避免大量拥有相同内容小类开销(如耗费内存)使大家共享个类(元类).
为什么使用?
面向对象语言原则就是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Software如果以每个文字都作为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"找出这些对象群共同点设计个元类封装可以被共享类另外还有些特性是取决于应用(context)是不可共享这也Flyweight中两个重要概念内部状态rinsic和外部状态extrinsic的分.
说白点就是先捏个原始模型然后随着区别场合和环境再产生各具特征具体模型很显然在这里需要产生区别新对象所以Flyweight模式中常出现Factory模式.Flyweight内部状态是用来共享Flyweight factory负责维护个Flyweight pool(模式池)来存放内部状态对象.
Flyweight模式是个提高效率和性能模式会大大加快运行速度.应用场合很多:比如你要从个数据库中读取系列串这些串中有许多是重复那么我们可以将这些串储存在Flyweight池(pool)中.
如何使用?
我们先从Flyweight抽象接口开始:
public erface Flyweight
{
public void operation( ExtrinsicState state );
}
//用于本模式抽象数据类型(自行设计)
public erface ExtrinsicState { }
下面是接口具体实现(ConcreteFlyweight) 并为内部状态增加内存空间 ConcreteFlyweight必须是可共享它保存任何状态都必须是内部(rinsic)也就是说ConcreteFlyweight必须和它应用环境场合无关.public ConcreteFlyweight implements Flyweight {
private IntrinsicState state;
public void operation( ExtrinsicState state )
{
//具体操作
}
}
当然并不是所有Flyweight具体实现子类都需要被共享所以还有另外种不共享ConcreteFlyweight:public UnsharedConcreteFlyweight implements Flyweight {
public void operation( ExtrinsicState state ) { }
}
Flyweight factory负责维护个Flyweight池(存放内部状态)当客户端请求个共享Flyweight时这个factory首先搜索池中是否已经有可适用如果有factory只是简单返回送出这个对象否则创建个新对象加入到池中再返回送出这个对象public FlyweightFactory {
//Flyweight pool
private Hashtable flyweights = Hashtable;
public Flyweight getFlyweight( Object key ) {
Flyweight flyweight = (Flyweight) flyweights.get(key);
( flyweight null ) {
//产生新ConcreteFlyweight
flyweight = ConcreteFlyweight;
flyweights.put( key flyweight );
}
flyweight;
}
}
至此Flyweight模式基本框架已经就绪我们看看如何:FlyweightFactory factory = FlyweightFactory;
Flyweight fly1 = factory.getFlyweight( "Fred" );
Flyweight fly2 = factory.getFlyweight( "Wilma" );
......
从上看好象是个纯粹Factory使用但奥妙就在于Factory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串其中肯定有重复那么我们使用Flyweight模式可以提高效率以唱片CD为例在个XML文件中存放了多个CD资料
每个CD有 3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个演唱者多个区别时期 区别曲目CD.我们将"歌唱者姓名"作为可共享ConcreteFlyweight.其他两个字段作为UnsharedConcreteFlyweight
首先看看数据源XML文件内容:
<?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>
虽然上面举例CD只有3张CD可看成是大量重复小类其中成分只有 3个字段而且有重复(歌唱者姓名)CD就是类似上面接口 Flyweight:
public CD {
private String title;
private year;
private Artist artist;
public String getTitle { title; }
public getYear { year; }
public Artist getArtist { artist; }
public void Title(String t){ title = t;}
public void Year( y){year = y;}
public void Artist(Artist a){artist = a;}
}
将"歌唱者姓名"作为可共享ConcreteFlyweight:public Artist {
//内部状态
private String name;
// note that Artist is immutable.
String getName{ name;}
Artist(String n){
name = n;
}
}
再看看Flyweight factory专门用来制造上面可共享ConcreteFlyweight:Artistpublic ArtistFactory {
Hashtable pool = Hashtable;
Artist getArtist(String key){
Artist result;
result = (Artist)pool.get(key);
////产生新Artist
(result null) {
result = Artist(key);
pool.put(keyresult);
}
result;
}
}
当你有几千张甚至更多CD时Flyweight模式将节省更多空间共享flyweight越多空间节省也就越大
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