观察者模式中一般有两种,一种是推数据的模式(被观测对象向观测对象 送出数据,即在被观测对象发生改变时,主动告知被观测对象),一种是拉模式(观测对象主动去被观测对象去查询是否发生了变化)。虽然方式略有不同,但是基本上实现的原理是相似的。
从java提供的观察者模式的实现入手很容易理解。java提供了一个Observable基类和一个Observer接口,分别是被观测对象基类和观察者需要实现的接口。
简要分析这两个类:1 被观察者对象基类,此基类为于java.util包中,下面贴源码看一下
写道
/**
* 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();
}
}
可以看出事实上,基类维护了一个同步的Vector来存放观察者,相关的提供了添加和删除观察者的方法,以及维护集合顺序之类的操作,这个基类中核心的方法就是notifyObservers(args),此方法调用观察者的公共接口update()方法。下面看一下观察者的接口 Observer ,此接口也位于java.util包下
/*
* @(#)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);
}
通过阅读这两个方法我们很容易理解观察者模式了。事实上就是莫中意义上的回调方法,当自己改变时通知关注此改变的监听者改变。
下面是一些demo示例
首先看被观察对象:
写道
public class ResourceBean extends Observable{
private String event;
//做点什么呢
public void doSomething(String event){
this.event= event;
setChanged();
System.out.println( "in resource bean event:"+event+" has done");
notifyObservers(event);
}
}
接下来看两个观察者
public class LockOberver implements Observer {
private static String INTREEST ="lock";
/* (non-Javadoc)
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
@Override
public void update(Observable o, Object arg) {
if(INTREEST.equalsIgnoreCase( (String) arg)){
System.out.println("收到锁定的通知 "+o.toString()+ "变化值:"+ arg.toString());
}
}
}
public class UnlockOberver implements Observer {
private static String INTREEST ="unlock";
/* (non-Javadoc)
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
@Override
public void update(Observable o, Object arg) {
if(INTREEST.equalsIgnoreCase( (String) arg)){
System.out.println("输出收到解锁的通知:"+o.toString()+" 参数值:"+arg.toString());
}
}
}
最后看一下如何 运行的示例
public class TestObserverPattern {
public static void main(String[] args){
ResourceBean bean = new ResourceBean();//被观测对象
LockOberver lock = new LockOberver();
UnlockOberver unlock = new UnlockOberver();
bean.addObserver(lock);
bean.addObserver(unlock);
bean.doSomething("lock");
bean.doSomething("freeAccess");
bean.doSomething("unlock");
}
}
运行的结果:
写道
in resource bean event:lock has done
收到锁定的通知 com.yasouth.ResourceBean@4a5ab2变化值:lock
in resource bean event:freeAccess has done
in resource bean event:unlock has done
输出收到解锁的通知:com.yasouth.ResourceBean@4a5ab2 参数值:unlock
这样一个简单的观察者示例就结束了
后续的工作是想了解一下,观察者模式中在java同步机制的具体应用,待续
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