如何使UDP服务器响应UDP客户端数据包？

Question

I'm having trouble getting the server to send data back to the client. Here is my code and my inputs and results.

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

public class UDPServer {

    public static void main(String[] args) throws IOException {
        InetAddress ip = InetAddress.getByName("127.0.0.1");
        int port = 1345;

        //Creates connection socket.
        DatagramSocket serverSocket = new DatagramSocket(port);

        System.out.println("Server Active");
        while(true) {
            //Receiving packet
            byte[] buffer = new byte[100];
            DatagramPacket receivePacket = new DatagramPacket(buffer, buffer.length);
            serverSocket.receive(receivePacket);
            String clientInput = new String(buffer);
            System.out.println("Server has Received : " + clientInput);

            //Sending packet
            byte[] data = clientInput.getBytes();
            DatagramPacket sendPack = new DatagramPacket(data, data.length, ip, port);
            serverSocket.send(sendPack);
            String serverInput = new String(data);
            System.out.println("Server has sent: " + serverInput);
        }

    }
}

~~~~~~~~~~~~~~~~~

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.DatagramSocket;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;


public class UDPClient
{
    public static void main(String args[]) throws IOException
    {
        DatagramSocket clientSocket = new DatagramSocket();
        InetAddress ip = InetAddress.getByName("127.0.0.1");
        BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
        int port = 1345;

        System.out.println("Would you like to continue?");
        while(inFromUser.readLine().equals("Yes") || inFromUser.readLine().equals("yes")) {
            System.out.println("Enter an expression to be evaluated");

            //gets userInput
            String userInput = inFromUser.readLine();
            byte[] buffer = userInput.getBytes();

            //sends packet to server
            DatagramPacket sendPacket = new DatagramPacket(buffer, buffer.length, ip, port);
            clientSocket.send(sendPacket);
            System.out.println("Client has sent: " + userInput);

            //receives packet from server
            byte[] data = new byte[100];
            DatagramPacket receivePacket = new DatagramPacket(data, data.length);
            clientSocket.receive(receivePacket);
            String serverInput = new String(data);
            System.out.println("Client has received: " + serverInput);
            System.out.println("Would you like to continue?");
        }
    }
}

I am not sure why the server does not echo back to the client? I'm not sure what I am doing wrong, or about the overall quality of this code.

Answer 1

The server is sending the reply to itself. When you create the sendPacket in the server, you're giving it ip and port but the ip and port are the server's own IP and bound (listening) port, not the port of your client.

Because the client created its DatagramSocket without specifying a port, the system will dynamically choose an unused port number for it. You need to obtain the client's port number with receivePacket.getSocketAddress() (in the server), then use that address in your construction of sendPacket . In fact, there is an alternate constructor for DatagramPacket that accepts buffer, length and SocketAddress which is ideal for constructing the reply packet.