Flyweight模式定義:
避免大量擁有相同內容的小類的開銷(如耗費內存),使大家共享一個類(元類).
為什么使用?
面向對象語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,對象數就是幾千,無疑耗費內存,那么我們還是要"求同存異",找出這些對象群的共同點,設計一個元類,封裝可以被共享的類,另外,還有一些特性是取決于應用(context),是不可共享的,這也Flyweight中兩個重要概念內部狀態intrinsic和外部狀態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境,再產生各具特征的具體模型,很顯然,在這里需要產生不同的新對象,所以Flyweight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Flyweight pool(模式池)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:比如你要從一個數據庫中讀取一系列字符串,這些字符串中有許多是重復的,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 Flyweight
{
public void operation( ExtrinsicState state );
}
//用于本模式的抽象數據類型(自行設計)
public interface ExtrinsicState { }
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下面是接口的具體實現(ConcreteFlyweight) ,并為內部狀態增加內存空間, ConcreteFlyweight必須是可共享的,它保存的任何狀態都必須是內部(intrinsic),也就是說,ConcreteFlyweight必須和它的應用環境場合無關.
public class ConcreteFlyweight implements Flyweight {
private IntrinsicState state;
public void operation( ExtrinsicState state )
{
//具體操作
}
} |
當然,并不是所有的Flyweight具體實現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 ) { }
}
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Flyweight factory負責維護一個Flyweight池(存放內部狀態),當客戶端請求一個共享Flyweight時,這個factory首先搜索池中是否已經有可適用的,如果有,factory只是簡單返回送出這個對象,否則,創建一個新的對象,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 ) {
//產生新的ConcreteFlyweight
flyweight = new ConcreteFlyweight();
flyweights.put( key, flyweight );
}
return flyweight;
}
}
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至此,Flyweight模式的基本框架已經就緒,我們看看如何調用:
FlyweightFactory factory = new FlyweightFactory();
Flyweight fly1 = factory.getFlyweight( "Fred" );
Flyweight fly2 = factory.getFlyweight( "Wilma" );
......
從調用上看,好象是個純粹的Factory使用,但奧妙就在于Factory的內部設計上.
Flyweight模式在XML等數據源中應用
我們上面已經提到,當大量從數據源中讀取字符串,其中肯定有重復的,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,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.
首先看看數據源XML文件的內容:
<?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>
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雖然上面舉例CD只有3張,CD可看成是大量重復的小類,因為其中成分只有三個字段,而且有重復的(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;}
}
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將"歌唱者姓名"作為可共享的ConcreteFlyweight:
public class Artist {
//內部狀態
private String name;
// note that Artist is immutable.
String getName(){return name;}
Artist(String n){
name = n;
}
}
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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);
////產生新的Artist
if(result == null) {
result = new Artist(key);
pool.put(key,result);
}
return result;
}
}
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當你有幾千張甚至更多CD時,Flyweight模式將節省更多空間,共享的flyweight越多,空間節省也就越大.
給個例子，coffee商店
package FlyWeight.coffeeshop;
public class Table {
?
?private int number;
?public int getNumber() {
??return number;
?}
?public void setNumber(int number) {
??this.number = number;
?}
?public Table(int number) {
??super();
??// TODO Auto-generated constructor stub
??this.number = number;
?}
}
package FlyWeight.coffeeshop;
public abstract class Order {
?
?public abstract void serve(Table table);
?
?public abstract String getFlavor();
}
package FlyWeight.coffeeshop;
public class Flavor extends Order {
?private String flavor;
?public Flavor(String flavor) {
??super();
??// TODO Auto-generated constructor stub
??this.flavor = flavor;
?}
?public String getFlavor() {
??return flavor;
?}
?public void setFlavor(String flavor) {
??this.flavor = flavor;
?}
?public void serve(Table table) {
??System.out.println("Serving table " + table.getNumber() + " with flavor " + flavor );
?}?
}
package FlyWeight.coffeeshop;
public class FlavorFactory {
?private Order[] flavors = new Flavor[10];
?private int ordersMade = 0;//已經處理好的訂單數
?private int totalFlavors = 0;//已購買的coffee風味種類數
?
?public Order getOrder(String flavorToGet){
??if(ordersMade > 0){
???for(int i=0; i<ordersMade; i++){
????if(flavorToGet.equalsIgnoreCase(flavors[i].getFlavor()))
?????return flavors[i];
???}
??}
??flavors[ordersMade] = new Flavor(flavorToGet);
??totalFlavors++;
??return flavors[ordersMade++];
?}
?
?public int getTotalFlavorsMade(){
??return totalFlavors;
?}
}
package FlyWeight.coffeeshop;
public class Client {
?
?private static Order[] flavors = new Flavor[100];
?
?private static int ordersMade = 0;
?private static FlavorFactory flavorFactory;
?
?private static void takeOrders(String aFlavor){
??flavors[ordersMade++] = flavorFactory.getOrder(aFlavor);
?}
?
?public static void main(String[] args){
??flavorFactory = new FlavorFactory();
??
???? takeOrders("Black Coffee");
???? takeOrders("Capucino");
???? takeOrders("Espresso");
???? takeOrders("Espresso");
???? takeOrders("Capucino");
???? takeOrders("Capucino");
???? takeOrders("Black Coffee");
???? takeOrders("Espresso");
???? takeOrders("Capucino");
???? takeOrders("Black Coffee");
???? takeOrders("Espresso");
????
???? for(int i=0; i<ordersMade; i++){
???? ?flavors[i].serve(new Table(i));
???? }
????
???? System.out.println("\nTotal Flavor objrcts made: " +
???? ??flavorFactory.getTotalFlavorsMade());
?}
?
}
//-------------------------------------------------------------------
運行結果：
Serving table 0 with flavor Black Coffee
Serving table 1 with flavor Capucino
Serving table 2 with flavor Espresso
Serving table 3 with flavor Espresso
Serving table 4 with flavor Capucino
Serving table 5 with flavor Capucino
Serving table 6 with flavor Black Coffee
Serving table 7 with flavor Espresso
Serving table 8 with flavor Capucino
Serving table 9 with flavor Black Coffee
Serving table 10 with flavor Espresso
Total Flavor objrcts made: 3