Observer与Observable

本文整理自：http://dev.21tx.com/2004/07/26/10822.html

  在Java中通过Observable类和Observer接口实现了观察者模式。一个Observer对象监视着一个Observable对象的变化，当Observable对象发生变化时，Observer得到通知，就可以进行相应的工作。例如在文档/视图结构中，文档被修改了，视图就会得到通知。
      java.util.Observable中有两个方法对Observer特别重要，一个是setChange()方法用来设置一个内部标志位注明数据发生了变化；一个是notifyObservers()方法会去调用一个列表中所有的Observer的update()方法，通知它们数据发生了变化。
      Observer通过Observable的addObserver()方法把自己添加到这个列表中。这个列表虽然由Observable拥有，但Observable并不知道到底有哪些Observer正在观察等待通知。Observable只提供一个方法让Observer能把自己添加进列表，并保证会去通知Observer发生了变化。通过这种机制，可以有任意多个Observer对Observable进行观察，而不影响Observable的实现。当然一个Observer也可以观察多个Observable。

Observer与Observable的简单使用可以参照实例1的SimpleObservable.java文件和SimpleObserver.java文件.

  Observable类有两个私有变量。一个boolean型的标志位，setChange()将它设为真，只有它为真时，notifyObservers方法才会调用Observer的update方法，clearChange()设标志位为假，hasChange返回当前标志位的值。另一个是一个Vector，保存着一个所有要通知的Observer列表，我们可以使用addObserver方法添加Observer到列表，deleteObserver从列表中删除指定Observer，deleteObservers清空列表，使用countObservers方法返回列表中Observer的数目，在Observer对象销毁前一定要用deleteObserver将其从列表中删除，不然因为还存在对象引用的关系，Observer对象不会被垃圾收集，造成内存泄漏，并且已死的Observer仍会被通知到，有可能造成意料外的错误，而且随着列表越来越大，notifyObservers操作也会越来越慢。
  
      Observable的所有方法都是同步的，保证了在一个线程对其标志位、列表进行操作时，不会有其它线程也在操作它。
      Observable的notifyObservers(Object obj)形式可以再调用update时将参数传进去。
Observer接收到通知的顺序是越晚加入列表的越先通知。Observer的update方法会被依次调用，由于Observable的notifyObservers方法和Observer的update方法其实是在同一个线程中被调用而且调用一个update方法返回后才进行下一个update方法的调用，这样当update中有大量操作时，最好将其中的工作给另一个线程来做，具体可以参照实例1的SimpleObserver2.java文件。

实例1

SimpleObservable.java文件

package com.lenovo.robin;
import java.util.Observable;
public class SimpleObservable extends Observable{ private int data = 0; private String name; public SimpleObservable(String name) { this.name=name; } public String getName() { return name; } public int getData() { return data; }
public void setData(int i) { if (this.data != i) { this.data = i; /* 只有在setChange()被调用后，notifyObservers()才会去调用update()，否则什么都不干。 */ setChanged(); notifyObservers();
} }}

SimpleObserver.java文件

package com.lenovo.robin;
import java.util.Observable;import java.util.Observer; public class SimpleObserver implements Observer{   private String name;   public SimpleObserver(String name){   this.name=name;   }   public void update(Observable o,Object arg){  SimpleObservable observable=(SimpleObservable)o;      System.out.println(name+" found that Data of "+observable.getName()+" has changed to " + observable.getData());   }}

SimpleObserver2.java

package com.lenovo.robin;
import java.util.Observable;import java.util.Observer;import java.util.concurrent.ArrayBlockingQueue;
public class SimpleObserver2 implements Observer,Runnable{   private String name;   public SimpleObserver2(String name){   this.name=name;   } /* 利用一个消息队列来接收Observable的通知，保证消息不会丢失 */ ArrayBlockingQueue<Message> queue = new ArrayBlockingQueue<Message>(100); Thread thread=null; public void run() { while (true) { Message msg; try { msg = queue.take(); Observable o = msg.observable; Object obj = msg.object; SimpleObservable observable=(SimpleObservable)o; System.out.println(name+" found that Data of "+observable.getName()+" has changed to " + observable.getData()); // …执行相应的工作 } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
public void update(Observable o, Object arg) { Message msg = new Message(o, arg); try { queue.put(msg); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } if(thread==null){ thread=new Thread(this); thread.start(); } }
class Message { Observable observable; Object object;
Message(Observable o, Object arg) { this.observable = o; this.object = arg; } }}

