Observer 模式及JAVA内置的observer示例

关于观察者模式

假设今天您设计一个图形分析算表程序，当中有一个资料物件，您可以用表格图形物件、柱状图形物件、圆饼图形物件等方式来 呈现物件，无论您是用哪种图形物件，重点是若资料物件的内容作了更改，则图形物件的内容也必须跟着修改，或许您的程式中有两个以上的图形物件来呈现资料，您在图形物件上更动资料，则另一个图形物 件也必须作出相对应的变化。

主题资料物件观察者柱状图形表格图形圆饼图形

又假设您今天设计一个网络游戏，您在服务器上维护一个连线客户端共享的资料物件，当其中一个客户端作了操作，将对此资料物件作修改，则伺服器必须通知其它 客户端作相对应的变化（像是人物位置走动、建了一个城堡等）。主题资料物件观察者客户端一客户端二客户端三

在Observer模式中的主角为主题（subject）与观察者（observer），观察者订阅它感兴趣的主题，一个主题可以被多个观察者订阅，当主题的状态发生变化时，它必须通知（notify）所有订阅它的观察者，观察者检视主题的状态变化，并作出对应的动作，所以Observer 模式也称之为Publish-Subscribe模式。 

Observer模式的 UML 图如下所示：

Subject类中有一个notify()方法，通常是在Subject的状态发生改变时呼叫它，notify()中会呼叫 Observer的update()方法，通常会先取得Subject的新状态，然后更新Observer的显示或行为，这个过程我们可以透过 Sequence Diagram来表达： 

在Java中支持观察者模式，要成为观察者的类必须实作Observer介面，这个介面中定义了一个update()方法，这个方法会被主题物件在通知状态变化时呼叫，您必须在这个方法中实作您所想要的对应行为。

主题物件会是Observable的子类，在这边注意两个重要的方法：setChanged()与notifyObserver()。 setChanged()是用来设定主题物件的状态已经被改变，而notifyObserver()方法会通知所要订阅主题物件的观察者，调用其 update()方法。

关于Java的相关源码分析

其中Java中的内置的模式的源码如下:

Observer.java接口类

仅有一个Observer的接口类,含有一个update抽象方法.

/* * Copyright (c) 1994, 1998, Oracle and/or its affiliates. All rights reserved. * ORACLE 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 * @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);}

observable.java类

具体的类内容,请看注释文档,这已经非常详细了.

/* * Copyright (c) 1994, 2004, Oracle and/or its affiliates. All rights reserved. * ORACLE 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 * @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();    }}<span style="font-size:18px;"></span>

实现观察者模式

实现观察者模式非常简单，

[1]创建被观察者类，它继承自java.util.Observable类；

[2]创建观察者类，它实现java.util.Observer接口；

[3]对于被观察者类，

添加它的观察者：

    /**     * 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);        }    }

addObserver()方法把观察者对象添加到观察者对象列表中。

 

当被观察事件发生时，执行：

/**     * 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);    }

以及

    /**     * 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;    }

只有在setChange()被调用后，notifyObservers()才会去调用update()。setChange()方法用来设置一个内部标志位注明数据发生了变化；notifyObservers()方法会去调用观察者对象列表中所有的Observer的update()方法，通知它们数据发生了变化。

[4]对于观察者类，实现Observer接口的唯一方法update

  void update(Observable o, Object arg);

形参Object arg，对应一个由notifyObservers(Object arg);传递来的参数，当执行的是notifyObservers();时，arg为null。

示例

1.NumObserable是一个被观察者，当它的成员变量data的数值发生变化时，会通知所有的观察者。

被观察者--主题类

/** * 被观察者--即是主题 * @author SuooL * @version 1.0.0 * <p> <a herf="suool.net" > SuooL's Blog </a> */package observer;import java.util.Observable;/** * 被观察者--主题类 * */public class numObservable extends Observable {    private int data = 0;    /**     * 获取数字值     *      * @return 取得的data     */    public int getData() {       return data;    }    /**     * 设置数字值     *      * @param i 要设置的数字值     */    public void setData(int i) {       data = i;       setChanged();       notifyObservers();    }}

观察者--订阅者

/** * 观察者--即是订阅者 * @author SuooL * @version 1.0.0 * <p> <a herf="suool.net" > SuooL's Blog </a> */package observer;import java.util.Observable;import java.util.Observer;/** * 继承自<code>Observer</code>接口类的观察者类 * @see java.util.Observer */public class numObserver implements Observer{/** * Observe接口类的的唯一抽象方<code>update</code>的实现 * <p>只有在setChange()被调用后，notifyObservers()才会去调用update()。 *@param o 主题类型 *@param arg Object类型参数, *       对应一个由<code>notifyObservers(Object arg);</code>传递来的参数, *       当执行的是<code>notifyObservers();</code>时，arg为<code>null</code>。 *        */public void update(Observable o, Object arg) {       numObservable myObserable=(numObservable) o;       System.out.println("Data has changed to " +myObserable.getData());    }}

test.

