Question

I’m trying to transfer an ArrayList from a client to a server using the UDP protocol.

The transfer starts at the “max” if statement.
Same in the server side

This is the client:

public class UdpClient {
protected DatagramPacket sendPacket;
protected DatagramPacket receivePacket;

public static void main(String args[]) throws IOException,
        ClassNotFoundException {
    UdpClient upd = new UdpClient();
    ArrayList<Integer> arr = new ArrayList<Integer>();
    BufferedReader inFromUser = new BufferedReader(new InputStreamReader(
            System.in));
    DatagramSocket clientSocket = new DatagramSocket();
    byte[] sendData = new byte[1024];
    byte[] receiveData = new byte[1024];
    ByteArrayOutputStream bStream = new ByteArrayOutputStream();
    ObjectOutput oo = new ObjectOutputStream(bStream);
    ByteArrayInputStream baos;
    ObjectInputStream oos;
    System.out
            .println("Commands: Time, Date, Weather, Sum-number, Max-number, Exit");
    while (true) {
        String fromUsr = inFromUser.readLine();
        if (fromUsr.equals("bye")) {
            break;
        } else if (fromUsr.equals("weather")) {
            sendData = fromUsr.getBytes();
            upd.sendPacket(sendData, clientSocket);
            System.out
                    .println("Please select a ctiy: Lund, Malmo, Stockholm");
            String weather = inFromUser.readLine();
            sendData = weather.getBytes();
            upd.sendPacket(sendData, clientSocket);
            upd.receivePacket(clientSocket, receiveData);
        } else if (fromUsr.equals("max")) {
            sendData = fromUsr.getBytes();
            upd.sendPacket(sendData, clientSocket);
            String max = inFromUser.readLine().trim();
            upd.nums(max, arr);
            oo.writeObject(arr);
            byte[] buf = new byte[bStream.toByteArray().length];
            buf = bStream.toByteArray();
            upd.sendPacket(buf, clientSocket);
            System.out.println(arr);
        } else {
            // send data that of the user
            sendData = fromUsr.getBytes();
            upd.sendPacket(sendData, clientSocket);
            upd.receivePacket(clientSocket, receiveData);
        }
        String fromServer = new String(upd.getData());
        System.out.println("Message from server:\n" + fromServer);
    }
}

private void sendPacket(byte[] sendData, DatagramSocket clientSocket)
        throws IOException {
    InetAddress IPAddress = InetAddress.getByName("ericman-PC");
    sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress,
            9876);
    clientSocket.send(sendPacket);

}

private void receivePacket(DatagramSocket serverSocket, byte[] receiveData)
        throws IOException {
    byte[] rData = new byte[1024];
    receiveData = rData;
    receivePacket = new DatagramPacket(receiveData, receiveData.length);
    serverSocket.receive(receivePacket);
}

This is the Server

    public class UdpServer {
protected DatagramPacket receivePacket;
protected DatagramPacket sendPacket;

public static void main(String args[]) throws IOException,
        ClassNotFoundException {
    UdpServer upd = new UdpServer();
    DatagramSocket serverSocket = new DatagramSocket(9876);
    DateFormat currentDate = new SimpleDateFormat("yyyy/MM/dd");
    DateFormat currentTime = new SimpleDateFormat("HH:mm:ss");
    Date date = new Date();
    ByteArrayInputStream baos;
    ObjectInputStream oos;
    byte[] sendData = new byte[1024];
    byte[] receiveData = new byte[1024];

    while (true) {
        System.out.println("****************************************"
                + "\nServer is connected");
        upd.receivePacket(serverSocket, receiveData);
        String str = new String(upd.getData()).trim();
        System.out.println("Message received is:" + " " + str);
        if (str.equals("time")) {
            str = currentTime.format(date);
            sendData = str.getBytes();
            upd.sendPacket(serverSocket, sendData);
        } else if (str.equals("date")) {
            str = currentDate.format(date);
            sendData = str.getBytes();
            upd.sendPacket(serverSocket, sendData);
        } else if (str.equals("weather")) {
            upd.receivePacket(serverSocket, receiveData);
            str = upd.weather(str = new String(upd.getData()).trim());
            sendData = str.getBytes();
            upd.sendPacket(serverSocket, sendData);

        } else if (str.equals("max")) {
            byte[] buf = new byte[1024];
            System.out.println("waitng for object to come");
            upd.receivePacket(serverSocket, buf);
            baos = new ByteArrayInputStream(buf);
            oos = new ObjectInputStream(baos);
            Object o = oos.readObject();
            System.out.println(o);
        } else {
            str = "Unknown command, please try again..";
            sendData = str.getBytes();
            upd.sendPacket(serverSocket, sendData);
        }

    }
}

This is the error i get in the Server side

Exception in thread "main" java.io.StreamCorruptedException: invalid stream header: 00000000
at java.io.ObjectInputStream.readStreamHeader(Unknown Source)
at java.io.ObjectInputStream.<init>(Unknown Source)
at UdpServer.main(UdpServer.java:54)

Line 54 that the error indicates is this line on the server

baos = new ByteArrayInputStream(buf);
            oos = new ObjectInputStream(baos);

If you could help me out of why this error is happening? thanx!

Answer 1

Your code creates alot of arrays it immediately discards. I would use a debugger to step through the code so you understand what it is doing.

private void receivePacket(DatagramSocket serverSocket, byte[] receiveData)
        throws IOException {
    // create a new array
    byte[] rData = new byte[1024];
    // throw away the orignal array so the new array will be update.
    receiveData = rData;
    // copy into the new array, not the old one.
    receivePacket = new DatagramPacket(receiveData, receiveData.length);
    serverSocket.receive(receivePacket);
}

In this case, the original receiveData will not me modified so it will contain lots of 0 bytes.

---

        byte[] buf = new byte[bStream.toByteArray().length];
        buf = bStream.toByteArray();

This creates three arrays when all you need is one. The first array bStream.toByteArray() is created just so you can determine what length it would be. You create a second array which is the same length but empty and put it in buf Finally you discard the second array and replace it with a copy of the first array.