi)部狀態(tài)intrinsic和外部狀態(tài)extrinsic之分.
說白點,就是先捏一個的原始模型,然后隨著不同場合和環(huán)境,再產(chǎn)生各具特征的具體模型,很顯然,在這里需要產(chǎn)生不同的新對象,所以Flyweight模式中常出現(xiàn)Factory模式.Flyweight的內(nèi)部狀態(tài)是用來共享的,Flyweight factory負(fù)責(zé)維護(hù)一個Flyweight pool(模式池)來存放內(nèi)部狀態(tài)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(yīng)用場合很多:比如你要從一個數(shù)據(jù)庫中讀取一系列字符串,這些字符串中有許多是重復(fù)的,那么我們可以將這些字符串儲存在Flyweight池(pool)中.
如何使用?
我們先從Flyweight抽象接口開始:
|
public interface Flyweightb
{
public void operation( ExtrinsicState state );
}
//用于本模式的抽象數(shù)據(jù)類型(自行設(shè)計)
public interface ExtrinsicState { } |
下面是接口的具體實現(xiàn)(ConcreteFlyweight) ,并為內(nèi)部狀態(tài)增加內(nèi)存空間, ConcreteFlyweight必須是可共享的,它保存的任何狀態(tài)都必須是內(nèi)部(intrinsic),也就是說,ConcreteFlyweight必須和它的應(yīng)用環(huán)境場合無關(guān).
|
public class ConcreteFlyweight implements Flyweight {
private IntrinsicState state;
public void operation( ExtrinsicState state )
{
//具體操作
}
} |
當(dāng)然,并不是所有的Flyweight具體實現(xiàn)子類都需要被共享的,所以還有另外一種不共享的ConcreteFlyweight:
|
public class UnsharedConcreteFlyweight implements Flyweight {
public void operation( ExtrinsicState state ) { }
} |
Flyweight factory負(fù)責(zé)維護(hù)一個Flyweight池(存放內(nèi)部狀態(tài)),當(dāng)客戶端請求一個共享Flyweight時,這個factory首先搜索池中是否已經(jīng)有可適用的,如果有,factory只是簡單返回送出這個對象,否則,創(chuàng)建一個新的對象,加入到池中,再返回送出這個對象.池
|
public class FlyweightFactory {
//Flyweight pool
private Hashtable flyweights = new Hashtable();
public Flyweight getFlyweight( Object key ) {
Flyweight flyweight = (Flyweight) flyweights.get(key);
if( flyweight == null ) {
//產(chǎn)生新的ConcreteFlyweight
flyweight = new ConcreteFlyweight();
flyweights.put( key, flyweight );
}
return flyweight;
}
} |
至此,Flyweight模式的基本框架已經(jīng)就緒,我們看看如何調(diào)用:
FlyweightFactory factory = new FlyweightFactory();
Flyweight fly1 = factory.getFlyweight( "Fred" );
Flyweight fly2 = factory.getFlyweight( "Wilma" );
......
從調(diào)用上看,好象是個純粹的Factory使用,但奧妙就在于Factory的內(nèi)部設(shè)計上.
Flyweight模式在XML等數(shù)據(jù)源中應(yīng)用
我們上面已經(jīng)提到,當(dāng)大量從數(shù)據(jù)源中讀取字符串,其中肯定有重復(fù)的,那么我們使用Flyweight模式可以提高效率,以唱片CD為例,在一個XML文件中,存放了多個CD的資料.
每個CD有三個字段:
1.出片日期(year)
2.歌唱者姓名等信息(artist)
3.唱片曲目 (title)
其中,歌唱者姓名有可能重復(fù),也就是說,可能有同一個演唱者的多個不同時期 不同曲目的CD.我們將"歌唱者姓名"作為可共享的ConcreteFlyweight.其他兩個字段作為UnsharedConcreteFlyweight.
首先看看數(shù)據(jù)源XML文件的內(nèi)容:
|
<?xml version="1.0"?>
<collection>
<cd>
<title>Another Green World</title>
<year>1978</year>
<artist>Eno, Brian</artist>
</cd>
<cd>
<title>Greatest Hits</title>
<year>1950</year>
<artist>Holiday, Billie</artist>
</cd>
<cd>
<title>Taking Tiger Mountain (by strategy)</title>
<year>1977</year>
<artist>Eno, Brian</artist>
</cd>
.......
</collection> |
雖然上面舉例CD只有3張,CD可看成是大量重復(fù)的小類,因為其中成分只有三個字段,而且有重復(fù)的(歌唱者姓名).
CD就是類似上面接口 Flyweight:
|
public class CD {
private String title;
private int year;
private Artist artist;
public String getTitle() { return title; }
public int getYear() { return year; }
public Artist getArtist() { return artist; }
public void setTitle(String t){ title = t;}
public void setYear(int y){year = y;}
public void setArtist(Artist a){artist = a;}
} |
將"歌唱者姓名"作為可共享的ConcreteFlyweight:
|
public class Artist {
//內(nèi)部狀態(tài)
private String name;
// note that Artist is immutable.
String getName(){return name;}
Artist(String n){
name = n;
}
} |
再看看Flyweight factory,專門用來制造上面的可共享的ConcreteFlyweight:Artist
|
public class ArtistFactory {
Hashtable pool = new Hashtable();
Artist getArtist(String key){
Artist result;
result = (Artist)pool.get(key);
////產(chǎn)生新的Artist
if(result == null) {
result = new Artist(key);
pool.put(key,result);
}
return result;
}
} |
當(dāng)你有幾千張甚至更多CD時,Flyweight模式將節(jié)省更多空間,共享的flyweight越多,空間節(jié)省也就越大.