使用的代码片段

       SimpleObservable observable01 = new SimpleObservable ("SimpleObservable01");        SimpleObservable observable02 = new SimpleObservable ("SimpleObservable02");        SimpleObserver observer1_0 = new SimpleObserver ("SimpleObserver1-0");        SimpleObserver observer1_1 = new SimpleObserver ("SimpleObserver1-1");        SimpleObserver2 observer2_0 = new SimpleObserver2 ("SimpleObserver2-0");        observable01.addObserver(observer1_0);        observable01.addObserver(observer1_1);        observable01.addObserver(observer2_0);        observable02.addObserver(observer1_0);        observable02.addObserver(observer1_1);        observable02.addObserver(observer2_0);        observable01.setData(1);        observable01.setData(2);        observable01.setData(2);        observable01.setData(3);        observable02.setData(100);        observable02.setData(110);

运行结果

SimpleObserver1-1 found that Data of SimpleObservable01 has changed to 1SimpleObserver1-0 found that Data of SimpleObservable01 has changed to 1SimpleObserver1-1 found that Data of SimpleObservable01 has changed to 2SimpleObserver1-0 found that Data of SimpleObservable01 has changed to 2SimpleObserver1-1 found that Data of SimpleObservable01 has changed to 3SimpleObserver1-0 found that Data of SimpleObservable01 has changed to 3SimpleObserver1-1 found that Data of SimpleObservable02 has changed to 100SimpleObserver1-0 found that Data of SimpleObservable02 has changed to 100SimpleObserver2-0 found that Data of SimpleObservable01 has changed to 3SimpleObserver2-0 found that Data of SimpleObservable01 has changed to 3SimpleObserver2-0 found that Data of SimpleObservable01 has changed to 3SimpleObserver2-0 found that Data of SimpleObservable02 has changed to 110SimpleObserver2-0 found that Data of SimpleObservable02 has changed to 110SimpleObserver1-1 found that Data of SimpleObservable02 has changed to 110SimpleObserver1-0 found that Data of SimpleObservable02 has changed to 110