/** * 测试 * @author SuooL * @version 1.0.0 * <p> <a herf="suool.net" > SuooL's Blog </a> */package observer;/** * 测试 */public class observerTest {    public static void main(String[] args) {    // 创建一个主题对象    numObservable number = new numObservable();    // 为主题对象增加订阅者    number.addObserver(new numObserver());    // 修改主题对象    number.setData(1);    number.setData(2);    number.setData(3);    }}

结果如下:

2.这个实例中，还是对data进行观察，拥有两个观察者，分别观察奇数和偶数的变化，通过notifyObservers(arg)中的参数arg来识别通知信息.

NumsObservable.java

/** * Description: * 被观察者--主题类 * <p>两个私有静态变量,分别变化 * <p>被两个订阅者订阅,分别关注不同的兴趣点--奇数变化 OR 偶数变化 * @author SuooL * @version 1.0.0 * <p> <a herf = "http://suool.net"> SuooL's Blog </a> */package observers;import java.util.Observable;/** * 主题类 */public class NumsObservable extends Observable {    public final static Integer ODD = 1;    public final static Integer EVEN = 2;    private int data = 0;    /**     * 获取对象的数据     * @return data 返回取得的数据     */    public int getData() {       return data;    }    /**     * 设置数据变化     * 根据数据的变化设置相应的标志变量,通知给订阅者     * @param i 要设置的数据     */    public void setData(int i) {       data = i;       Integer flag = EVEN;       if ((data & 0x0001) == 1)           flag = ODD;       setChanged();       // 将变化的变化的标识变量通知给订阅者       notifyObservers(flag);    }}

/** * Description: * 偶数观察者类 * <p>订阅主题的内容的偶数变化 * @author SuooL * @version 1.0.0 * <p> <a herf = "http://suool.net"> SuooL's Blog </a> */package observers;import java.util.Observable;import java.util.Observer;/** * 偶数内容订阅类 * @author SuooL * */public class evenObserver implements Observer{/** * 继承自Observer接口类,update的方法的实现 * @param o 主题对象 * @param arg notifyObservers(flag);传来的参数,即是标识变量 */public void update(Observable o, Object arg) {if (arg == NumsObservable.EVEN) {NumsObservable myObserable=(NumsObservable) o;    System.out.println("Data has changed to EVEN number " +myObserable.getData());    }}}

/** * Description: * 奇数观察者类 * <p>订阅主题的内容的奇数变化 * @author SuooL * @version 1.0.0 * <p> <a herf = "http://suool.net"> SuooL's Blog </a> */package observers;import java.util.Observable;import java.util.Observer;/** * 奇数内容订阅类 * @author SuooL * */public class oddObserver implements Observer{/** * 继承自Observer接口类,update的方法的实现 * @param o 主题对象 * @param arg notifyObservers(flag);传来的参数,即是标识变量 */public void update(Observable o, Object arg) {if (arg == NumsObservable.ODD) {NumsObservable myObserable=(NumsObservable) o;    System.out.println("Data has changed to ODD number " +myObserable.getData());    }}}

测试类:

/** * Description: * 被观察者--主题类 * <p>两个私有静态变量,分别变化 * <p>被两个订阅者订阅,分别关注不同的兴趣点--奇数变化 OR 偶数变化 * @author SuooL * @version 1.0.0 * <p> <a herf = "http://suool.net"> SuooL's Blog </a> */package observers;public class NumsTest {public static void main(String[] args) {// TODO Auto-generated method stub// 创建主题对象和订阅者对象NumsObservable observerNum = new NumsObservable();oddObserver oddObserver = new oddObserver();evenObserver evenObserver = new evenObserver();// 为主题增加订阅者observerNum.addObserver(oddObserver);observerNum.addObserver(evenObserver);// 修改主题对象内容observerNum.setData(12);observerNum.setData(11);observerNum.setData(10);}}

运行结果: