实现原理：

长连接的维持，是要客户端程序，定时向服务端程序，发送一个维持连接包的。
如果，长时间未发送维持连接包，服务端程序将断开连接。

客户端：

Client通过持有Socket的对象，可以随时（使用sendObject方法）发送Massage Object(消息)给服务端。
如果keepAliveDelay毫秒（程序中是2秒）内未发送任何数据，则自动发送一个KeepAlive Object(心跳)给服务端，用于维持连接。
由于，我们向服务端，可以发送很多不同的消息对象，服务端也可以返回不同的对象。所以，对于返回对象的处理，要编写具体的ObjectAction实现类进行处理。通过Client.addActionMap方法进行添加。这样，程序会回调处理。

服务端：

由于客户端会定时（keepAliveDelay毫秒）发送维持连接的信息过来，所以，服务端要有一个检测机制。
即当服务端receiveTimeDelay毫秒（程序中是3秒）内未接收任何数据，则自动断开与客户端的连接。
ActionMapping的原理与客户端相似（相同）。
通过添加相应的ObjectAction实现类，可以实现不同对象的响应、应答过程。

心跳反映的代码：

package com.java.excel.keepalive;import java.io.Serializable;import java.text.SimpleDateFormat;import java.util.Date;/** * * 维持连接的消息对象（心跳对象） */public class KeepAlive implements Serializable { private static final long serialVersionUID = -2813120366138988480L; /* 覆盖该方法，仅用于测试使用。 * @see java.lang.Object#toString() */ @Override public String toString() { return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date())+"\t维持连接包"; }}

服务端

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 { /** * 要处理客户端发来的对象，并返回一个对象，可实现该接口。 */ 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("处理并返回："+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("关闭："+s.getRemoteSocketAddress()); } } }

客户端

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 { /** * 处理服务端发回的对象，可实现该接口。 */ 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("处理：\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; //连接状态 private long lastSendTime; //最后一次发送数据的时间 //用于保存接收消息对象类型及该类型消息处理的对象 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(); //保持长连接的线程，每隔2秒项服务器发一个一个保持连接的心跳消息 new Thread(new ReceiveWatchDog()).start(); //接受消息的线程，处理消息 } public void stop(){ if(running)running=false; } /** * 添加接收对象的处理对象。 * @param cls 待处理的对象，其所属的类。 * @param action 处理过程对象。 */ 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("发送：\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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