java - 为什么我的回显服务器有时会在消息末尾出现垃圾数据？

Question

嗨，我使用数据报制作了一个回显客户端-服务器代码。代码正在运行，从客户端发送的消息在服务器端被接收，然后它们被回显给客户端。以下场景显示了我遇到的问题

1->客户端:嗨，服务器:嗨， 回应:嗨

2->客户端:你好，服务器:你好， 回复:你好 现在，当客户端输入一条比上一条短的消息时，会发生以下情况

3->客户端:K，服务器:Kello， 回显:K

我不明白为什么服务器打印上一条消息的其余字符，这是服务器端代码:

    import java.awt.BorderLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.io.IOException;
    import java.net.DatagramPacket;
    import java.net.DatagramSocket;
    import java.net.InetAddress;
    import java.net.SocketException;
    import java.net.UnknownHostException;

    import javax.swing.SwingUtilities;
    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JScrollBar;
    import javax.swing.JScrollPane;
    import javax.swing.JTextArea;
    import javax.swing.JTextField;

    public class Server extends JFrame {
int port;
// JTextField textField;
JButton resendPacket;
JTextArea chatWindow;
DatagramSocket socket;
private final int PACKET_SIZE = 124;
private final int port_Number = 6789;
DatagramPacket packet =  new DatagramPacket(new byte[PACKET_SIZE], PACKET_SIZE);

    InetAddress host;


public Server() {
    super("INSTANT ECHO - SERVER");
    resendPacket = new JButton("Echo");
    chatWindow = new JTextArea();

//  add(resendPacket, BorderLayout.SOUTH);
    add(new JScrollPane(chatWindow));
    setSize(300, 300);
    setVisible(true);
}

public void startRunning() {

    try {
        port = 6777;
        socket = new DatagramSocket(port);
        showMessage("Server is Ready... \n");
        /*resendPacket.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                // TODO Auto-generated method stub
                try {
                    socket.send(packet);
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }
        }); */
        while(true)
        {
            processPacket();
        }


    } catch (SocketException e) {
        // TODO: handle exception
        showMessage("\n Could not send packet!\n");
        e.printStackTrace();
    } 
}

private void processPacket() {




    // create a packet

    // receive a packet

    try {
        socket.receive(packet);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        showMessage("\nCould not receive Packet");
        e.printStackTrace();
    }
    try {
        socket.send(packet);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }


    // display the contents of the packet
    showMessage(new String(packet.getData()));



    // send the packet again to the sender

}

public void showMessage(final String text) {
    SwingUtilities.invokeLater(

    new Runnable() {

        public void run() {
            chatWindow.append("\n Packet Data : " + text);
        }
    });

}

}

这是客户端代码

    import java.awt.BorderLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.io.IOException;
    import java.net.DatagramPacket;
    import java.net.DatagramSocket;
    import java.net.InetAddress;
    import java.net.SocketException;
    import java.net.UnknownHostException;

    import javax.swing.JButton;
    import javax.swing.JFrame;
    import javax.swing.JScrollPane;
    import javax.swing.JTextArea;
    import javax.swing.JTextField;
    import javax.swing.SwingUtilities;

    public class Client extends JFrame {
int port = 6777;
JTextField textField;
String data = "hello" ;
// JButton sendPacket;
JTextArea chatWindow;
DatagramSocket socket;
DatagramPacket packet;
private final int PACKET_SIZE = 124;
InetAddress host;

public Client() {
    super("INSTANT ECHO - CLIENT");

    textField = new JTextField();
    try {
        host = InetAddress.getByName("127.0.0.1");
    } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        showMessage("\n Could not find local host");
        e.printStackTrace();
    }

尝试{

        socket = new DatagramSocket();

    } catch (SocketException e) {
        // TODO Auto-generated catch block
        showMessage("\n Could not find Socket");
        e.printStackTrace();
    }
    // textField.setEditable(false);
    textField.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent event) {
            // TODO Auto-generated method stub
            data = event.getActionCommand();
            showMessage(event.getActionCommand());
            startRunning();
            textField.setText("");

        }
    });

    chatWindow = new JTextArea();
    add(textField, BorderLayout.SOUTH);


    add(new JScrollPane(chatWindow));
    setSize(300, 300);
    setVisible(true);

}

public void startRunning() {



    byte[] bytes_to_send = data.getBytes();

    DatagramPacket packet = new DatagramPacket(bytes_to_send,
            bytes_to_send.length, host, port);
    try {


        sendPacket(packet);
        setTime();
        receivePacket(packet);

        socket.receive(packet);
        showMessage("\n" + new String(packet.getData()));
    } catch (IOException e) {
        // TODO Auto-generated catch block

        e.printStackTrace();
    } finally {
        //closeCrap();
    }

}

private void sendPacket(DatagramPacket packet) {
    try {
        textField.setEditable(true);

        socket.send(packet);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        showMessage("\nUnable to send Packet !");
        e.printStackTrace();
    }
}

private void setTime() {
    try {
        socket.setSoTimeout(5);
    } catch (SocketException e) {
        // TODO Auto-generated catch block
        showMessage("\nRequest Time Out");
        e.printStackTrace();
    }
}

private void receivePacket(DatagramPacket packet) {
    packet = new DatagramPacket(new byte[PACKET_SIZE], PACKET_SIZE);

    try {
        socket.receive(packet);
    } catch (IOException e) {
        // TODO Auto-generated catch block
        showMessage("\n Unable to receive packet\n");
        e.printStackTrace();
    }
    showMessage("\n Echoed Message --> "+new String(packet.getData()));
}

private void closeCrap() {
    socket.close();
}

private void showMessage(final String data) {
    SwingUtilities.invokeLater(

    new Runnable() {

        @Override
        public void run() {
            // TODO Auto-generated method stub
            chatWindow.append("\n" + data);

        }
    });

}

}

谁能告诉我为什么前面的字符会被打印出来吗？

Answer 1

DatagramPacket.getLength()已使用的缓冲区的长度。

尝试

showMessage(new String(packet.getData(), 0, packet.getLength()));