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。

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： 

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);
}

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

    /**
     * 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);
        }
    }

/**
     * 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);
    }

    /**
     * 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;
    }

  void update(Observable o, Object arg);

示例

/**
 * 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 = http://www.mamicode.com/0;>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());
    }
}

/**
 * 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);
    }
}

/**
 * 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 = http://www.mamicode.com/0;>
/**
 * 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);
	}
}

Observer 模式及JAVA内置的observer示例