實現原理：

長連接的維持，是要客戶端程序，定時向服務端程序，發送一個維持連接包的。如果，長時間未發送維持連接包，服務端程序將斷開連接。

客戶端：

Client通過持有Socket的對象，可以隨時（使用sendObject方法）發送Massage Object(消息)給服務端。如果keepAliveDelay毫秒（程序中是2秒）內未發送任何數據，則自動發送一個KeepAlive Object(心跳)給服務端，用于維持連接。由于，我們向服務端，可以發送很多不同的消息對象，服務端也可以返回不同的對象。所以，對于返回對象的處理，要編寫具體的ObjectAction實現類進行處理。通過Client.addActionMap方法進行添加。這樣，程序會回調處理。

服務端：

由于客戶端會定時（keepAliveDelay毫秒）發送維持連接的信息過來，所以，服務端要有一個檢測機制。即當服務端receiveTimeDelay毫秒（程序中是3秒）內未接收任何數據，則自動斷開與客戶端的連接。ActionMapping的原理與客戶端相似（相同）。通過添加相應的ObjectAction實現類，可以實現不同對象的響應、應答過程。

心跳反映的代碼：

package com.java.excel.keepalive;import java.io.Serializable;import java.text.SimpleDateFormat;import java.util.Date;/** * * 維持連接的消息對象（心跳對象） */public class KeepAlive implements Serializable { private static final long serialVersionUID = -2813120366138988480L; /* 覆蓋該方法，僅用于測試使用。 * @see java.lang.Object#toString() */ @Override public String toString() { return new SimpleDateFormat('yyyy-MM-dd HH:mm:ss').format(new Date())+'t維持連接包'; }}

服務端

package com.java.excel.keepalive;import java.io.IOException;import java.io.InputStream;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.net.ServerSocket;import java.net.Socket;import java.util.concurrent.ConcurrentHashMap;public class Server { /** * 要處理客戶端發來的對象，并返回一個對象，可實現該接口。 */ public interface ObjectAction{ Object doAction(Object rev, Server server); } public static final class DefaultObjectAction implements ObjectAction{ public Object doAction(Object rev,Server server) { System.out.println('處理并返回：'+rev); return rev; } } public static void main(String[] args) { int port = 65432; Server server = new Server(port); server.start(); } private int port; private volatile boolean running=false; private long receiveTimeDelay=3000; private ConcurrentHashMap<Class, ObjectAction> actionMapping = new ConcurrentHashMap<Class,ObjectAction>(); private Thread connWatchDog; public Server(int port) { this.port = port; } public void start(){ if(running)return; running=true; connWatchDog = new Thread(new ConnWatchDog()); connWatchDog.start(); } @SuppressWarnings('deprecation') public void stop(){ if(running)running=false; if(connWatchDog!=null)connWatchDog.stop(); } public void addActionMap(Class<Object> cls,ObjectAction action){ actionMapping.put(cls, action); } class ConnWatchDog implements Runnable{ public void run(){ try {ServerSocket ss = new ServerSocket(port,5);while(running){ Socket s = ss.accept(); new Thread(new SocketAction(s)).start();} } catch (IOException e) {e.printStackTrace();Server.this.stop(); } } } class SocketAction implements Runnable{ Socket s; boolean run=true; long lastReceiveTime = System.currentTimeMillis(); public SocketAction(Socket s) { this.s = s; } public void run() { while(running && run){if(System.currentTimeMillis()-lastReceiveTime>receiveTimeDelay){ overThis();}else{ try { InputStream in = s.getInputStream(); if(in.available()>0){ ObjectInputStream ois = new ObjectInputStream(in); Object obj = ois.readObject(); lastReceiveTime = System.currentTimeMillis(); System.out.println('接收：t'+obj); ObjectAction oa = actionMapping.get(obj.getClass()); oa = oa==null?new DefaultObjectAction():oa; Object out = oa.doAction(obj,Server.this); if(out!=null){ObjectOutputStream oos = new ObjectOutputStream(s.getOutputStream());oos.writeObject(out);oos.flush(); } }else{ Thread.sleep(10); } } catch (Exception e) { e.printStackTrace(); overThis(); } } } }private void overThis() { if(run)run=false; if(s!=null){try { s.close();} catch (IOException e) { e.printStackTrace();} } System.out.println('關閉：'+s.getRemoteSocketAddress()); } } }

客戶端

package com.java.excel.keepalive;import java.io.IOException;import java.io.InputStream;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.net.Socket;import java.net.UnknownHostException;import java.util.concurrent.ConcurrentHashMap;public class Client { /** * 處理服務端發回的對象，可實現該接口。 */ public static interface ObjectAction{ void doAction(Object obj,Client client); } public static final class DefaultObjectAction implements ObjectAction{ public void doAction(Object obj,Client client) { System.out.println('處理：t'+obj.toString()); } } public static void main(String[] args) throws UnknownHostException, IOException { String serverIp = '127.0.0.1'; int port = 65432; Client client = new Client(serverIp,port); client.start(); } private String serverIp; private int port; private Socket socket; private boolean running=false; //連接狀態 private long lastSendTime; //最后一次發送數據的時間 //用于保存接收消息對象類型及該類型消息處理的對象 private ConcurrentHashMap<Class, ObjectAction> actionMapping = new ConcurrentHashMap<Class,ObjectAction>(); public Client(String serverIp, int port) { this.serverIp=serverIp; this.port=port; } public void start() throws UnknownHostException, IOException { if(running)return; socket = new Socket(serverIp,port); System.out.println('本地端口：'+socket.getLocalPort()); lastSendTime=System.currentTimeMillis(); running=true; new Thread(new KeepAliveWatchDog()).start(); //保持長連接的線程，每隔2秒項服務器發一個一個保持連接的心跳消息 new Thread(new ReceiveWatchDog()).start(); //接受消息的線程，處理消息 } public void stop(){ if(running)running=false; } /** * 添加接收對象的處理對象。 * @param cls 待處理的對象，其所屬的類。 * @param action 處理過程對象。 */ public void addActionMap(Class<Object> cls,ObjectAction action){ actionMapping.put(cls, action); } public void sendObject(Object obj) throws IOException { ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream()); oos.writeObject(obj); System.out.println('發送：t'+obj); oos.flush(); } class KeepAliveWatchDog implements Runnable{ long checkDelay = 10; long keepAliveDelay = 1000; public void run() { while(running){if(System.currentTimeMillis()-lastSendTime>keepAliveDelay){ try { Client.this.sendObject(new KeepAlive()); } catch (IOException e) { e.printStackTrace(); Client.this.stop(); } lastSendTime = System.currentTimeMillis();}else{ try { Thread.sleep(checkDelay); } catch (InterruptedException e) { e.printStackTrace(); Client.this.stop(); }} } } } class ReceiveWatchDog implements Runnable{ public void run() { while(running){try { InputStream in = socket.getInputStream(); if(in.available()>0){ ObjectInputStream ois = new ObjectInputStream(in); Object obj = ois.readObject(); System.out.println('接收：t'+obj); ObjectAction oa = actionMapping.get(obj.getClass()); oa = oa==null?new DefaultObjectAction():oa; oa.doAction(obj, Client.this); }else{ Thread.sleep(10); }} catch (Exception e) { e.printStackTrace(); Client.this.stop();} } } } }

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