UDP 服务器无法有效地向客户端发送响应

Question

目前我的 UDP 客户端只向服务器发送两个命令 (td & TEMP)。 td 返回当前时间和日期。 TEMP 10.30 将温度 10.30 转换为华氏度并将结果返回给客户端。

问题：

当我首先输入 td 命令时，它工作正常。之后，如果我输入 TEMP 命令，它不会转换为华氏度，而是转换为当前日期和时间（有些奇怪的回复是这样的：Message Returned from Server : 50.5480-09 18:45:53）。那不是我想要的。另一方面，输入 td 和 TEMP 反之亦然似乎工作正常。我不知道 while 循环中我的逻辑问题出在哪里。

UDP服务器：

    public static void main(String arg[]) throws Exception {
    DatagramSocket serversocket = new DatagramSocket(9999);
    UDPServer udpserver = new UDPServer();
    byte[] receivedBuffer; // = new byte[1024];
    byte[] sentBuffer; //= new byte[1024];
    while (true) {
        receivedBuffer = new byte[1024];
        sentBuffer = new byte[1024];
        DatagramPacket receivedpacket = new DatagramPacket(receivedBuffer, receivedBuffer.length);
        System.out.println("Server Waiting for a message from Client.....");

        serversocket.receive(receivedpacket);
        String fromClient = new String(receivedpacket.getData());
        // enter td command to display the curerct date and time
        if (fromClient != null && fromClient.startsWith("td")) {
            InetAddress clientIP = receivedpacket.getAddress();
            System.out.println("Message received from client : " + fromClient + " at IP Address = "
                    + clientIP.getHostAddress() + ", Host Name = " + clientIP.getHostName());

            String toClient = udpserver.dateAndTime();
            sentBuffer = toClient.getBytes();
            DatagramPacket sendpacket = new DatagramPacket(sentBuffer, sentBuffer.length, clientIP, 8888);
            serversocket.send(sendpacket);
            System.out.println(" Reply Message is sent to client " + clientIP.getHostAddress());
        }

        // converting the TEMPERATURE into Farenheit
        if (fromClient != null && fromClient.startsWith("TEMP") && !fromClient.startsWith("td")) {

            InetAddress clientIP = receivedpacket.getAddress();
            System.out.println("Message received from client : " + fromClient + " at IP Address = "
                    + clientIP.getHostAddress() + ", Host Name = " + clientIP.getHostName());
            float temp = Float.parseFloat(fromClient.substring(fromClient.indexOf(' ') + 1));
            float tempInFaren = (float) (temp * 1.8 + 32.0);
            //float toClient = tempInFaren ;
            String convertIntoFarenheit = String.valueOf(tempInFaren);

            sentBuffer = convertIntoFarenheit.getBytes();
            DatagramPacket sendpacket = new DatagramPacket(sentBuffer, sentBuffer.length, clientIP, 8888);
            serversocket.send(sendpacket);
            System.out.println(" Reply Message is sent to client " + clientIP.getHostAddress());

        }
        try {
            Thread.sleep(2000);
        } catch (InterruptedException ie) {
        }
    }
}

//method for returning current date and time
public String dateAndTime() {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date d = new Date();
    String s = sdf.format(d);

    return s;

}

UDP客户端：

public static void main(String args[]) throws Exception {
    byte[] sendData = new byte[1024];
    byte[] receiveData = new byte[1024];
    BufferedReader inFromKeyboard = new BufferedReader(new InputStreamReader(System.in));
    try {
        DatagramSocket clientSocket = new DatagramSocket(8888);
        InetAddress IPAddress = InetAddress.getByName("localhost");
        while (true) {
            System.out.println("Please enter the message to send to server: ");
            String sentence = inFromKeyboard.readLine();
            //sending time and date command
            if (sentence.startsWith("td") && !sentence.startsWith("TEMP")) {
                sendData = sentence.getBytes();
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9999);
                clientSocket.send(sendPacket);

