TCP Java client sending Data to a C++ Server

Question

I got a Server with a ClientHandle which accepts requests if there are some and i'd like to send data from Java to it. (the C++ client does work without any issues)

I have read the other posts about this but they didnt help me in any way. Post1,Post2,Post3

bool ClientHandle::getData(std::shared_ptr<std::string> ptr)
{
    int n;
    ptr->clear();

    char size[4];
    n = recv(m_sock, size, sizeof(size), 0);

    //check if retval passed
    if (!checkRetValRecv(n))
        return false;

    //calc how much data
    auto msgLen = atoi(size);
    LOG_INFO << "recv a message with size: " << msgLen;

    //create buffer for the data
    auto buffer = new char[msgLen];

    //read buffer
    n = recv(m_sock, buffer, msgLen, 0);

    //check retval if it passes
    if (!checkRetValRecv(n))
        return false;
    ptr->append(buffer, msgLen);
    LOG_INFO << "Message: " << *ptr;

    delete[] buffer; 
    return true;
}
bool ClientHandle::checkRetValRecv(const int& n)
{
    if (n == SOCKET_ERROR)
    {
        size_t err = WSAGetLastError();
        if (err == WSAECONNRESET)
        {
            LOG_ERROR << "Failed to recv! Disconnecting from client: " << "Connection reset by peer.";
        }
        else
        {
            LOG_ERROR << "Disconnecting from client. WSAError: " << err;
        }
        m_connected = false;
        return false;
    }

    if (n == 0)
    {
        LOG_INFO << "Client disconnected.";
        m_connected = false;
        return false;
    }
    return true;
}

Now i would like to write a Java client for this to send data but i cant even send the size of the incoming data.

public class JClient{
    String m_host;
    int m_port;
    Socket m_sock;
    OutputStreamWriter m_out; 
    BufferedReader m_in;

    public JClient(String host, int port)
    {
        m_host = host;
        m_port = port;
    }

    private void connect() throws IOException
    {
        m_sock = new Socket(m_host, m_port);
        m_out = new OutputStreamWriter(m_sock.getOutputStream());
        m_in = new BufferedReader(new InputStreamReader(m_sock.getInputStream()));
    }

    private void close() throws IOException
    {
        m_sock.close();
    }

    public void send(String json) throws IOException {
        this.connect();
        m_out.write(json.length());
        m_out.flush();
        m_out.write(json);
        m_out.flush();

        //wait for result
        m_in.read();
        this.close();
    }
}

If i try to send Data like so:

JClient client = new JIMClient("localhost", 6060);
String toSend = "{\"hello\":\"someJson\"}";
try {
    client.send(toSend);
} catch (IOException e) {
    e.printStackTrace();
}

I do not receive anything usefull. recv a message with size: 0 , Client disconnected.

The send method does not work. I already tried other Streams and using a different encoding, which i guess is the reason for this. So the Question is how do i resolve this issue with the encoding here? I'd like to leave the C++ client as it is if possible or get it to some kind of standart.

Answer 1

Java code is only sending only a single character for the length but C++ expects a 4 byte string. You might try

m_out.write(String.format("%4d%s",json.length(),json));
m_out.flush();