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)。