Java 中观察者模式的使用
????? 在一对多依赖的对象关系中, 如果这个'一'对象状态发生了变化,那么它所有依赖的'多'对象都应该被通知,然后做相应的变化,这就是观察者模式.? 就如同'多'对象一直在观察'一'对象的状态变化一样.
????? 在观察者模式中最重要的俩个对象分别是:Observable和Observer对象.它们的关系可总结如下:
1. Observable和Observer对象是一对多的关系,也就是说一旦Observable对象状态变化,它就要负责通知所有和它有关系的Observer对象,然后做相应的改变.
1. Observable对象不会主动去通知各个具体的Observer对象其状态发生了变化,而是提供一个注册接口供Observer对象使用,任何一个Observer对象如果想要被通知,则可以使用这个接口来注册.
3. 在Observable中有一个集合和一个状态控制开关,所有注册了通知的Observer对象会被保存在这个集合中.这个控制开关就是用来控制Observable是否发生了变化,一旦发生了变化,就通知所有的Observer对象更新状态.
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在java? api中分别提供了Observable对象:java.util.Observable和Observer接口:java.util.Observer. 下面用实例来实现一下观察者模式: 股票系统
所有的类如下:
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StockData???? (Observable对象,也就是所股票数据发生了变化,它就要通知所有和它有关系的交易实体做相应的变化)
BigBuyer???????(Observer对象, 实现了Observer接口)
TradingFool?? (Observer对象, 实现了Observer接口)
StockQuote?? 测试类
在这个例子中一旦StockData对象的状态发生了变化,那BigBuyer和TradingFool都应该受到通知:
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StockData.java:
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import java.util.Observable;public class StockData extends Observable{private String symbol;private float close;private float high;private float low;private long volume;public StockData(){}public String getSymbol(){return symbol;}public float getClose(){return close;}public float getHigh(){return high;}public float getLow(){return low;}public long getVolume(){return volume;}public void sendStockData(){setChanged();notifyObservers();}public void setStockData(String symbol,float close,float high,float low,long volume){this.symbol = symbol;this.close = close;this.high = high;this.low = low;this.volume = volume;sendStockData();}}?
BigBuyer.java:
public class BigBuyer implements Observer{private String symbol;private float close;private float high;private float low;private long volume;public BigBuyer(Observable observable){observable.addObserver(this); //注册关系}public void update(Observable observable,Object args){if(observable instanceof StockData){StockData stockData = (StockData)observable;this.symbol = stockData.getSymbol();this.close = stockData.getClose();this.high = stockData.getHigh();this.low = stockData.getLow();this.volume = stockData.getVolume();display();}}public void display(){DecimalFormatSymbols dfs = new DecimalFormatSymbols();DecimalFormat volumeFormat = new DecimalFormat("###,###,###,###",dfs);DecimalFormat priceFormat = new DecimalFormat("###.00",dfs);System.out.println("Big Buyer reports... ");System.out.println("\tThe lastest stock quote for " + symbol + " is:");System.out.println("\t$" + priceFormat.format(close) + " per share (close).");System.out.println("\t$" + priceFormat.format(high) + " per share (high).");System.out.println("\t$" + priceFormat.format(low) + " per share (low).");System.out.println("\t" + volumeFormat.format(volume) + " shares traded.");System.out.println();}}?
?TradingFool.java:
public class TradingFool implements Observer{private String symbol;private float close;public TradingFool(Observable observable){observable.addObserver(this);//注册关系}public void update(Observable observable,Object args){if(observable instanceof StockData){StockData stockData = (StockData)observable;this.symbol = stockData.getSymbol();this.close = stockData.getClose();display();}}public void display(){DecimalFormatSymbols dfs = new DecimalFormatSymbols();DecimalFormat priceFormat = new DecimalFormat("###.00",dfs);System.out.println("Trading Fool says... ");System.out.println("\t" + symbol + " is currently trading at $" + priceFormat.format(close) + " per share.");System.out.println();}}?
StokeQuote.java
public class StockQuotes{public static void main(String[] args){System.out.println();System.out.println("-- Stock Quote Application --");System.out.println();StockData stockData = new StockData();// register observers...new TradingFool(stockData);new BigBuyer(stockData);// generate changes to stock data...stockData.setStockData("JUPM",16.10f,16.15f,15.34f,(long)481172);stockData.setStockData("SUNW",4.84f,4.90f,4.79f,(long)68870233);stockData.setStockData("MSFT",23.17f,23.37f,23.05f,(long)75091400);}}?
在测试类中我们可以看到俩个Observer对象都注册了Observable对象,而当Observable对象发生改变时,这俩个Observable对象就会做相应的更新了, 运行结果如下:
Big Buyer reports... The lastest stock quote for JUPM is:$16.10 per share (close).$16.15 per share (high).$15.34 per share (low).481,172 shares traded.Trading Fool says... JUPM is currently trading at $16.10 per share.Big Buyer reports... The lastest stock quote for SUNW is:$4.84 per share (close).$4.90 per share (high).$4.79 per share (low).68,870,233 shares traded.Trading Fool says... SUNW is currently trading at $4.84 per share.Big Buyer reports... The lastest stock quote for MSFT is:$23.17 per share (close).$23.37 per share (high).$23.05 per share (low).75,091,400 shares traded.Trading Fool says... MSFT is currently trading at $23.17 per share.
我们通过Observable源码可以看到,其实Observable对象不关心具体的Observer的实例类型. 只要是实现了Observer接口的Observer对象都可以得到通知,这就为我们如果想要对模型进行扩展提供了方便,使Observable对象和Observer对象实现了松耦合. 如果我们需要添加一个新的Observer对象时,我们只要注册一下,当Observable对象发生变化时就可以得到通知,而不要做其他任何改变,非常方便.
1 楼 仅此而已 2011-04-28 学习了， 我想问一句 update(Observable observable,Object args)