观测者模式已在博客""中介绍，下面说下如何将观察者模式应用在实际工作中。
问题描述
某业务系统会定期接收到传回来的数据，数据放在一个目录下。由于业务的需要，当有新的数据产生时，需要将数据上传到多台机器上。你如何设计这个业务逻辑呢？
功能设计
放在目录下的数据时不断更新的，我们需要一个守护线程来监控目录下数据的变化，当有新数据时就通知观测者observers。这里的观测者是需要将数据上传到FTP服务器的对象，当有新数据产生时，就上传数据到FTP服务器。
这里很适合用观测者模式来解决，其中subject的功能是监控目录变化，和通知观测者变化的数据。观测者的功能是上传新的数据到FTP服务器，这里有多个观测者，而且虽这业务的发展，观察者的数目是变化的。
采用观测者模式，可以在不修改代码的情况下，很容易的添加观测者。
详细设计
监控目录变化的subject：
import java.io.File;import java.util.HashMap;import java.util.HashSet;import java.util.Iterator;import java.util.Map;import java.util.Observable;import zyang.designPattern.observerPattern.Observer;import zyang.designPattern.observerPattern.Subject;/** * Fuction: * 一个守护线程，用户监控目录下是否有新数据 * 如果有新数据，通知监听对象observers * @author zhonghua * @version 2013-3-20 下午9:25:56 * @since 1.0 */public class DirectoryMonitorSubject extends Observable implements Runnable { // ------------------------------------------------- // properties // ------------------------------------------------- /** * Whether or not this thread is active. */ private boolean active = false; /** * The interval in seconds to run this thread */ private int interval = -1; /** * The name of this thread */ private String threadName; /** * This instance's thread */ private Thread runner; /** * 监控目录 */ private String directoryFullPath; /** * The map of last recorded files and their timestamps (String fileName => Long lastMod) */ private Map prevDatas=new HashMap (); /** * The map of current files and their timestamps (String fileName => Long lastMod) */ private Map currentDatas=new HashMap (); /** * The map of new files and their timestamps (String fileName => Long lastMod) */ private Map newDatas=new HashMap (); // ------------------------------------------------- // constructor // ------------------------------------------------- /** * Construct a new interval thread that will run on the given interval * with the given name. * @param threadName the name of the thread * @param directoryFullPath * @param interval the number of seconds to run the thread on */ public DirectoryMonitorSubject(String threadName,String directoryFullPath, int interval) { this.threadName=threadName; this.directoryFullPath=directoryFullPath; this.interval=interval; System.out.println("staring moditing direcotry "+directoryFullPath); } // ------------------------------------------------- // public method // ------------------------------------------------- /** * Start the thread on the specified interval. */ public void start() { active = true; //If we don't have a thread yet, make one and start it. if (runner == null && interval > 0) { runner = new Thread(this); runner.start(); } }//end start() /** * Stop the interval thread. */ public void stop() { active = false; } //end stop() public void run() { //Make this a relatively low level thread Thread.currentThread().setPriority(Thread.MIN_PRIORITY); //Pause this thread for the amount of the interval while(active){ try { setNewDatas(); Thread.sleep(interval); //监控时间间隔 } catch (InterruptedException e) { e.printStackTrace(); } }//end while }//end run public void direcotryChanged(){ setChanged(); notifyObservers(newDatas); //将新增加的数据传给observers } //end temperatureChanged() //监控目录下是否有新数据，如果有新数据就传给observers public void setNewDatas(){ if(checkNewDatas()){ //目录下有新数据 System.out.println("subject notice:have "+newDatas.size()+" data in "+directoryFullPath); direcotryChanged(); }//end if } //end setNewDatas() // ------------------------------------------------- // private method // ------------------------------------------------- /** * 检查目录下是否有新的数据（线程会反复调用该方法），并将新数据放入newDatas * @return 如果有新的数据返回ture，否则返回false */ private boolean checkNewDatas(){ boolean isHaveNewData=false; //清空先前的数据 prevDatas.clear(); newDatas.clear(); //将上次的数据先保存在 prevDatas prevDatas.putAll(currentDatas); currentDatas.clear(); //清空数据 //添加当前目录下的数据到currentDatas File direcotryFile=new File(directoryFullPath); File[] filesList=direcotryFile.listFiles(); for(File file:filesList){ currentDatas.put(file.getAbsolutePath(), new Long(file.lastModified())); }//end for //将当前目录下数据与先前目录下数据进行比较 Iterator currentIt=currentDatas.keySet().iterator(); while(currentIt.hasNext()){ String fileName=(String)currentIt.next(); Long lastModified = (Long) currentDatas.get(fileName); if(!prevDatas.containsKey(fileName)){ newDatas.put(fileName, lastModified); }//end if else if(prevDatas.containsKey(fileName)){ Long