在 java 中无法从 UDP 服务器接收到正确的消息

Question

我正在尝试使用 udp 客户端将消息发送到 udp 服务器，然后将消息发送回客户端，其中包含有关接收到的消息的各种元数据。我有这两个类:

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

public class UdpDateClient {
    public static void main(String[] args) throws IOException {
        String host = "localhost";
        if (args.length > 0)
            host = args[0];
        // get a datagram socket on any available port
        try {
            DatagramSocket socket = new DatagramSocket();
            // send request
            byte[] buf = new byte[2048];
            buf="hi it's Max".getBytes();
            InetAddress address = InetAddress.getByName(host);
            DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4445);
            socket.send(packet);
            // get response
            packet = new DatagramPacket(buf, buf.length);
            socket.receive(packet);
            // display response
            buf = packet.getData();
            int len = packet.getLength();
            String received = (new String(buf)).substring(0, len);
            System.out.println("From server: " + received);
            socket.close();
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;
import java.util.Date;

public class UdpDateServer {
    private DatagramSocket socket = null;
    private boolean moreClients = true;

    public static void main(String[] args) {
        UdpDateServer server = new UdpDateServer();
        server.start();
    }

    public UdpDateServer() {
        try {
            socket = new DatagramSocket(4445);
            System.out.println("server ready...");
        } catch (SocketException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void start() {
        DatagramPacket packet;
        while (moreClients) {
            try {
                byte[] buf = new byte[2048];
                // receive request
                packet = new DatagramPacket(buf, buf.length);
                socket.receive(packet);
                System.out.println("recieved packet from client: "+new String(packet.getData()));
                // prepare response
                String responseString = "Client's message received: "+new String(packet.getData())+"\n"
                        + "Received on:" +new Date().toString();

                buf = responseString.getBytes();
                System.out.println("buf to be sent from server: "+(new String(buf)));
                System.out.println("buf.length="+buf.length);
                // send the response to "address" and "port"
                InetAddress address = packet.getAddress();
                int port = packet.getPort();
                packet = new DatagramPacket(buf, buf.length, address, port);
                socket.send(packet);
            } catch (IOException e) {
                e.printStackTrace();
                moreClients = false;
            }
        }
        socket.close();
    }
}

我在客户端得到的只是服务器发送的消息的前 11 个字节：

客户端：

From server: Client's me

服务器端：

server ready...

recieved packet from client: hi it's Max

buf to be sent from server: Client's message received: hi it's Max

Received on:Fri Aug 14 16:00:20 IDT 2015

buf.length=2116

如您所见，客户端收到的来自服务器的消息在 11 个字符后被截断。

我做错了什么？

Answer 1

问题出在客户端代码的第 buf="hi it's Max".getBytes(); 行。此处将缓冲区长度设置为 11。您的客户端代码应如下所示

            DatagramSocket socket = new DatagramSocket();
            // send request
            byte[] buf = "hi it's Max".getBytes();
            InetAddress address = InetAddress.getByName(host);
            DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4445);
            socket.send(packet);
            // get response
            buf = new byte[2048];
            packet = new DatagramPacket(buf, buf.length);
            socket.receive(packet);
            // display response
            buf = packet.getData();
            int len = packet.getLength();
            String received = (new String(buf)).substring(0, len);
            System.out.println("From server: " + received);
            socket.close();

Answer 2

byte[] buf = new byte[2048];
buf="hi it's Max".getBytes();

此后 buf.length 将是 11 而不是 2048！

在你回来的路上

packet = new DatagramPacket(buf, buf.length);
socket.receive(packet);

只会收到 11 个字节，因为 buf.length 是 11。将第一个块更改为

byte[] buf = new byte[2048];
System.arraycopy("hi it's Max".getBytes(), 0, buf, 0, 11);

在 java 中无法从 UDP 服务器接收到正确的消息

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