Observable.java源码

/* * @(#)Observable.java 1.39 05/11/17 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */
package java.util;
/** * This class represents an observable object, or "data" * in the model-view paradigm. It can be subclassed to represent an  * object that the application wants to have observed.  * <p> * An observable object can have one or more observers. An observer  * may be any object that implements interface <tt>Observer</tt>. After an  * observable instance changes, an application calling the  * <code>Observable</code>'s <code>notifyObservers</code> method   * causes all of its observers to be notified of the change by a call  * to their <code>update</code> method.  * <p> * The order in which notifications will be delivered is unspecified.   * The default implementation provided in the Observable class will * notify Observers in the order in which they registered interest, but  * subclasses may change this order, use no guaranteed order, deliver  * notifications on separate threads, or may guarantee that their * subclass follows this order, as they choose. * <p> * Note that this notification mechanism is has nothing to do with threads  * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>  * mechanism of class <tt>Object</tt>. * <p> * When an observable object is newly created, its set of observers is  * empty. Two observers are considered the same if and only if the  * <tt>equals</tt> method returns true for them. * * @author  Chris Warth * @version 1.39, 11/17/05 * @see     java.util.Observable#notifyObservers() * @see     java.util.Observable#notifyObservers(java.lang.Object) * @see     java.util.Observer * @see     java.util.Observer#update(java.util.Observable, java.lang.Object) * @since   JDK1.0 */public class Observable {    private boolean changed = false;    private Vector obs;       /** Construct an Observable with zero Observers. */
    public Observable() { obs = new Vector();    }
    /**     * Adds an observer to the set of observers for this object, provided      * that it is not the same as some observer already in the set.      * The order in which notifications will be delivered to multiple      * observers is not specified. See the class comment.     *     * @param   o   an observer to be added.     * @throws NullPointerException   if the parameter o is null.     */    public synchronized void addObserver(Observer o) {        if (o == null)            throw new NullPointerException(); if (!obs.contains(o)) {    obs.addElement(o); }    }
    /**     * Deletes an observer from the set of observers of this object.      * Passing <CODE>null</CODE> to this method will have no effect.     * @param   o   the observer to be deleted.     */    public synchronized void deleteObserver(Observer o) {        obs.removeElement(o);    }
    /**     * If this object has changed, as indicated by the      * <code>hasChanged</code> method, then notify all of its observers      * and then call the <code>clearChanged</code> method to      * indicate that this object has no longer changed.      * <p>     * Each observer has its <code>update</code> method called with two     * arguments: this observable object and <code>null</code>. In other      * words, this method is equivalent to:     * <blockquote><tt>     * notifyObservers(null)</tt></blockquote>     *     * @see     java.util.Observable#clearChanged()     * @see     java.util.Observable#hasChanged()     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)     */    public void notifyObservers() { notifyObservers(null);    }
    /**     * If this object has changed, as indicated by the      * <code>hasChanged</code> method, then notify all of its observers      * and then call the <code>clearChanged</code> method to indicate      * that this object has no longer changed.      * <p>     * Each observer has its <code>update</code> method called with two     * arguments: this observable object and the <code>arg</code> argument.     *     * @param   arg   any object.     * @see     java.util.Observable#clearChanged()     * @see     java.util.Observable#hasChanged()     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)     */    public void notifyObservers(Object arg) { /*         * a temporary array buffer, used as a snapshot of the state of         * current Observers.         */        Object[] arrLocal;
synchronized (this) {    /* We don't want the Observer doing callbacks into     * arbitrary code while holding its own Monitor.     * The code where we extract each Observable from      * the Vector and store the state of the Observer     * needs synchronization, but notifying observers     * does not (should not).  The worst result of any      * potential race-condition here is that:     * 1) a newly-added Observer will miss a     *   notification in progress     * 2) a recently unregistered Observer will be     *   wrongly notified when it doesn't care     */    if (!changed)                return;            arrLocal = obs.toArray();            clearChanged();        }
        for (int i = arrLocal.length-1; i>=0; i--)            ((Observer)arrLocal[i]).update(this, arg);    }
    /**     * Clears the observer list so that this object no longer has any observers.     */    public synchronized void deleteObservers() { obs.removeAllElements();    }
    /**     * Marks this <tt>Observable</tt> object as having been changed; the      * <tt>hasChanged</tt> method will now return <tt>true</tt>.     */    protected synchronized void setChanged() { changed = true;    }
    /**     * Indicates that this object has no longer changed, or that it has      * already notified all of its observers of its most recent change,      * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.      * This method is called automatically by the      * <code>notifyObservers</code> methods.      *     * @see     java.util.Observable#notifyObservers()     * @see     java.util.Observable#notifyObservers(java.lang.Object)     */    protected synchronized void clearChanged() { changed = false;    }
    /**     * Tests if this object has changed.      *     * @return  <code>true</code> if and only if the <code>setChanged</code>      *          method has been called more recently than the      *          <code>clearChanged</code> method on this object;      *          <code>false</code> otherwise.     * @see     java.util.Observable#clearChanged()     * @see     java.util.Observable#setChanged()     */    public synchronized boolean hasChanged() { return changed;    }
    /**     * Returns the number of observers of this <tt>Observable</tt> object.     *     * @return  the number of observers of this object.     */    public synchronized int countObservers() { return obs.size();    }}

Observer.java源码

/* * @(#)Observer.java 1.20 05/11/17 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */package java.util;
/** * A class can implement the <code>Observer</code> interface when it * wants to be informed of changes in observable objects. * * @author  Chris Warth * @version 1.20, 11/17/05 * @see     java.util.Observable * @since   JDK1.0 */public interface Observer {    /**     * This method is called whenever the observed object is changed. An     * application calls an <tt>Observable</tt> object's     * <code>notifyObservers</code> method to have all the object's     * observers notified of the change.     *     * @param   o     the observable object.     * @param   arg   an argument passed to the <code>notifyObservers</code>     *                 method.     */    void update(Observable o, Object arg);}
结束！