UDP传输的实现与检测重发机制

 java建立UDP连接和建立TCP/IP连接一样简单，只用到了SocketAddress（绑定主机的IP地址和应用程序端口，包括自己的和发送地）、DatagramSocket（数据套接字，接收和发送数据包都靠它，同时在new该对象时需要try…catch，而SocketAddress则无需）、DatagramPacket（数据包对象，UDP数据都是一个包一个包发送的）这三个类。然后实现起来是非常简单的。首先是发送方new两个SocketAddress对象，一个参数写自己的机子，另一个写要发送的目标主机。然后把自己的SocketAddress对象传入DatagramSocket创建其对象，假设对象名为ds。再将要发送的数据连同目标主机的SocketAddress对象一起传入DatagramPacket创建其对象，设为dp。最后调用方法ds.send(dp);就OK了。这是发送方的，接收方差不多，实现起来没多大难度。不过众所周知，UDP有一个特点却是比不上TCP/IP的，那就是建立的连接不一定非常可靠，经常会出现丢包的情况，所以需要在发送过程中添加检测机制，而这也就大大加大了实现的难度。基本检测机制是：

      发送方发送一个数据包，接收方接收后，发回一个应答包告诉发送方已收到消息，此应答包包含发送的消息的唯一序列号。关于这个序列号的唯一性设置我还不是很明白，所以在测试中就把第n条消息的n作为序列号了。若发送方在一定时间内（时间长短可自行设置）未收到应答包则重发消息。重发消息有两个原因：①接收方未收到，此时重发是应该的。②接收方收到消息但是发回的应答包丢失了，此时重发消息则重复了，所以在接收方还得添加一个机制：若收到的消息与以前发过的消息重复，则再次发送应答包。在这些机制下，能够初步的保证UDP传输的完整性。以下是实现代码：

 

发送端：

 

 import java.io.IOException;
 import java.net.DatagramPacket;
 import java.net.DatagramSocket;
 import java.net.InetSocketAddress;
 import java.net.SocketAddress;
 import java.util.ArrayList;
 import java.util.List;

 public class UDPServer {

     public UDPServer() {
         try {
             sa = new InetSocketAddress("127.0.0.1", 9999);
             ds = new DatagramSocket(sa);
             saa = new InetSocketAddress("127.0.0.1", 1234);
             String msg = "";
             startThread();
             new Thread() {
                 public void run() {
                     while (true) {
                         recMsg();
                     }
                 }
             }.start();
             for (int i = 1;; i++) {
                 msg = "第" + i + "条消息";
                 Msg m = new Msg(msg, i);
                 sendMsg(m);
                 list.add(m);
                 Thread.sleep(1000);
             }
         } catch (Exception e) {
         }
     }

     public void sendMsg(Msg m) {
         DatagramPacket dp;
         try {
             dp = new DatagramPacket(m.getmsg().getBytes(), m.getmsg()
                     .getBytes().length, saa);
             ds.send(dp);
             m.setsendtime(System.currentTimeMillis());
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

     public void recMsg() {
         byte[] buffer = new byte[20];
         DatagramPacket dpp = new DatagramPacket(buffer, buffer.length);
         try {
             ds.receive(dpp);
         } catch (IOException e) {
             e.printStackTrace();
         }
         String rec = new String(dpp.getData()).trim();
         int id = Integer.parseInt(rec.substring(4, rec.length() - 3));
         for (int i = 0; i < list.size(); i++) {
             if (id == list.get(i).getid()) {
                 System.out.println("确认对方已收到第" + list.get(i).getid()
                         + "条消息，从队列中除去");
                 list.remove(i);
             }
         }
     }

     public void startThread() {
         new Thread() {
             public void run() {
                 while (true) {
                     // 如果list里面有元素
                     if (list.size() > 1) {
                         for (int i = 0; i < list.size(); i++) {
                             if (System.currentTimeMillis()
                                     - list.get(i).getsendtime() > 3000
                                     && list.get(i).gettime() < 4) {
                                 sendMsg(list.get(i));
                                 System.out.println("重发第" + list.get(i).getid()
                                         + "条消息第" + list.get(i).gettime() + "次");
                                 if (list.get(i).gettime() == 3) {
                                     System.out.println("重发第"
                                             + list.get(i).getid()
                                             + "条消息已超过4次，丢弃包");// System.exit(0);
                                     list.remove(i);
                                 } else {
                                     list.get(i).settime(
                                             list.get(i).gettime() + 1);
                                 }
                             }
                         }
                     }
                     try {
                         Thread.sleep(1000);
                     } catch (InterruptedException e) {
                         e.printStackTrace();
                     }
                 }
             }
         }.start();
     }

     public static void main(String[] args) {
         new UDPServer();
     }

     class Msg {
         public Msg(String msg, int id) {
             this.msg = msg;
             this.id = id;
         }

         public void setid(int i) {
             id = i;
         }

         public int getid() {
             return id;
         }

         public void setmsg(String newmsg) {
             msg = newmsg;
         }

         public String getmsg() {
             return msg;
         }

         public void settime(int i) {
             time = i;
         }

         public int gettime() {
             return time;
         }

         public void setsendtime(long time) {
             lastsendtime = time;
         }

         public long getsendtime() {
             return lastsendtime;
         }

         private int id;
         private String msg;
         private int time;
         private long lastsendtime;
     }

