java设计模式---观察者模式

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.

2. 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.

 

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:

 

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:

 

StockData.java:

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.