观察者模式
定义
定义了对象之间的一对多依赖，让多个观察者对象同时监听某一个主题对象。当主题对象发生变化时，它的所有依赖者（观察者）都会收到通知并更新。
适用场景
关联行为场景，建立一套触发机制
提出需求
用户订阅微信公众号，公众号更新，用户接收到通知
需求分析
观察者模式又叫发布-订阅模式。需求中，用户订阅微信公众号，当微信公众号有新的消息时，所有订阅这个微信公众号的用户都可以接收到这个消息。这个就是很典型的发布订阅模式。微信公众号作为被订阅者（也叫被观察者），用户订阅这个公众号。用户就是作为观察者的角色存在的。
代码实现
jdk中已经帮我们实现了观察者模式。java.util.Observable和java.util.Observer。
我们先看下jdk源码
- java.util.Observable 被观察者
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 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<Observer> 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();
}
}
- java.util.Observer 观察者
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);
}
由于代码比较简单，此处就不做解读了。
下面我们来实现我们的需求
代码结构
被观察者（Observable）和观察者（Observer）之间是一对多的关系
- WeixinPublic 被观察者（发布者）
import java.util.Observable;
/**
* @Author: ming.wang
* @Date: 2019/3/7 10:46
* @Description:
*/
@Getter
@Setter
@ToString
public class WeixinPublic extends Observable {
private String name;
public WeixinPublic(String name) {
this.name = name;
}
public void publishNews(String content){
System.out.println(name+"发布新消息来，内容是:"+content+"。");
setChanged();
notifyObservers(content);
}
}
- ZhangSan 观察者（订阅者）
import java.util.Observable;
import java.util.Observer;
/**
* @Author: ming.wang
* @Date: 2019/3/7 10:46
* @Description:
*/
@Getter
@Setter
@ToString
public class ZhangSan implements Observer {
private String name;
public ZhangSan(String name) {
this.name = name;
}
@Override
public void update(Observable o, Object arg) {
WeixinPublic weixinPublic= (WeixinPublic) o;
String content= (String) arg;
System.out.println(name+"，您关注的"+weixinPublic.getName()+"有新的更新，内容是:"+content);
}
}
- Lisi 观察者（订阅者）
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;
import java.util.Observable;
import java.util.Observer;
/**
* @Author: ming.wang
* @Date: 2019/3/7 10:46
* @Description:
*/
@Getter
@Setter
@ToString
public class Lisi implements Observer {
private String name;
public Lisi(String name) {
this.name = name;
}
@Override
public void update(Observable o, Object arg) {
WeixinPublic weixinPublic= (WeixinPublic) o;
String content= (String) arg;
System.out.println(name+"，您关注的"+weixinPublic.getName()+"有新的更新，内容是:"+content);
}
}
- Test
/**
* @Author: ming.wang
* @Date: 2019/3/7 11:02
* @Description:
*/
public class Test {
public static void main(String[] args) {
WeixinPublic weixinPublic=new WeixinPublic("技术狂奔路");
weixinPublic.addObserver(new ZhangSan("张三"));
weixinPublic.addObserver(new Lisi("李四"));
weixinPublic.publishNews("java设计模式该怎么学？");
}
}
运行结果