prevModified = (Long) prevDatas.get(fileName); if (prevModified.compareTo(lastModified) != 0){ newDatas.put(fileName, lastModified); }//end if }//end if }//end while if(newDatas.size()>0) isHaveNewData=true; return isHaveNewData; }//end checkNewDatas() }//end DirectoryWatcher 观测者类：上传数据到服务器 import java.util.HashMap;import java.util.Iterator;import java.util.Map;import java.util.Observable;import java.util.Observer;/** * Fuction: * * @author zhonghua * @version 2013-3-20 下午9:38:54 * @since 1.0 */public class DataObserver implements Observer { private Observable observable; private Map newDatas=new HashMap (); //新增加的数据 // ------------------------------------------------- public DataObserver(Observable observable){ this.observable=observable; observable.addObserver(this); } // ------------------------------------------------- //当得到subject的通知，做something，上传数据到FTP public void update(Observable obs, Object dataFromSubject) { if(obs instanceof DirectoryMonitorSubject){ DirectoryMonitorSubject dw=(DirectoryMonitorSubject)obs; newDatas.clear(); //先清空数据 newDatas.putAll((Map)dataFromSubject); uploadData(); }//end if }//end update() /** * 上传新数据到FTP */ private void uploadData(){ System.out.println("starting upload new data to ftp"); //这里就不写真正上传的代码了，直接输出 System.out.println("have upload "+newDatas.size()+" number data to ftp,they are:"); Iterator newDatasIt=newDatas.keySet().iterator(); while(newDatasIt.hasNext()){ String fileName=(String)newDatasIt.next(); Long lastModified=(Long)newDatas.get(fileName); System.out.println("fileName="+fileName+",lastModified="+lastModified.toString()); }//end while }//end loadNewData()} //end class FileListener 现在看下如何使用 import zyang.DirectoryMonitor.DataObserver;import zyang.DirectoryMonitor.DirectoryMonitorSubject;/** * Fuction: * How to use subject and observers * @author zhonghua * @since 1.0 */public class WatcheNewDataApp { public static void main(String[] args) { //subject DirectoryMonitorSubject wp=new DirectoryMonitorSubject("moniteDirectory","E:\\temp", 2000); //observers DataObserver fl=new DataObserver(wp); wp.start(); //开启监控守护线程 }//end main()}//end class WatcheNewDataApp 运行main函数，结果如下 
观测者模式模版
下面写了一个通用的观测者模式模版代码，用户只需要在对应地方加入自己的业务逻辑即可
sunbject类：只需要修改logicMethod方法中的业务逻辑即可。
public class YourSubject extends Observable { // ------------------------------------------------- // constructor // ------------------------------------------------- public YourSubject() { } /** * Must have this method's content, this method is called by the logicMethod. * of course,you can change this method's name */ public void informationChanged(){ setChanged(); notifyObservers(); //该方法的参数用于subject和observers传递数据，向observers传递数据，observers在其update方法中使用传过来的数据// notifyObservers(dataSendToObservers) } //end informationChanged() /** * the subject only need do one thing: write your logic in this method */ public void logicMethod(){ //write your logic code here //TODO System.out.println("subject notice:monitor information has changed, observers can do their things now."); //for example //end TODO informationChanged(); } //end logicMethod() } //end class YourSubject observer类： 只需要在 update方法中写入你的业务逻辑即可 public class OneObserver implements Observer { private Observable observable; // ------------------------------------------------- // constructor // ------------------------------------------------- public OneObserver(Observable observable){ this.observable=observable; observable.addObserver(this); } /** * the observer only need do one thing: write your logic in this method */ public void update(Observable obs, Object dataFromSubject) { if(obs instanceof YourSubject){ YourSubject yourSubject=(YourSubject)obs; //write your logic code here //TODO System.out.println("observer:have upload to ftp"); //for example }//end if }//end update}//end class OneObserver 如何使用呢？见代码 /** * Fuction: * the example shows how to use subject object and observers objects * @author zhonghua * @version 2013-8-26 下午9:25:56 * @since 1.0 */public class UseApp { public static void main(String[] args) { //subject YourSubject wp=new YourSubject(); //observers OneObserver clientA=new OneObserver(wp); //notify observers,and observers do their things after have the notice wp.logicMethod(); }//end main()} 上面的例子和模版已共享，。 推送功能
现在很多手机软件的推送功能，比如百度新闻，微信公众平台，其实很适合用观测者模式。发消息的服务端即时subject，接收消息的观测者observers即手机软件使用者。服务端监控消息，当有消息时通知多个观测者，并发送消息给观测者。
