观察者模式的定义：
在对象之间定义了一对多的依赖，这样一来，当一个对象改变状态，依赖它的对象会收到通知并自动更新。
大白话：
其实就是发布订阅模式，发布者发布信息，订阅者获取信息，订阅了就能收到信息，没订阅就收不到信息。
举个例子，如订阅了天气，那么天气突发，就会发通知到你的手机上面。记得是天气发现了变化，才发通知的。
这里我直接用java提供的观察者模式实现
1.建立目标对象
package dyl.study.observer;
import java.util.Observable;
public class ConcreteWeatherSubject extends Observable {
//天气情况的内容
private String content;
public String getContent() {
return content;
}
public void setContent(String content) {
this.content = content;
this.setChanged();
this.notifyObservers(content); //推方式
}
}可以看到setContent，只要改变目标状态，就会推送信息给观察者。
Observable在这个类中，其实也就是存观察者对象。类里面就是注册观察者，删除观察者，保存观察者的数组。、
可以看下源码
/*
* Copyright (c) 1994, 2012, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
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*
*
*
*/
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.创建观察者对象
package dyl.study.observer;
import java.util.Observable;
import java.util.Observer;
//具体的观察者对象
public class ConcreteObserver implements Observer {
private String observerName;
@Override
public void update(Observable o, Object arg) {
//推方式
System.out.println(observerName+"收到了"+arg);
//拉方式
System.out.println(observerName+"收到了消息，主动到目标对象那去取得数据，取得的数据为："+((ConcreteWeatherSubject)o).getContent());
}
public String getObserverName() {
return observerName;
}
public void setObserverName(String observerName) {
this.observerName = observerName;
}
} this.notifyObservers(content);目标对象的这个方法会调用观察者的update方法
另外还涉及到
退方式和拉方式
这里我解释一下
退就目标者把信息退给你，这里是要需求分析的，因为推给你的你不一定想要。
而拉就是观察者自己去获取自己想要的信息。所以传递的是目标的对象过去
可以看下的Observer源码
/*
* 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);
}
3.客户端实现
package dyl.study.observer;
public class Client {
public static void main(String[] args) {
ConcreteWeatherSubject subject = new ConcreteWeatherSubject();
ConcreteObserver p1 = new ConcreteObserver();
ConcreteObserver p2 = new ConcreteObserver();
p1.setObserverName("我");
p2.setObserverName("你");
subject.addObserver(p1);
subject.addObserver(p2);
subject.setContent("QQ天气：【某地】阴7°/10°，09:05更新~");
}
}客户端实现的过程是先创建目标，再创建观察者，设置观察者，最后更改目标的状态。
最后说下
区别对待观察者
有个需求是目标对象改变不用每次都去通知观察者，观察者会很烦
那就要在目标对象中的
public void setContent(String content) {
this.content = content;
this.setChanged();
this.notifyObservers(content); //推方式
}加判断语句，如果是简单的话。