一、天氣預報項目需求分析
天氣預報的需求，具體要求如下:
1. 氣象站可以將每天測到的溫度、溼度、氣壓、PM2.5等以公告的形式發佈到自己的網站或者第三方。
2. 有對外的接口可以被其他系統所調用，比如新浪等能夠接入該接口來獲取數據。
3. 提供關鍵數據的接口，溫度、溼度、第三方等信息。
4.測量數據更新時，要能實時地通知給第三方。
二、方案實現
1.普通方案
currentConditions類主要負責顯示當前的天氣信息，接收氣象站發過來新的溫度、溼度等信息並更新當前信息。
package com.exam.observer;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午6:50:20
*當前情況類
*/
public class CurrentConditions {
private String temperature;
private String humidity;
private String pressure;
/**
*
* 顯示天氣信息
*/
private void display() {
System.out.println("當前溫度爲:"+this.temperature);
System.out.println("當前溼度爲:"+this.humidity);
System.out.println("當前大氣壓爲:"+this.pressure);
}
/**
*更新天氣信息,更新完後將天氣信息顯示出來
*/
public void update(String temperature,String humidity,String pressure) {
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
display();
}
}
WeatherData類，主要提供獲取信息的接口，然後將信息的更新傳給currentConditions:
package com.exam.observer;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午6:47:35
*天氣信息類
*/
public class WeatherData {
private String temperature;
private String humidity;
private String pressure;
private CurrentConditions current;
public WeatherData(CurrentConditions current){
this.current=current;
}
public String getTemperature() {
return temperature;
}
public String getHumidity() {
return humidity;
}
public String getPressure() {
return pressure;
}
public void setData(String temperature,String humidity,String pressure) {
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
//設置信息時也要調用dataChange()方法。
dataChange();
}
public void dataChange() {
current.update(getTemperature(), getHumidity(), getPressure());
}
}
測試類:
package com.exam.observer;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午6:53:29
*測試類
*/
public class Test {
public static void main(String[] args) {
CurrentConditions current=new CurrentConditions();
WeatherData data=new WeatherData(current);
data.setData("1", "2", "3");
data.setData("4", "5", "6");
}
}
打印結果爲:
當前溫度爲:1
當前溼度爲:2
當前大氣壓爲:3
當前溫度爲:4
當前溼度爲:5
當前大氣壓爲:6
此方案實現了基本的功能，氣象站(WeatherData)可以發佈信息(setData())，然後能夠實時的更新信息dataChange()。
但此方案存在一定的問題，分析:
1）其他三方無法進入氣象站獲取到數據的問題。
2) 無法在運行時動態的添加第三方(新浪)。
3) 違反OCP原則(軟件應該對擴展開放，對修改關閉)--> 觀察者模式。
2.改進方案---觀察者模式
在普通方案的基礎上進行改進，新增subject接口和observer接口，subject接口用來管理多個奶站或者氣象站的信息，將信息發佈出去。 observer接口則可以用來給多個第三方接入來獲取奶站或者氣象站的數據。
UML圖如下:
奶站或者氣象站接口(Subject):
package com.exam.observer.improve;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午7:58:18
*/
public interface Subject {
/**
* 註冊observer
*/
public void registerObserver(Observer o);
/**
* 移除observer
*/
public void removeObserver(Observer o);
/**
* 通知所有的observer
*/
public void notifyAllObserver();
}
天氣氣象站(WeatherData): 管理氣象信息和所有的觀察者。
package com.exam.observer.improve;
import java.util.ArrayList;
import java.util.List;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午7:51:26
*核心信息，管理觀察者
*
*/
public class WeatherData implements Subject{
private String temperature;
private String humidity;
private String pressure;
private List<Observer> observers;
public WeatherData() {
observers=new ArrayList<Observer>();
}
public void registerObserver(Observer o) {
observers.add(o);
}
public void removeObserver(Observer o) {
observers.remove(o);
}
public String getTemperature() {
return temperature;
}
public String getHumidity() {
return humidity;
}
public String getPressure() {
return pressure;
}
/**
* 發佈信息
*/
public void setData(String temperature,String humidity,String pressure) {
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
update();
}
/**
* 通知觀察者修改信息
*/
public void update() {
notifyAllObserver();
}
/**
* 通知所有的觀察者
*/
public void notifyAllObserver() {
for(int i=0;i<observers.size();i++) {
observers.get(i).update(getTemperature(), getHumidity(), getPressure());
}
}
}
觀察者接口(Observer):
package com.exam.observer.improve;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午7:51:53
*觀察者接口，負責接收奶站或者氣象站的數據並更新當前擁有的數據。
*/
public interface Observer {
void update(String temperature,String humidity,String pressure);
}
不同的觀察者:
package com.exam.observer.improve;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午7:54:33
*/
public class CurrentConditions implements Observer{
private String temperature;
private String humidity;
private String pressure;
public void display() {
System.out.println("當前溫度爲:"+this.temperature);
System.out.println("當前溼度爲:"+this.humidity);
System.out.println("當前大氣壓爲:"+this.pressure);
}
public void update(String temperature, String humidity, String pressure) {
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
display();
}
}
百度:
package com.exam.observer.improve;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午8:13:24
*/
public class Baidu implements Observer{
private String temperature;
private String humidity;
private String pressure;
public void display() {
System.out.println("百度當前溫度爲:"+this.temperature);
System.out.println("百度當前溼度爲:"+this.humidity);
System.out.println("百度當前大氣壓爲:"+this.pressure);
}
public void update(String temperature, String humidity, String pressure) {
this.temperature=temperature;
this.humidity=humidity;
this.pressure=pressure;
display();
}
}
測試類:
package com.exam.observer.improve;
/**
*author:bingbing
*日期:2020年5月4日
*時間:下午8:11:12
*/
public class Test {
public static void main(String[] args) {
WeatherData data=new WeatherData();
data.registerObserver(new CurrentConditions());
data.setData("1", "2", "3");
data.registerObserver(new Baidu());
data.setData("3", "4", "5");
}
}
打印結果:
當前溫度爲:1
當前溼度爲:2
當前大氣壓爲:3
當前溫度爲:3
當前溼度爲:4
當前大氣壓爲:5
百度當前溫度爲:3
百度當前溼度爲:4
百度當前大氣壓爲:5
三、觀測者模式在JDK源碼中的應用
Jdk的Observable類就使用了觀察者模式
Observable類的源碼如下:
/*
* 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();
}
}
Observer接口:
/*
* 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);
}