观察者模式（Observer）

一：基本概念

1.什么是观察者模式

• Observer模式是行为模式之一，它的作用是当一个对象的状态发生变化时，能够自动通知其他关联对象，自动刷新对象状态。
• Observer模式提供给关联对象一种同步通信的手段，使某个对象与依赖它的其他对象之间保持状态同步。

2.观察者模式的UML结构

3.观察者模式的角色和职责

• Subject（被观察者）    被观察的对象。当需要被观察的状态发生变化时，需要通知队列中所有观察者对象。Subject需要维持（添加，删除，通知）一个观察者对象的队列列表。
• ConcreteSubject    被观察者的具体实现。包含一些基本的属性状态及其他操作。
• Observer（观察者）    接口或抽象类。当Subject的状态发生变化时，Observer对象将通过一个callback函数得到通知。
• ConcreteObserver    观察者的具体实现。得到通知后将完成一些具体的业务逻辑处理。

4.观察者模式的典型应用

• Observer模式的典型应用
  • 侦听事件驱动程序设计中的外部事件
  • 侦听/监视某个对象的状态变化
  • 发布者/订阅者(publisher/subscriber)模型中，当一个外部事件（新的产品，消息的出现等等）被触发时，通知邮件列表中的订阅者

二：具体例子

1.功能需求描述：发布一条新闻的时候，观察者可以观察到。

2.具体实现：

2.1.效果：

　　

2.2.项目结构

• Article.java-文章bean
• MyObservable.java-被观察者要实现的基类
• MyObserver.java-观察者要实现的接口
• NewsMainClass.java-启动方法
• ObserverNews.java-实现了观察者接口的类
• ObserverNews1.java-实现了观察者接口的类
• PublisherArticle.java-实现了被观察者的类

　　

2.3.运行流程

2.3.1.NewsMainClass.java类主要将，观察者对象注入到被观察者对象中。

package com.witwicky.myobserver;

public class NewsMainClass {
	public static void main(String[] args) {
		Article article = new Article("title1", "content1");

		PublisherArticle pa = new PublisherArticle();

		ObserverNews obn = new ObserverNews();
		ObserverNews1 obn1 = new ObserverNews1();

		pa.addOberserver(obn);
		pa.addOberserver(obn1);

		pa.removeOberserver(obn1);
		// pa.removeAll(); //可以使用

		pa.publisherArt(article);
	}
}

2.3.2.Article.java-文章bean

package com.witwicky.myobserver;

public class Article {
	private String title;
	private String content;

	public Article(String title, String content) {
		this.title = title;
		this.content = content;
	}

	public String getTitle() {
		return title;
	}

	public void setTitle(String title) {
		this.title = title;
	}

	public String getContent() {
		return content;
	}

	public void setContent(String content) {
		this.content = content;
	}

	@Override
	public String toString() {
		return "Article [title=" + title + ", content=" + content + "]";
	}

}

2.3.3.MyObservable.java-被观察者要实现的基类

自己实现：

package com.witwicky.myobserver;

import java.util.Vector;

public class MyObservable {
	private Vector<MyObserver> observerList = new Vector<>();

	public void notifyObservers() {
		notifyObservers(null);
	}

	public void notifyObservers(Object object) {
		for (MyObserver myObserver : observerList) {
			myObserver.update(myObserver, object);
		}
	}

	public void addOberserver(MyObserver observer) {
		if(!observerList.contains(observer)) {
			observerList.addElement(observer);
		}
	}

	public void removeOberserver(MyObserver observer) {
		observerList.remove(observer);
	}

	public void removeAll() {
		observerList.removeAllElements();
	}

	public Integer obsCount() {
		return observerList.size();
	}
}

 JDK中官方实现：

/*
 * Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

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();
    }
}

2.3.4.MyObserver.java-观察者要实现的接口

自己实现：

package com.witwicky.myobserver;

public interface MyObserver {
	public void update(MyObserver myobserver, Object object);
}

JDK官方实现：

/*
 * Copyright (c) 1994, 1998, Oracle and/or its affiliates. All rights reserved.
 * ORACLE 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
 * @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);
}

2.3.5.ObserverNews.java-实现了观察者接口的类

package com.witwicky.myobserver;

public class ObserverNews implements MyObserver {

	@Override
	public void update(MyObserver myobserver, Object object) {
		System.out.println("观察者：观察到又发布了一条新闻，内容如下："+object.toString());		
	}
	
}

2.3.6.ObserverNews1.java-实现了观察者接口的类

package com.witwicky.myobserver;

public class ObserverNews1 implements MyObserver {

	@Override
	public void update(MyObserver myobserver, Object object) {
		System.out.println("观察者1：观察到又发布了一条新闻，内容如下："+object.toString());		
	}
	
}

2.3.7.PublisherArticle.java-实现了被观察者的类

package com.witwicky.myobserver;

public class PublisherArticle extends MyObservable {
	public void publisherArt(Article article) {
		System.out.println("发布者：发布了新文章：" + article.toString());
		this.notifyObservers(article);
		
		System.out.println(this.obsCount());
	}
}

注：最关键的两个类是 MyObserver与MyObservalbe，自己实现代码更灵活一些，如果只是使用基本功能的话，官方JDK版本，更健壮。