exchange both string and binary data using socket without closing it

i'm working on an instant messaging project which it's client side is android and server is java
i need to use socket with streams
here is my protocol (something like HTTP) :

Method : attachment \n
Content-Length : {some-int-value} \n
\r\n
binary data bla bla bla...

lets assume i want to send this message from client to server
by doing so exchanging header section goes pretty well
but reading binary data at the server side never complete and server goes into hang for good

Client side code :

    Socket socket = new Socket();
    SocketAddress address = new InetSocketAddress(SERVER_ADDRESS, SERVER_PORT);
    try {
        socket.connect(address);
        InputStream in = socket.getInputStream();
        OutputStream out = socket.getOutputStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(in, "UTF-8"));
        BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(out, "UTF-8"));
        byte[] data = getSomeBinaryData();
        writer.write("Method : attachment" + "\n");
        writer.write("Content-Length : " + data.length + "\n");
        writer.write("\r\n");
        writer.flush();
        out.write(data); // write binary data
        // do more exchange later
    } catch (IOException ex) {
        // handle exception
    }

Server starter code :

public static void main(String[] args){
    ExecutorService pool = Executors.newFixedThreadPool(50);
    try (ServerSocket server = new ServerSocket(PORT_NUMBER)) {
        while (true) {
            try {
                Socket connection = server.accept();
                Callable<Void> task = new ClientTask(connection);
                pool.submit(task);
            } catch (IOException ex) {}
        }
    } catch (IOException ex) {
        System.err.println("Couldn't start server");
    }
}

Server Task thread for each client :

class ClientTask implements Callable<Void> {

private Socket connection;
private HashMap<String, String> header = new HashMap<>();
private byte[] content;

ClientTask(Socket c) {
    this.connection = c;
}

@Override
public Void call() throws Exception {
    InputStream in = connection.getInputStream();
    BufferedReader reader = new BufferedReader(new InputStreamReader(in, "UTF-8"));
    readHeader(reader);
    System.out.println("incoming message : " + header.get("Method"));
    int contentLength = Integer.parseInt(header.get("Content-Length"));
    content = new byte[contentLength];
    int bytesRead = in.read(content, 0, contentLength);
    System.out.print(bytesRead);
    return null;
}

private void readHeader(BufferedReader reader){
    try {
        char c;
        StringBuilder builder = new StringBuilder();
        while ((c = (char) reader.read()) != '\r'){
            if(c == '\n'){
                String line = builder.toString();
                line = line.replaceAll(" ", "");
                String[] sections = line.split(":");
                header.put(sections[0], sections[1]);
                builder = new StringBuilder();  // clear builder
            }else {
                builder.append(c);
            }
        }
        reader.read();  // skip the last \n character after header
    } catch (IOException e) {
        e.printStackTrace();
    }
}