Java Programming

I've written a client-server combo using sockets, and the client seems to continuallly get stuck on a particular loop, after receiving a file.

The server sends a file, and then 3 strings once the file transfer is complete (which each are preceded by a byte "1". The receives the file successfully, and then continually gets stuck on the following loop:

                    while (instruction == 0) {
                        instruction = in.readByte();
                        sentString = new byte[in.readShort()];
                        in.read(sentString);
                        message.concat(new String(sentString, "UTF-8"));
                    }

After stepping through it, I have noticed that it continually reads the zero followed by an empty string, but while stepping through the server and continually setting break points, I have observed that it never (in this example) sends 0, so why does the client keep receiving it?

I have included all the code for both classes below. Any help would be greatly appreciated.

The complete server code is:

import java.net.*;
import java.io.*;

public class WebServer
{
    private File file;
    private Socket client = null;
    private DataOutputStream terminal;
    private byte info;
    //The above byte contains the two information bits of the protocol.
    //The second bit is the type of transfer, and the first bit is whether
    //the transfer is complete or not. So, b00 == 0 == "Message transfer, not complete."
    //b01 == 1 == "Message Transfer, complete.", b10 == 2 == "File transfer, not complete.",
    //and b11 == 3 == "File Transfer, complete."
    private byte sendBuffer[];
    private short size;
    
    public static void main(String[] args) throws IOException {
        
        WebServer webServer = new WebServer();
        
    }
        

    public WebServer() throws IOException {
        
        ServerSocket server = null;
        try {
            server = new ServerSocket(8008);
        } catch (IOException e) {
            System.out.println("Unable to listen to Port 8008.");
            System.exit(1);
        }
        
        try {
            client = server.accept();
        } catch (IOException e) {
            System.err.println("Connection failed.");
            System.exit(1);
        }

        sendBuffer = new byte[client.getReceiveBufferSize()];
        terminal = new DataOutputStream(client.getOutputStream());
        BufferedReader command = new BufferedReader(new InputStreamReader(client.getInputStream()));
        writeMessage("Welcome to Jimmy's webserver.\n", false);
        String input;
            while ((input = command.readLine()) != null) {
                if (input.toLowerCase().equals("exit")) {
                    break;
                }
                else if (input.toLowerCase().equals("help")) {
                    writeMessage("Commands:\n", false);
                    writeMessage("GET <filename> - JWS will send <filename> to you.\n", false);
                    writeMessage("EXIT - exits Jimmy's Webserver.\n", false);
                }
                else if (input.toLowerCase().startsWith("get ")) {
                    sendFile(new File(input.substring(4)));
                }
            }
        terminal.close();
        command.close();
        client.close();
        server.close();
    }
    
    private void sendFile(File file) throws FileNotFoundException, IOException {
        if (file == null) {
            writeMessage("Please use the command: GET filename\n", false);
        }
        else if (!file.exists()) {
            writeMessage(file.toString() + " could not be found in the current directory.\n", false);
        }
        else {
            writeMessage("Sending file: " + file.toString() + "\n", false);
            writeMessage("File size: " + file.length() + "bytes.\n", false);
            Long time = System.currentTimeMillis();
            
            //message type: String or File (1 bit)
            //transmission complete: (1 bit)
            //sent size: 2 bytes
            //

            FileInputStream fileReader = new FileInputStream(file);
            writeMessage(file.toString(), true);
            long fileRemaining = file.length();
            while (fileRemaining != 0) {
                size = (short) fileReader.read(sendBuffer);
                if (file.length() < sendBuffer.length) {
                    size = (short)file.length();
                    terminal.write(3);
                }
                else {
                    terminal.write(2);
                }
                terminal.writeChar(size);
                fileReader.read(sendBuffer);
                terminal.write(sendBuffer);
                fileRemaining -= size;
            }
            fileReader.close();
            writeMessage("Transfer of " + file.toString() + " is complete.\n", false);
            time = System.currentTimeMillis() - time;
            if (time == 0) { time++; }
            writeMessage("Transfer took: " + time + " milliseconds.\n", false);
        }
    }
    
    
    private void writeMessage(String s, boolean sendingFile) throws UnsupportedEncodingException, IOException { //replace these with try/catch.
        byte stringBytes[] = s.getBytes("UTF-8");
        if (sendBuffer.length > stringBytes.length) {
            if (!sendingFile) {
                terminal.write(1);
            }
            else {
                terminal.write(2);
            }
            size = (short)stringBytes.length;
            terminal.writeChar(size);
            terminal.write(stringBytes);
        }
        else {
            terminal.write(0);
            size = (short)sendBuffer.length;
            long sent = 0;
            terminal.writeChar(size);
            while (sent < (stringBytes.length - sendBuffer.length)) {
                terminal.write(0);
                terminal.writeChar(size);
                for (int i = 0; i < sendBuffer.length; i++, sent++) {
                    sendBuffer[i] = stringBytes[(int)sent];
                }
                terminal.write(sendBuffer);
            }
            terminal.write(1);
            size = (short)(stringBytes.length - sendBuffer.length);
            terminal.writeChar(size);
            for (int i = 0; i < size; i++) {
                sendBuffer[i] = stringBytes[i + (int)sent]; //try and find out why these sents need casting to ints?
            }
            terminal.write(sendBuffer);
        }
    }
}