java，一个被观察者多个观察者

import java.util.Observable;
import java.util.Observer;

/**
 * 一个被观察者多个观察者
 * @author zengst
 * @date
 */
public class ObserverTest1 {
 public static void main(String args[]){
  
  BeingWatched bw = new BeingWatched();
  
  Watch1 w1 = new Watch1();
  Watch2 w2 = new Watch2();
  
  //光标定位到某一行，crtl+Alt+向下箭头，复制当前行内容到下一行
  bw.addObserver(w1);
  bw.addObserver(w2);
  
  bw.counter();
 }
}

class Watch1 implements Observer{
 
 //这里的args是被观察者调用notifyObservers方法时传递的参数
 //这里的o是被观察者的父类，因为一个观察者可以观者多个被观察者
 //所以这里无法直接通过o调用某一个被观察者定义的方法
 @Override
 public void update(Observable o, Object arg) {
  
  System.out.println("我是观察者一，被观察的人变化了");
 }
}

class Watch2 implements Observer{
 
 //这里的args是被观察者调用notifyObservers方法时传递的参数
 //这里的o是被观察者的父类，因为一个观察者可以观者多个被观察者
 //所以这里无法直接通过o调用某一个被观察者定义的方法
 @Override
 public void update(Observable o, Object arg) {
  
  System.out.println("我是观察者二，被观察的人变化了");
 }
}

class BeingWatched extends Observable{
 public int j =888;
 
 public int cry(){
  System.out.println("I am crying--"+j);
  return j;
 }
 void counter(){
  try{
   for(int i=0; i<5;i++){
    setChanged();
    notifyObservers(i);
    Thread.sleep(1000);
   }
  }catch(Exception e){
   e.printStackTrace();
  }
 }
}

 

 

Observable类源码：

/*
 * @(#)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();
    }
}