Java设计模式之行为型模式(观察者模式)
2017-02-09 15:35
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观察者模式(Observer)
1、背景
在软件开发中,当一个类发生变化,需要通知其他类,并让其他类做某些逻辑操作,观察者模式应运而生。2、概述
①定义
将主类与其他多个类建立一种“一对多”的通信关系,当“主类“发生某些操作时,与之建立从属关系的类作出反应。②应用场景
监听某个类的运行情况被监听类分发信息、监听类订阅信息场景
多个程序的同步更新
同步产生程序运行日志
③优势
一次修改处处运行,提高程序运行时的扩展性能够监听被监听对象的运行状态,有利于后期维护
有利于完善代码的相关功能,如:消息订阅发布。
④结构
附图:Observable为抽象类,是所有被监听类的父类。observers属性用于存储观察类;addObserver、removeObserver为observers这一属性的相关操作;modify为Observable抽象类的改变方法
Observer为观察者接口,update为接口方法。
Target类为Observable的子类
Observer1、Observer2为Observer接口的实现类。实现了update方法
3、实例分析
观察者模式分为以下几种:缺省模式
推模式
拉模式
复合模式
①缺省模式
附图:缺省模式与上面讲的观察者模式结构图一致。拥有Observable、Observer、Target、Observer1、Observer2这些单位。观察者模式中的缺省模式只注重观察者的方法调用,并不注重观察者对被观察者的数据交互。事实上:只调用方法,不涉及数据侵入。
下面是实例代码:
import java.util.Vector; /** * @author Hanlin Wang */ public class ObserverMode { public static void main(String[] args) { //创建两种类型的观察者 Observer1 ob1 = new Observer1(); Observer2 ob2 = new Observer2(); //交付给Target管理 Target.addObserver(ob1); Target.addObserver(ob2); //Target发生改变,观察者发出响应。 Target.modify(); } } //添加可观察抽象类 abstract class Observable{ private static Vector<Observer> observers = new Vector<Observer>(); public static void addObserver(Observer ob){ observers.add(ob); } public static void removeObserver(Observer ob){ observers.remove(ob); } public static void notifyObservers(){ for (Observer observer : observers) { observer.update(); } } public static void modify(){ System.out.println("Observable has been modified"); notifyObservers(); } } //被观察实际类 class Target extends Observable{ } //观察者接口 interface Observer{ void update(); } //添加观察者1、2 class Observer1 implements Observer{ public void update(){ System.out.println("Observer1 has been awakened"); } } class Observer2 implements Observer{ public void update(){ System.out.println("Observer2 has been awakened"); } }
打印结果:
Observable has been modified
Observer1 has been awakened
Observer2 has been awakened
分析:创建两个Observer类对象:ob1、ob2。利用Target的静态方法addObserver将ob1、ob2在Target中注册,至此,ob1、ob2就成为了Target的观察者,监听着Target中modify方法的调用。从打印结果可以知晓,当Target中的modify方法调用时,ob1、ob2的方法也相继被调用。Target只是担任了ob1、ob2的update方法的触发器,并没有改变ob1、ob2的update方法中的内部逻辑。这就是缺省状态下的观察者模式。
②推模式
推模式较缺省模式,被观察者会对观察者推送额外的信息。附图:
代码:
import java.util.Vector; /** * @author Hanlin Wang */ public class ObserverMode { public static void main(String[] args) { //创建两种类型的观察者 Observer1 ob1 = new Observer1(); Observer2 ob2 = new Observer2(); //交付给Target管理 Target.addObserver(ob1); Target.addObserver(ob2); //Target发生改变,观察者发出响应。 Target.modify(); } } //添加可观察抽象类 abstract class Observable{ private static Vector<Observer> observers = new Vector<Observer>(); public static void addObserver(Observer ob){ observers.add(ob); } public static void removeObserver(Observer ob){ observers.remove(ob); } public static void notifyObservers(){ for (Observer observer : observers) { observer.update("Target向观察者发送的message"); } } public static void modify(){ System.out.println("Observable has been modified"); notifyObservers(); } } //被观察实际类 class Target extends Observable{ } //观察者接口 interface Observer{ void update(String message); } //添加观察者1、2 class Observer1 implements Observer{ public void update(String message){ System.out.println("Observer1 : "+message); } } class Observer2 implements Observer{ public void update(String message){ System.out.println("Observer1 : "+message); } }
分析:推模式较缺省模式,Observer1、Observer2中的update的方法需要传入一个String类型的参数,打印的结果将附加该参数的值,这样就体现了被观察者对观察者的信息交互推送,而不是缺省模式下的只调用方法。
③拉模式
