Question

I’m having a socket problem. This problem occurs when I’m running the server and client on the same PC i.e. using “localhost” parameter. But problem is not seen when different PCs are being used.
Client sends a file with these codes:

output_local.write(buffer, 0, bytesRead);
output_local.flush();

And after that in another method I’m sending a command with these:

outputStream.write(string);
outputStream.flush();

Server appends the command to the end of the file. So it thinks it hasn’t received the command from the client yet. do you have an idea what might causing this problem? How can I solve the defect? below is the file receive method at the server:

    while (true) {
        try {
            bytesReceived = input.read(buffer);
        } catch (IOException ex) {
            Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
            System.out.println("exception occured");
            break;
        }
        System.out.println("received:" + bytesReceived);
        try {
            /* Write to the file */
            wr.write(buffer, 0, bytesReceived);
        } catch (IOException ex) {
            Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
        }
        total_byte = total_byte + bytesReceived;
        if (total_byte >= filesizeInt) {
            break;
        }
    }

Answer 1

If you want message-like support, you need a create a protocol to clarify what you’re going to send and receive.

In TCP, you can’t rely on separate “packets” being received separately (e.g., sending 4 chunks of 10 bytes may be received as 1 chunk of 40, or of 2 chunks of 20, or one chunk of 39 and one chunk of 1). TCP guarantees in order delivery, but not any particular ‘packetization’ of your data.

So, for example, if you’re sending a string you need to first send the string length then its bytes. The logic in pseudocode would be something like:

Client:

1. Send the command indicator
2. Send the payload length
3. Send the payload

Server:

1. Read the command indicator
2. Read the payload length
3. Loop reading payload until the complete length has been read