设计模式之观察者模式（Java实现）
观察者模式是设计模式中常见的一种模式，他的思想犹如他的名字一样，就是想构建一个观察者观察他所关注的主题，当主题发生变化了，就会通知观察者。观察者模式也叫发布订阅模式，这种设计模式用在了很多地方，比如zookeeper的节点观察机制。这里我们构建一个如下模型，Subject<-AbstractSubject<-TopSubject 与 Observer<-Watcher，他们的关系如下图所示：
通过java实现：
Observer.java
package com.xxx.design.watcher;public interface Observer {void update();}
Watcher.java
package com.xxx.design.watcher;/*** 观察者**/public class Watcher implements Observer {private String name;public Watcher(String name) {this.name = name;}@Overridepublic void update() {System.out.println("watcher "+name+" update state.");}}
Subject.java
package com.xxx.design.watcher;public interface Subject {void add(Observer observer);void delete(Observer observer);void notifyObserver();void action();}
AbstractSubject.java
package com.xxx.design.watcher;import java.util.Enumeration;import java.util.Vector;public abstract class AbstractSubject implements Subject{private Vector<Observer> obs = new Vector<Observer>();@Overridepublic void add(Observer observer) {obs.add(observer);}@Overridepublic void delete(Observer observer) {obs.remove(observer);}@Overridepublic void notifyObserver() {Enumeration<Observer> eles = obs.elements();while(eles.hasMoreElements()){eles.nextElement().update();}}}
TopSubject.java
package com.xxx.design.watcher;public class TopSubject extends AbstractSubject {@Overridepublic void action() {System.out.println("i am a subject,update now.");notifyObserver();}public static void main(String[] args) {Subject subject = new TopSubject();subject.add(new Watcher("aa"));subject.add(new Watcher("bb"));subject.add(new Watcher("cc"));subject.action();}}运行程序，打印结果如下：i am a subject,update now.watcher aa update state.watcher bb update state.watcher cc update state.观察者需要注意的是：避免循环引用，如果是顺序执行，考虑采用异步实现，避免因为一个处理异常导致整个系统卡壳。另外，现在java.util包中api已经提供了观察者模式的实现。我们可以利用Observable类和Observer接口来实现一个观察者模式：
Subscribe.java
package com.xxx.design.watcher.jdk;import java.util.Observable;import java.util.Observer;public class Subscribe implements Observer {private String name;public Subscribe(String name){this.name = name;}public void add(Observable o){o.addObserver(this);}@Overridepublic void update(Observable o, Object arg) {System.out.println(name+" 收到通知："+((Publish)o).getData());}}
Publish.java
package com.xxx.design.watcher.jdk;import java.util.Observable;public class Publish extends Observable {private String data = "";public String getData() {return data;}public void setData(String data) {if(!this.data.equals(data)){this.data = data;setChanged();}notifyObservers();}public static void main(String[] args) {Publish publish = new Publish();Subscribe subscribe = new Subscribe("aaa");subscribe.add(publish);Subscribe subscribe2 = new Subscribe("bbb");subscribe2.add(publish);publish.setData("start");publish.setData("end");}}
运行程序，打印信息如下：
bbb 收到通知：startaaa 收到通知：startbbb 收到通知：endaaa 收到通知：end
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