推模式虽然能推送信息,但是这种推送方式具有局限性:每次只能统一推送一定的消息,无法做到具体化,个例化推送,并且观察者无法真正对被观察者的数据进行操作。拉模式就解决了推模式推送信息单一的问题。拉模式将当前被观察对象作为update的参数,即update(Observable o),这样观察者就可以直接操纵被观察者中的相关数据。拉模式相较推模式需要修改的地方:Target类中定义成员变量data,用于代表被观察者类的数据信息;update方法改为update(Observable o),即观察者接受一个参数。
附图:
代码:
import java.util.Vector; /** * @author Hanlin Wang */ public class ObserverMode { public static void main(String[] args) { //创建两种类型的观察者 Observer1 ob1 = new Observer1(); Observer2 ob2 = new Observer2(); //交付给Target管理 Target.addObserver(ob1); Target.addObserver(ob2); //Target发生改变,观察者发出响应。 Target.modify(); } } //添加可观察抽象类 abstract class Observable{ private static Vector<Observer> observers = new Vector<Observer>(); public static void addObserver(Observer ob){ observers.add(ob); } public static void removeObserver(Observer ob){ observers.remove(ob); } public static void notifyObservers(){ for (Observer observer : observers) { observer.update(new Target()); } } public static void modify(){ System.out.println("Observable has been modified"); notifyObservers(); } } //被观察实际类 class Target extends Observable{ private static String data = "zhangsan"; public static String getData() { return data; } public static void setData(String data) { Target.data = data; } } //观察者接口 interface Observer{ void update(Observable o); } //添加观察者1、2 class Observer1 implements Observer{ public void update(Observable o){ System.out.println(((Target)o).getData()); } } class Observer2 implements Observer{ public void update(Observable o){ System.out.println(((Target)o).getData()); } }
private static String data = “zhangsan”为Targe类中的静态常量,修改update方法修改为update(Observable o)。
运行结果:
Observable has been modified
zhangsan
zhangsan
4、Java对观察者模式的支持
在java.util包下存在两个工具类:Observable、Observer。Observable为普通类,Observer为接口。Observable原生支持复合模式的观察者模式,即:被观察者既要对观察者推送信息,观察者也可以访问、操作被观察者的相关属性或方法,既有“推”,又有“拉”。附图:
代码:
import java.util.Observable; import java.util.Observer; public class Q { public static void main(String[] args) { Subject subject = new Subject("iPhone 7"); Ob1 ob1 = new Ob1(); Ob2 ob2 = new Ob2(); subject.addObserver(ob1); subject.addObserver(ob2); subject.change(); //观察类对象个数 System.out.println(subject.countObservers()); } } class Subject extends Observable{ private String data; public Subject(String data) { super(); this.data = data; } public String getData() { return data; } public void setData(String data) { this.data = data; } public void change(){ setChanged(); notifyObservers("hahaha"); } } class Ob1 implements Observer{ public void update(Observable o, Object arg) { System.out.println("the message is : "+ arg +", the Subject's data is : "+((Subject)o).getData()); } } class Ob2 implements Observer{ public void update(Observable o, Object arg) { System.out.println("the message is : "+ arg +", the Subject's data is : "+((Subject)o).getData()); } }
运行结果:
the message is : hahaha, the Subject’s data is : iPhone 7
the message is : hahaha, the Subject’s data is : iPhone 7
2
如果大家对Observable类不太了解,下面附带JDK源码:
/* * Copyright (c) 1994, 2004, 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 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 * @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(); } }
代码量的确很多,大家只需要关注setChanged和notifyObservers这个方法,在使用时需要定义一个Subject类继承Observable类,你可以自己定义一个方法(如我自己定义的change()方法),该方法中执行逻辑必须要有setChanged()方法调用和notifyObservers()方法调用,且setChanged必须在notifyObservers方法之前调用。具体实现的逻辑可以参考notifyObservers(null)和notifyObservers(Object arg)。
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