Do sockets clean part of their buffer every time I read a segment of the incoming message?

Question

I've started working with Java and sockets and I'm having some problems with DataInputStream. I'm receiving a telegram which contains the message length in the first 4 bytes of the message itself, so at the first iteration I just read this memory portion. When I read again the incoming message I've noticed that the first 4 bytes are gone, so I need to subtract those 4 bytes in the method I created to compute the message length itself. The question is: does the buffer of the incoming data lose the bytes I've already read? I can't find anything in the Java doc, but I'm probably missing something due to my inexperience.

This is the method for the data reading:

/**
 * It receives data from a socket.
 *
 * @param socket The communication socket.
 * @param lengthArea The area of the header containing the length of the message to be received.
 * @return The received data as a string.
 */
 static String receiveData(Socket socket, int lengthArea) {
    byte[] receivedData = new byte[lengthArea];

    try {
        DataInputStream dataStream = new DataInputStream(socket.getInputStream());
        int bufferReturn = dataStream.read(receivedData, 0, lengthArea);

        System.out.println("Read Data: " + bufferReturn);
    } catch (IOException e) {
        // Let's fill the byte array with '-1' for debug purpose.
        Arrays.fill(receivedData, (byte) -1);

        System.out.println("IO Exception.");
    }

    return new String(receivedData);
}

And this one is the method I use to compute the message length:

/**
 * It converts the message length from number to string, decreasing the calculated length by the size of the message
 * read in the header. The size is defined in 'Constants.java'.
 *
 * @param length The message size.
 * @return The message size as an integer.
 */
static int calcLength(String length) {
    int num;

    try {
        num = Integer.parseInt(length) + 1 - MESSAGE_LENGTH_AREA_FROM_HEADER;
    } catch (Exception e) {
        num = -1;
    }

    return num;
}

Constants.java

MESSAGE_LENGTH_AREA_FROM_HEADER = 4;

Answer 1

does the buffer of the incoming data lose the bytes I've already read

Yes, of course it does. TCP presents a byte stream. You consume part of it, it's gone. No different from reading from a file.

You should be using DataInputStream.readInt() to read the length word if it's in binary, and then DataInputStream.readFully() to read the data.