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一、观察者模式定义

类型：行为型模式
目的：当一个对象的状态发生改变时，其所有依赖者（观察者）都会收到通知并自动更新。

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二、例子

2.1 菜鸟教程例子

2.1.1 定义观察者

//观察者
public abstract class Observer {//被观察对象protected Subject subject;public abstract void update();
}

public class BinaryObserver extends Observer{public BinaryObserver(Subject subject){this.subject = subject;this.subject.attach(this);}@Overridepublic void update() {System.out.println( "Binary String: " + Integer.toBinaryString( subject.getState() ) ); }
}

public class HexaObserver extends Observer{public HexaObserver(Subject subject){this.subject = subject;this.subject.attach(this);}@Overridepublic void update() {System.out.println( "Hex String: "  + Integer.toHexString( subject.getState() ).toUpperCase() ); }
}

2.1.2 定义被观察对象

import java.util.ArrayList;
import java.util.List;
//被观察对象 
public class Subject {//观察者列表private List<Observer> observers = new ArrayList<Observer>();private int state;public int getState() {return state;}public void attach(Observer observer){observers.add(observer);      }//通知所有观察者public void notifyAllObservers(){for (Observer observer : observers) {observer.update();}}  //修改数据时，通知所有观察者public void setState(int state) {this.state = state;notifyAllObservers();}
}

2.1.3 使用

public class ObserverPatternDemo {public static void main(String[] args) {//创建被观察对象Subject subject = new Subject();//创建观察者对象new HexaObserver(subject);new BinaryObserver(subject);//创建被观察对象调用setState时，通知国有观察者System.out.println("First state change: 15");   subject.setState(15);System.out.println("Second state change: 10");  subject.setState(10);}
}

2.2 JDK源码 —— Observable

2.2.1 观察者接口Observer

public interface Observer {void update(Observable o, Object arg);
}

2.2.1 被观察者对象Observable

public class Observable {private boolean changed = false;private Vector<Observer> obs;//添加观察者public synchronized void addObserver(Observer o) {if (o == null)throw new NullPointerException();if (!obs.contains(o)) {obs.addElement(o);}}//通知观察者public void notifyObservers(Object arg) {Object[] arrLocal;synchronized (this) {if (!changed)return;arrLocal = obs.toArray();clearChanged();}for (int i = arrLocal.length-1; i>=0; i--)((Observer)arrLocal[i]).update(this, arg);}
}

2.3 Spring源码 —— AbstractApplicationContext

2.3.1 观察者

public class ContextStartedEvent extends ApplicationContextEvent {public ContextStartedEvent(ApplicationContext source) {super(source);}
}

public class ContextStoppedEvent extends ApplicationContextEvent {public ContextStoppedEvent(ApplicationContext source) {super(source);}
}

public class ContextRefreshedEvent extends ApplicationContextEvent {public ContextRefreshedEvent(ApplicationContext source) {super(source);}
}

2.3.2 被观察者

public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {//观察者集合private Set<ApplicationEvent> earlyApplicationEvents;//添加观察者Eventpublic void publishEvent(ApplicationEvent event) {this.publishEvent(event, (ResolvableType)null);}//通知观察者Eventprotected void registerListeners() {Iterator var1 = this.getApplicationListeners().iterator();while(var1.hasNext()) {ApplicationListener<?> listener = (ApplicationListener)var1.next();this.getApplicationEventMulticaster().addApplicationListener(listener);}String[] listenerBeanNames = this.getBeanNamesForType(ApplicationListener.class, true, false);String[] var7 = listenerBeanNames;int var3 = listenerBeanNames.length;for(int var4 = 0; var4 < var3; ++var4) {String listenerBeanName = var7[var4];this.getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);}Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;this.earlyApplicationEvents = null;if (!CollectionUtils.isEmpty(earlyEventsToProcess)) {Iterator var9 = earlyEventsToProcess.iterator();while(var9.hasNext()) {ApplicationEvent earlyEvent = (ApplicationEvent)var9.next();this.getApplicationEventMulticaster().multicastEvent(earlyEvent);}}}
}

2.3 Guava源码 —— EventBus

public class EventBus {//注册订阅者（观察者）public void register(Object object) {this.subscribers.register(object);}//通知订阅者（观察者）public void post(Object event) {Iterator<Subscriber> eventSubscribers = this.subscribers.getSubscribers(event);if (eventSubscribers.hasNext()) {this.dispatcher.dispatch(event, eventSubscribers);} else if (!(event instanceof DeadEvent)) {this.post(new DeadEvent(this, event));}}
}

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三、其他设计模式

创建型模式
结构型模式

• 1、设计模式——装饰器模式（Decorator Pattern）+ Spring相关源码

行为型模式

• 1、设计模式——访问者模式（Visitor Pattern）+ Spring相关源码
• 2、设计模式——中介者模式（Mediator Pattern）+ JDK相关源码
• 3、设计模式——策略模式（Strategy Pattern）+ Spring相关源码
• 4、设计模式——状态模式（State Pattern）
• 5、设计模式——观察者模式（Observer Pattern）+ Spring相关源码