java - 为什么我的简单 UDPServer 仅在第一次运行时才接收来自 UDPClient 的数据包？

Question

我的UDP客户端将不断发送“Hello Server!”每 2 秒发送一次到 UDP 服务器(仅回显消息，但以大写形式)。他们都会打印收到的内容。

我的问题是我总是必须先运行服务器然后再运行客户端。如果我先运行客户端，然后运行服务器，服务器将根本不会收到任何内容。我希望服务器可以随时启动，并且仍然接收来自客户端的数据包。我怎样才能做到这一点？

UDP客户端:

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

class UDPClient
{
   public static void main(String args[]) throws Exception
   {
      while(true) {
         DatagramSocket clientSocket = new DatagramSocket();
         InetAddress IPAddress = InetAddress.getByName("localhost");
         byte[] sendData = new byte[100];
         byte[] receiveData = new byte[100];
         String sentence = "Hello Server!";
         sendData = sentence.getBytes();
         DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
         clientSocket.send(sendPacket);
         DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
         clientSocket.receive(receivePacket);
         String modifiedSentence = new String(receivePacket.getData());
         System.out.println("FROM SERVER:" + modifiedSentence);
         clientSocket.close();
         Thread.sleep(2000);
      }

   }
}

UDP服务器:

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

class UDPServer
{
   public static void main(String args[]) throws Exception
      {
         DatagramSocket serverSocket = new DatagramSocket(9876);
            byte[] receiveData = new byte[100];
            byte[] sendData = new byte[100];
            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);
               }
      }
}

Answer 1

如果您先运行客户端，它将收到异常 PortUnreachableException 并且客户端程序将关闭，为了防止这种情况发生，您必须捕获该异常，以便客户端继续运行。

尝试:

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

class UDPClient
{
   public static void main(String args[]) throws Exception
   {
      while(true) {
         try{
         DatagramSocket clientSocket = new DatagramSocket();
         InetAddress IPAddress = InetAddress.getByName("localhost");
         byte[] sendData = new byte[100];
         byte[] receiveData = new byte[100];
         String sentence = "Hello Server!";
         sendData = sentence.getBytes();
         DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
         clientSocket.send(sendPacket);
         DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
         clientSocket.receive(receivePacket);
         String modifiedSentence = new String(receivePacket.getData());
         System.out.println("FROM SERVER:" + modifiedSentence);
         clientSocket.close();
         }
         catch (PortUnreachableException pe)
         {
           System.out.println("COULDN'T CONNECT TO SERVER:" + pe.getMessage());
         }
         catch (Exception ex)
         {
           System.out.println("COULDN'T CONNECT TO SERVER:" + ex.getMessage());
         }
         Thread.sleep(2000);
      }

   }
}