1.用意
观察者模式定义了一种一对多的依赖关系,让多个观察者对象同时监听某一个主题对象,这个主题对象在状态上发生变化时,会通知所有观察者对象,使它们能够自动更新自己。
2.类图
第一种简单方式
第二种方式
第三种方式 Java的内在支持
3.代码
第一种方式代码：
/*
* 抽象观察者角色
*/
public interface Observer {
/*
* 调用这个方法会更新自己
*/
void update();
}
/*
* 具体观察者角色
*/
public class ConcreteObserver implements Observer{
/*
* 调用这个方法会更新自己
* @see com.javapatterns.observer.Observer#update()
*/
public void update() {
System.out.println("I am notified.");
}
}
/*
* 抽象主题角色
*/
public interface Subject {
/*
* 调用这个方法登记一个新的观察者对象
*/
public void attach(Observer observer);
/*
* 调用这个方法删除一个已经登记过的观察者对象
*/
public void detach(Observer observer);
/*
* 调用这个方法通知所有登记过的观察者对象
*/
void notifyObserver();
}
/*
* 具体主题角色
*/
public class ConcreteSubject implements Subject {
private Vector observersVector = new Vector();
public void attach(Observer observer) {
observersVector.addElement(observer);
}
public void detach(Observer observer) {
observersVector.removeElement(observer);
}
public Enumeration observers() {
return ((Vector) observersVector.clone()).elements();
}
public void notifyObserver() {
Enumeration enumeration = observers();
while (enumeration.hasMoreElements()) {
((Observer) enumeration.nextElement()).update();
}
}
}
/*
* 测试类
*/
public class Test {
public static void main(String[] args) {
Subject subject = new ConcreteSubject();
subject.attach(new ConcreteObserver());
subject.attach(new ConcreteObserver());
subject.attach(new ConcreteObserver());
subject.notifyObserver();
}
}
第二种方式代码：
public abstract class Subject {
private Vector observersVector = new Vector();
public void attach(Observer observer) {
observersVector.addElement(observer);
S.s("Attached an observer.");
}
public void detach(Observer observer) {
observersVector.removeElement(observer);
S.s("Detached an observer.");
}
public Enumeration observers() {
return ((Vector) observersVector.clone()).elements();
}
public void notifyObserver() {
Enumeration enumeration = observers();
while (enumeration.hasMoreElements()) {
((Observer) enumeration.nextElement()).update();
}
}
}
/*
* 具体主题角色，现在只有一个改变内部状态的商业方法
*/
public class ConcreteSubject extends Subject{
private String state;
public void change(String newState){
state = newState;
this.notifyObserver();
}
}
/*
* 测试类
*/
public class Test {
private static ConcreteSubject subject;
private static Observer observer;
public static void main(String[] args) {
subject = new ConcreteSubject();
observer = new ConcreteObserver();
subject.attach(observer);
subject.change("new state");
}
}
第3种方式，Java的内在支持：
/*
* 被观察者对象
*/
public class Watched extends Observable {
private String data = "";
public String retrieveData() {
return data;
}
public void changeDate(String data) {
if (!this.data.equals(data)) {
this.data = data;
//只有setChange过观察者才会执行update方法，详见Observable源代码
setChanged();
}
//通知观察者执行update方法
notifyObservers();
}
}
/*
* 观察者
*/
public class Watcher implements Observer {
public Watcher(Watched w) {
w.addObserver(this);
}
public void update(Observable o, Object arg) {
S.s("Data has been changed to: " + ((Watched) o).retrieveData() + "");
}
}
/*
* 测试类
*/
public class Test {
static private Watched watched;
static private Observer watcher;
public static void main(String[] args) {
watched = new Watched();
watcher = new Watcher(watched);
watched.changeDate("In C, we create bugs.");
watched.changeDate("In Java, we inherit bugs.");
watched.changeDate("In Java, we inherit bugs.");
watched.changeDate("In visual Basic, we visualize bugs.");
}
}
输出：
Observer源代码：
/*
* %W% %E%
*
* Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package sam.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 %I%, %G%
* @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源代码：
/*
* %W% %E%
*
* Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package sam.java.util;
import java.util.Vector;
/**
* 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 %I%, %G%
* @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();
}
}
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