                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                System.out.println(
                        "Message Sent to Server : " + sentence + "\n Now waiting for reply from Server....");
                clientSocket.receive(receivePacket);
                String fromServer = new String(receivePacket.getData());
                System.out.println("Message Returned from Server : " + fromServer);

            }
            //sending TEMP command

            if (sentence.startsWith("TEMP") && !sentence.startsWith("td")) {
                sendData = sentence.getBytes();
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9999);
                clientSocket.send(sendPacket);

                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                System.out.println(
                        "Message Sent to Server : " + sentence + "\n Now waiting for reply from Server....");
                clientSocket.receive(receivePacket);
                String fromServer = new String(receivePacket.getData());
                System.out.println("Message Returned from Server : " + fromServer);

            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Answer 1

你一直在发送和接收原始数据，但你把它当作字符串。 getData() returns 你的 1024 字节缓冲区，你只用 50.5480 填充了前 7 个字节。其余的都是上次通话的垃圾。

当你在时间之前获得温度时，它似乎对你有用（它实际上没有用，但问题并没有表现出来），因为时间字符串比温度字符串长并且它完全覆盖它。您会看到与在接收到时间后再次尝试获取温度相同的不愉快效果。

解决此问题的一种方法是将字符串的长度作为第一个条目包含在 UDP 消息中，并在从响应构造字符串时使用此字符串长度。

Answer 2

问题是客户端用来接收数据的字节数组也包含了之前接收到的内容。当它从服务器收到响应时，只有一部分被覆盖，因此其余部分也会与实际响应一起打印。

解决这个问题的方法有很多种。您可以在 UDP 数据包中包含字符串的长度，也可以在每次从服务器收到响应后简单地刷新缓冲区，以便下次响应时缓冲区为空。再次有不同的方法可以做到这一点。一种是将缓冲区的所有内容清零，或者只是每次都创建一个新缓冲区（您已经在服务器中这样做了 class).

查看客户端代码的变化。只有while循环的开始部分需要改动。

UDPClient:

public static void main(String args[]) throws Exception {
    byte[] sendData;  
    byte[] receiveData;
    BufferedReader inFromKeyboard = new BufferedReader(new InputStreamReader(System.in));
    try {
        DatagramSocket clientSocket = new DatagramSocket(8888);
        InetAddress IPAddress = InetAddress.getByName("127.0.0.1");
        while (true) {

             /*********************************
              create a new buffer each time */
                sendData = new byte[1024];
                receiveData = new byte[1024];
             /*********************************/

            System.out.println("Please enter the message to send to server: ");
            String sentence = inFromKeyboard.readLine();
            //sending time and date command
            if (sentence.startsWith("td") && !sentence.startsWith("TEMP")) {
                sendData = sentence.getBytes();
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9999);
                clientSocket.send(sendPacket);

                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                System.out.println(
                        "Message Sent to Server : " + sentence + "\n Now waiting for reply from Server....");
                clientSocket.receive(receivePacket);
                String fromServer = new String(receivePacket.getData());
                System.out.println("Message Returned from Server : " + fromServer);

            }
            //sending TEMP command

            if (sentence.startsWith("TEMP") && !sentence.startsWith("td")) {
                sendData = sentence.getBytes();
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9999);
                clientSocket.send(sendPacket);

                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                System.out.println(
                        "Message Sent to Server : " + sentence + "\n Now waiting for reply from Server....");
                clientSocket.receive(receivePacket);
                String fromServer = new String(receivePacket.getData());
                System.out.println("Message Returned from Server : " + fromServer);

            }

        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

如果您不想每次都创建缓冲区，也可以使用全零重新初始化缓冲区。您需要在每次迭代中执行此操作。为此，您需要导入数组 class.

Arrays.fill( receiveData, (byte) 0 );
Arrays.fill( sendData, (byte) 0 );

UDP 服务器无法有效地向客户端发送响应

UDP server can't send response to the client in efficient way

java

sockets

udp

client-server

bufferedreader