     List list = new ArrayList();
     SocketAddress sa;
     DatagramSocket ds;
     SocketAddress saa;

 }

 

 

接收端：

 import java.awt.Dimension;
 import java.awt.FlowLayout;
 import java.awt.Font;
 import java.io.IOException;
 import java.net.DatagramPacket;
 import java.net.DatagramSocket;
 import java.net.InetSocketAddress;
 import java.net.SocketAddress;
 import java.util.ArrayList;
 import java.util.List;

 import javax.swing.JFrame;
 import javax.swing.JScrollPane;
 import javax.swing.JTextArea;
 import javax.swing.UIManager;

 public class UDPClient {

     JFrame f;
     JTextArea jta, jta2;
     SocketAddress sa;
     SocketAddress saa;
     DatagramSocket ds;
     List list = new ArrayList();

     public UDPClient() {
         try {
             UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
         } catch (Exception e) {
         }
         Font font = new Font("Dialog", Font.PLAIN, 12);
         f = new JFrame();
         jta = new JTextArea();
         jta.setFont(font);
         jta2 = new JTextArea();
         jta2.setFont(font);
         JScrollPane jsp = new JScrollPane(jta);
         JScrollPane jsp2 = new JScrollPane(jta2);
         jsp.setPreferredSize(new Dimension(500, 400));
         jsp2.setPreferredSize(new Dimension(300, 400));
         f.setLayout(new FlowLayout());
         f.add(jsp);
         f.add(jsp2);
         f.pack();
         f.setLocationRelativeTo(null);
         f.setDefaultCloseOperation(3);
         f.setVisible(true);
         sa = new InetSocketAddress("127.0.0.1", 1234);
         saa = new InetSocketAddress("127.0.0.1", 9999);
         new Thread() {
             public void run() {
                 recMsg();
             }
         }.start();
         startThread();
     }

     public void recMsg() {
         try {
             ds = new DatagramSocket(sa);
             while (true) {
                 byte[] buffer = new byte[20];
                 DatagramPacket dpp = new DatagramPacket(buffer, buffer.length);
                 jta.append("等待数据...\n");
                 ds.receive(dpp);
                 String rec = new String(dpp.getData()).trim();
                 if (check(rec)) {
                     jta.append(rec + "\n");
                     int id = Integer
                             .parseInt(rec.substring(1, rec.length() - 3));
                     Msg m = new Msg(rec, id);
                     list.add(m);
                     send(m);
                 }
             }
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

     public boolean check(String msg) {
         if (list.size() > 0) {
             for (int i = 0; i < list.size(); i++) {
                 if (list.get(i).getmsg().equals("msg")) {
                     send(list.get(i));
                     return false;
                 }
             }
         }
         return true;
     }

     public void send(Msg m) {
         String msg = "已收到第" + m.getid() + "条消息";
         try {
             DatagramPacket dp = new DatagramPacket(msg.getBytes(),
                     msg.getBytes().length, saa);
             ds.send(dp);
             jta2.append(msg + "\n");
             m.setrectime(System.currentTimeMillis());
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

     public void startThread() {
         new Thread() {
             public void run() {
                 while (true) {
                     if (list.size() > 0) {
                         for (int i = 0; i < list.size(); i++) {
                             if (System.currentTimeMillis()
                                     - list.get(i).getrectime() > 5000) {
                                 list.remove(i);
                                 jta2.append("确认已收到第" + list.get(i).getid()
                                         + "条消息，从队列中除去\n");
                             }
                         }
                     }
                     try {
                         Thread.sleep(1000);
                     } catch (InterruptedException e) {
                         e.printStackTrace();
                     }
                 }
             }
         }.start();
     }

     public static void main(String[] args) {
         new UDPClient();
     }

     class Msg {
         public Msg(String msg, int id) {
             this.msg = msg;
             this.id = id;
         }

         public void setid(int i) {
             id = i;
         }

         public int getid() {
             return id;
         }

         public void setmsg(String newmsg) {
             msg = newmsg;
         }

         public String getmsg() {
             return msg;
         }

         public void settime(int i) {
             time = i;
         }

         public int gettime() {
             return time;
         }

         public void setrectime(long time) {
             lastrectime = time;
         }

         public long getrectime() {
             return lastrectime;
         }

         private int id;
         private String msg;
         private int time;
         private long lastrectime;
     }

 }