Java监听UDP端口

Question

I want to listen to a UDP port to receive the information it is transferring in wireshark it looks like this

all that I found was people creating ServerSockets but when i try that I dont get any output from the console.

package me.xenopyax.albionmarket;

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

public class Main {

    public static void main(String[] args) throws IOException {

        DatagramSocket serverSocket = new DatagramSocket(5056);
        byte[] receiveData = new byte[1024];
        byte[] sendData = new byte[1024];
        while(true)
            {
          DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
          serverSocket.receive(receivePacket);
          String sentence = new String( receivePacket.getData());
          System.out.println("RECEIVED: " + sentence);
          InetAddress IPAddress = receivePacket.getAddress();
          int port = receivePacket.getPort();
          String capitalizedSentence = sentence.toUpperCase();
          sendData = capitalizedSentence.getBytes();
          DatagramPacket sendPacket =
          new DatagramPacket(sendData, sendData.length, IPAddress, port);
          serverSocket.send(sendPacket);
       }

    }

}

Am I corret with using ServersSockets to listen to the port. or what am I doing wrong?

Answer 1

As the comments state, ServerSocket is for stream-based protocol and UDP is not a stream-based protocol. You need to use DatagramSocket to read UDP packets.

However DatagramSocket can only be used to read UDP packets that are addressed to the current machine ... one way or another.

And that doesn't match what Wireshark does. In reality, Wireshark

• receives raw network packets (at the ISO layer 2) aka IP packets, and
• often puts the NIC into "promiscuous" mode so that it can pick up packets that are not addressed to the host.

You can't do either of those things using the Java SE libraries, but there some third-party alternatives; eg

• How do I read and write raw ip packets from java on a mac?
• Can I do low level network programming in Java?