Java 中的 UDP 客户端-服务器都在本地计算机上，客户端程序没有响应

Question

我正在使用 UDP 协议在 Java 中模拟一个基本的客户端-服务器聊天应用程序，问题是我的客户端程序没有响应服务器程序发送的消息，首先，服务器发送一条启动消息，客户端应该从其各自的 PowerShell 终端读取一个字符串，该消息将被发送到服务器程序，当我调试代码时，我意识到服务器程序能够发送启动消息，但客户端程序没有从它的终端获取输入，你能看到这个吗，如果这听起来有点傻，请原谅我，但我是计算机网络课程的初学者。（我本地机器上的客户端和服务器运行）

服务器程序

import java.io.*;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.nio.charset.*;



public class ServerUDP {
    private static int SERVERPORT;
    private static int CLIENTPORT;
    private static InetAddress SERVERADDRESS;
    public DatagramSocket ServerSocket;
    public BufferedReader br;
    private int MSGLEN;
    
    void ConnectionSetup() throws Exception{
        SERVERPORT = 3001;
        CLIENTPORT = 3000;
        MSGLEN = 100;
        SERVERADDRESS = InetAddress.getLocalHost();
        br = new BufferedReader(new InputStreamReader(System.in));
        ServerSocket = new DatagramSocket(SERVERPORT);
    }

    void ConnectionClose() throws Exception{
        br.close();
        ServerSocket.close();
    }

    void SendMessage(String mString) throws Exception{
        byte[] BuffToClient = new byte[this.MSGLEN];
        BuffToClient = mString.getBytes();
        DatagramPacket PktToClient = new DatagramPacket(BuffToClient, BuffToClient.length,SERVERADDRESS,CLIENTPORT);
        ServerSocket.send(PktToClient);
    }

    String GetMessage() throws Exception{
        byte[] BuffFromClient = new byte[this.MSGLEN];
        DatagramPacket PktFromClient = new DatagramPacket(BuffFromClient,BuffFromClient.length);
        ServerSocket.receive(PktFromClient);
        BuffFromClient = PktFromClient.getData();
        return new String(BuffFromClient,StandardCharsets.UTF_8);
    }

    void Converse() throws Exception{
        this.SendMessage("Hello Client, How are you?");
        System.out.println("msg sent");
        while(true){
            String CliMsg = this.GetMessage();
            if(CliMsg.equals("bye")){
                this.SendMessage("Ok bye");
                break;
            }else{
                System.out.println("Client Says : "+CliMsg);
                this.SendMessage(br.readLine());
            }
        }
    }

    public static void main(String[] args) throws Exception{
        ServerUDP Server = new ServerUDP();
        Server.ConnectionSetup();
        Server.Converse();
        Server.ConnectionClose();
    }    
}

客户端程序

import java.io.*;
import java.net.InetAddress;
import java.nio.charset.*;
import java.net.*;


public class ClientUDP {
    public static int CLIENTPORT;
    public static int SERVERPORT;
    public InetAddress CLIENTADD;
    public InetAddress SERVERADDRESS;
    public int MSGLEN;
    public static BufferedReader br;
    public DatagramSocket clientSocket;

    void ConnectionSetup() throws Exception{
        CLIENTPORT = 3000;
        SERVERPORT = 3001;
        MSGLEN = 100;
        CLIENTADD = InetAddress.getLocalHost();
        SERVERADDRESS = InetAddress.getLocalHost();
        br = new BufferedReader(new InputStreamReader(System.in));
        clientSocket = new DatagramSocket(CLIENTPORT);
    }

    void ConnectionClose() throws Exception{
        br.close();
        clientSocket.close();
    }

    void SendMessage(String mString) throws Exception{
        byte[] BuffToServer = new byte[this.MSGLEN];
        BuffToServer = mString.getBytes();
        DatagramPacket PktToServer = new DatagramPacket(BuffToServer,BuffToServer.length,CLIENTADD,SERVERPORT);
        clientSocket.send(PktToServer);
    }

    String GetMessage() throws Exception{
        byte[] BuffFromServer = new byte[this.MSGLEN];
        DatagramPacket PktFromServer = new DatagramPacket(BuffFromServer, BuffFromServer.length);
        clientSocket.receive(PktFromServer);
        BuffFromServer = PktFromServer.getData();
        System.out.println("msg recived");
        for(byte b : BuffFromServer){
            System.out.println(b);
        }
        return new String(BuffFromServer,StandardCharsets.UTF_8);
    }

    void Converse() throws Exception{
        System.out.println(this.GetMessage());
        System.out.println("got the msg");
        while(true){
            String MsgToSrvr = br.readLine();
            this.SendMessage(MsgToSrvr);
            System.out.println("Server Says : "+this.GetMessage());
            if(MsgToSrvr.equals("bye")){
                break;
            }
        }
    }

    public static void main(String[] args) throws Exception{
        ClientUDP Client = new ClientUDP();
        Client.ConnectionSetup();
        Client.Converse();
        Client.ConnectionClose();
    }
}

Answer 1

我猜你的问题是你先开了服务器，再开了客户端。

如果您先打开客户端，它将阻塞在线 clientSocket.receive(PktFromServer); 等待服务器发送内容。之后，你打开服务器，它在启动后立即发送消息，客户端收到消息，打印出来并等待用户在控制台打印一些东西。与此同时，服务器等待客户端发送一些东西。然后客户端等待服务器，服务器等待用户等

但是，如果您先打开服务器，它会将消息发送到任何地方，因为 UDP 不需要在发送内容之前建立连接。之后，您打开开始等待服务器的客户端，但它不会收到任何东西，因为来自服务器的初始消息丢失了。所以他们会卡住等待对方送东西。

这就是你的情况的原因。你没有提到你希望客户端和服务器如何相互行为，所以我现在不建议任何东西。您可以指定，我会建议修改。

Java 中的 UDP 客户端-服务器都在本地计算机上，客户端程序没有响应

UDP Client-Server in Java both on local machine, The Client Program is not responding

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