Flyweight模式定义:
避免大量拥有相同内容的小类的开销(如耗费内存),使大家共享一个类(元类).
为什么使用?
面向对象语言的原则就是一切都是对象,但是如果真正使用起来,有时对象数可能显得很庞大,比如,字处理软件,如果以每个文字都作为一个对象,几千个字,对象数就是几千,无疑耗费内存,那么我们还是要"求同存异",找出这些对象群的共同点,设计一个元类,封装可以被共享的类,另外,还有一些特性是取决于应用(context),是不可共享的,这也Flyweight中两个重要概念内部状态intrinsic和外部状态extrinsic之分.
说白点,就是先捏一个的原始模型,然后随着不同场合和环境,再产生各具特征的具体模型,很显然,在这里需要产生不同的新对象,所以Flyweight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Flyweight pool(模式池)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:比如你要从一个数据库中读取一系列字符串,这些字符串中有许多是重复的,那么我们可以将这些字符串储存在Flyweight池(pool)中.
如何使用?
我们先从Flyweight抽象接口开始:
public interface Flyweight
{
public void operation( ExtrinsicState state );
}
//用于本模式的抽象数据类型(自行设计)
public interface ExtrinsicState { }
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下面是接口的具体实现(ConcreteFlyweight) ,并为内部状态增加内存空间, ConcreteFlyweight必须是可共享的,它保存的任何状态都必须是内部(intrinsic),也就是说,ConcreteFlyweight必须和它的应用环境场合无关.
public class ConcreteFlyweight implements Flyweight {
private IntrinsicState state;
public void operation( ExtrinsicState state )
{
//具体操作
}
} |
当然,并不是所有的Flyweight具体实现子类都需要被共享的,所以还有另外一种不共享的ConcreteFlyweight:
public class UnsharedConcreteFlyweight implements Flyweight {
public void operation( ExtrinsicState state ) { }
}
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Flyweight factory负责维护一个Flyweight池(存放内部状态),当客户端请求一个共享Flyweight时,这个factory首先搜索池中是否已经有可适用的,如果有,factory只是简单返回送出这个对象,否则,创建一个新的对象,加入到池中,再返回送出这个对象.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 ) {
//产生新的ConcreteFlyweight
flyweight = new ConcreteFlyweight();
flyweights.put( key, flyweight );
}
return flyweight;
}
}
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至此,Flyweight模式的基本框架已经就绪,我们看看如何调用:
FlyweightFactory factory = new FlyweightFactory();
Flyweight fly1 = factory.getFlyweight( "Fred" );
Flyweight fly2 = factory.getFlyweight( "Wilma" );
......
从调用上看,好象是个纯粹的Factory使用,但奥妙就在于Factory的内部设计上.
Flyweight模式在XML等数据源中应用
我们上面已经提到,当大量从数据源中读取字符串,其中肯定有重复的,那么我们使用Flyweight模式可以提高效率,以唱片CD为例,在一个XML文件中,存放了多个CD的资料.
每个CD有三个字段:
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复,也就是说,可能有同一个演唱者的多个不同时期 不同曲目的CD.我们将"歌唱者姓名"作为可共享的ConcreteFlyweight.其他两个字段作为UnsharedConcreteFlyweight.
首先看看数据源XML文件的内容:
<?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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虽然上面举例CD只有3张,CD可看成是大量重复的小类,因为其中成分只有三个字段,而且有重复的(歌唱者姓名).
CD就是类似上面接口 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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将"歌唱者姓名"作为可共享的ConcreteFlyweight:
public class Artist {
//内部状态
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,专门用来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);
////产生新的Artist
if(result == null) {
result = new Artist(key);
pool.put(key,result);
}
return result;
}
}
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当你有几千张甚至更多CD时,Flyweight模式将节省更多空间,共享的flyweight越多,空间节省也就越大.
给个例子，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;//已经处理好的订单数
private int totalFlavors = 0;//已购买的coffee风味种类数
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());
}
}
//-------------------------------------------------------------------
运行结果：
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