Question

I am trying to write a client-server system using Sockets in java, however I cannot seem to read data sent from the server to the client.

Here is the code for the client:

public class ClientSocket 
{
    Socket clientSocket = null;
    PrintWriter out = null;
    BufferedReader in = null;


    // establish a connection to All Care's server application through socket 4444 (adjust localhost to reflect the IP address that the server
    // is being run from)
    public ClientSocket()
    {
        try
        {
            clientSocket = new Socket("localhost", 4445);

            out = new PrintWriter(clientSocket.getOutputStream());
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        }
        catch (IOException e)
        {
            System.out.println("Could not connect to All Care Server Application");
        }
    }

    public void closeClientSocket()
    {   
        try
        {
            clientSocket.close();
        }
        catch (IOException e)
        {
            System.out.println("Could not close connection to All Care Server Application");
        }
    }

    public String getMessageFromServer()
    {
        try
        {
            String input = in.readLine();

            return input;
        }
        catch (IOException e)
        {
            System.out.println("Could not read message from server");
        }

        return "No Data";
    }

    public void sendMessageToServer(String message)
    {
        out.write(message);
    }
}

And here is the Server code:

public class ArFileServer {

    public static void main(String[] args) 
    {
        ServerSocket serverSocket = null;
        boolean listening = true;

        try
        {
            serverSocket = new ServerSocket(4445);

            // infinite loop to continually listen for connection requests made by clients
            while (listening)
            {
                new ClientConnection(serverSocket.accept()).start();

                if (serverSocket != null)
                {
                    System.out.println("Connection to client established");
                }
            }

            serverSocket.close();
        }
        catch (IOException e)
        {
            System.out.println("Error could not create socket connection to port");
        }
    }
}

public class ClientConnection extends Thread
{
    private Socket socket = null;

    public ClientConnection(Socket socket) 
    {
        super("ClientConnection");
        this.socket = socket; 
    }

    // the thread that runs after a connection to the server has been accepted
    @Override
    public void run()
    {
        try
        {
            PrintWriter out = new PrintWriter(socket.getOutputStream());
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

            sendMessagetoClient(out, "CONNECTION SUCCESS");

            // check login credentials sent from client to the server

            // if valid send back their encrypted password, otherwise output a login error message

            // wait for user input and then do various processes based on their requests

            in.close();
            out.close();
            socket.close();
        }
        catch (IOException e)
        {
            System.out.println("Client socket connection error");
        }
    }

    // sends a message to the client
    void sendMessagetoClient(PrintWriter out, String message)
    {
        out.write(message);
    } 

    // listens for a message from the client
    String getMessageFromClient(BufferedReader in)
    {      
        try
        {
            String input = in.readLine();
            return input;
        }
        catch (IOException e)
        {
            System.out.println("Could not read message from client");
        }

        return "No Data";
    }

And here is the line of code im using to see if the data is being sent.

System.out.println(clientSocket.getMessageFromServer());

Answer 1

In your sendMessageToClient() method, you need to flush:

void sendMessagetoClient(PrintWriter out, String message)
{
    out.write(message);
    out.flush();
} 

---

Or, when you create the PrintWriter, use the constructor with autoflush:

PrintWriter out = new PrintWriter(socket.getOutputStream(),true);

And when you write, instead of out.write(message) use printf() or println().