Java套接字编程（客户端，桥，服务器）

Question

The task is to

• (1) Send message from Client to Server via Bridge

• (2) Send back the message in UPPER CASE from server to client via Bridge

(1) is done I am have problem with sending the message back to the client

Here are the classes:

UDPCLIENT

import java.io.*;
import java.net.*;

class UDPClient
{
   public static void main(String args[]) throws Exception
   {

       //getting input from the user and sending to Bridge
      BufferedReader inFromUser =
         new BufferedReader(new InputStreamReader(System.in));
      DatagramSocket clientSocket = new DatagramSocket();
      InetAddress IPAddress = InetAddress.getByName("localhost");
      byte[] sendData = new byte[1024];
      byte[] receiveData = new byte[1024];
      String sentence = inFromUser.readLine();


      sendData = sentence.getBytes();
      DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 4000);
      clientSocket.send(sendPacket);

      //Getting data from the Bridge
      DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
      clientSocket.receive(receivePacket);
      String modifiedSentence = new String(receivePacket.getData());
      System.out.println("C: FROM SERVER:" + modifiedSentence);
      clientSocket.close();
   }
}

UDPSERVER

import java.io.*;
import java.net.*;

class UDPServer
{
   public static void main(String args[]) throws Exception
      {
         DatagramSocket serverSocket = new DatagramSocket(5000);
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];

            while(true)
               {
                   //receiveing data from the bridge
                  DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                  serverSocket.receive(receivePacket);
                  String sentence = new String( receivePacket.getData());
                  System.out.println("S:RECEIVED: " + sentence);

                  InetAddress IPAddress = InetAddress.getByName("localhost");


                  // Sending data to the bridge
                  int port = 4000;
                  String capitalizedSentence = sentence.toUpperCase();
                  sendData = capitalizedSentence.getBytes();
                  DatagramPacket sendPacket =
                  new DatagramPacket(sendData, sendData.length, IPAddress, port);
                  serverSocket.send(sendPacket);
               }
      }
   }

Bridge

import java.io.*;
import java.net.*;

/**
 * Write a description of class Bridge here.
 *
 * @author (your name)
 * @version (a version number or a date)
 */
public class Bridge
{
   public static void main(String args[]) throws Exception{

       DatagramSocket bridgeSocket1 = new DatagramSocket(4000);
       DatagramSocket bridgeSocket2 = new DatagramSocket();

       byte[] receiveData = new byte[1024];
       byte[] sendData = new byte[1024];

       DatagramPacket  receivePacket;
       DatagramPacket  sendPacket;
       InetAddress  IPAddress = InetAddress.getByName("localhost");


       while(true){

           //Receiveing data from the UDPClient
           receivePacket = new DatagramPacket(receiveData, receiveData.length);
           bridgeSocket1.receive(receivePacket);  


           //Sending data to UDPServer
           String sentence = new String(receivePacket.getData());
           System.out.println("B: Data Received:" + sentence); 
           int port = 5000;
           sendData = sentence.getBytes();
           sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port);
           bridgeSocket2.send(sendPacket);


           // Receiving Data from the UDPServer
           receivePacket = new DatagramPacket(receiveData,receiveData.length);
           bridgeSocket1.receive(receivePacket);


           String capitalizedSentence = new String(receivePacket.getData());
           System.out.println("Capitalized Sentence in the Bridge Class: " + capitalizedSentence);


           //Sending data to the UDPClient

           sendData = capitalizedSentence.getBytes();
           sendPacket = new DatagramPacket(sendData,sendData.length,IPAddress,5000);
           bridgeSocket2.send(sendPacket);
        }
        }
    }

Answer 1

You are sending the response from the server back to the server because you are using port 5000 as the destination port. But the server is running on 5000 , not your client. You have to assign your client to a port as well and send the message received from the server back to the client on the defined port.

For now your sequence looks like this:

 (C) ---> 4000
               ---> 5000
          4000 <---
               ---> 5000
          4000 <---
               ---> 5000
          4000 <---
               ---> 5000
          4000 <---
          [...]

But it should look like this:

 (C) ---> 4000
               ---> 5000
          4000 <---
4500 <---

(assuming your client is listening on port 4500)