java观察者模式

       今天在查看项目中Map模块，学到了java观察者模式，再次记录下，以便以后查阅。

      JDK1.2后，Java提供了对观察者模式的支持接口和实现类。

       其中接口 java.util.Observer 用来指定观察者，观察者必须实现 void update(Observable o, Object arg) 方法。

       而 java.util.Observable 用来指定观察物（被观察者、可被观察的），并且提供了一系列的方法。读者可以很轻易的使用这个接口和实现类来实现观察者模式。

       java.util.Observer 只有一个简单的方法 void update(Observable o, Object arg)

       其中，参数 Observable o 用于指定触发 update 方法的对象， Object arg 用于指定触发 update 方法时候的附加参数。

       如果有桌面应用开发的读者应该很了解，这跟事件处理机制是完全一样的，其中 Observable o 可被看作事件源。 Object arg 可被看作消息。

       具体Map是这么用的，Map需要做一个放大缩小的功能。ZoomState作为被观察者，MapZoomView作为观察者，当ZoomState发声变化，MapZoomView就根据ZoomState的参数发生变化。

       这边做一个小例子：

     

package com.jeff;import java.util.Observable;public class Publisher extends Observable {private String magazineName;public String getMagazineName() {return magazineName;}public void publish(String magazineName) {this.magazineName = magazineName;setChanged();notifyObservers(this);}}

package com.jeff;import java.util.Observable;import java.util.Observer;public class Reader implements Observer {@Overridepublic void update(Observable o, Object arg1) {Publisher p = (Publisher) o;System.out.println("我要订阅" + p.getMagazineName());}}

 

 

这边附上java.util.Observable的源码：

/** * @(#)Observable.java1.39 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();    }}

java.util.Observer的源码：

/** * @(#)Observer.java1.20 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);}
	
				
	
					   
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