Reputation: 71

Socket problem - readline won't work properly

I try to code a client and server connection using socket. The problem is my client can't read the response from the server (it hangs on the readline).

Here is some of the code.

Server:

    try {
        // Create the server socket.
        portNumber = Integer.parseInt(myParam.get("socket.portNumber"));
        System.out.println(portNumber);
        mainSocket = new ServerSocket(portNumber);

    } catch (IOException ioe) {
        System.out.println("Error Message : "+ioe.getMessage());
    }

    while(true)
    {     
        try
        {
            // Accept connections
            Socket clientSocket = mainSocket.accept();
            SocketServerThread st = new SocketServerThread (clientSocket);
            st.start();
        }
        catch(IOException ioe)
        {
            System.out.println("Error message :"+ioe.getMessage());
        }
    }

The Thread:

public void run() {

    BufferedReader in = null;
    PrintWriter out = null;
    String clientResponse = null;

    try {
        in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        out = new PrintWriter(new OutputStreamWriter(clientSocket.getOutputStream()));

        //Read The Message
        String clientRequest = in.readLine();
        System.out.println("Message recieved : " + clientRequest);

        //Process the message

        // Send response
        out.println(clientResponse+"\n");
        out.flush();
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        // Clean up
        try {
            in.close();
            out.close();
            clientSocket.close();
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }

The client:

    try {
        // Create the server socket.
        simSocket = new Socket("192.168.52.27", portNumber);
    } catch (IOException ioe) {
        System.out.println("Error Message : " + ioe.getMessage());
    }
    BufferedReader in = null;
    PrintWriter out = null;
    try {
        in = new BufferedReader(new InputStreamReader(simSocket.getInputStream()));
        out = new PrintWriter(new OutputStreamWriter(simSocket.getOutputStream()));

            out.write("My message");
            out.flush();

            do{
            response = in.readLine(); //This is where the code hang
            }while (response.length()<= 0);

            System.out.print(response);

    } catch (IOException ioe) {
        System.out.println("Error message :" + ioe.getMessage());
    } finally {
        try {
            in.close();
            out.close();
            simSocket.close();
        } catch (IOException ioException) {
            ioException.printStackTrace();
        }
    }

Could you guys tell me what's the problem? Thank you very much for any help

Upvotes: 1

Answers (5)

Willy

Reputation: 71

OK, I made several editing in the code and now it run nicely :

The server :

    try {
        // Create the server socket.
        portNumber = Integer.parseInt(myParam.get("socket.portNumber"));
        mainSocket = new ServerSocket(portNumber);

    } catch (IOException ioe) {
        System.out.println("Error Message : " + ioe.getMessage());
    }

    // Accept connections    
    try {
        clientSocket = mainSocket.accept();
    } catch (IOException ioe) {
        System.out.println("Error Message : " + ioe.getMessage());
    }
    BufferedReader in = null;
    PrintWriter out = null;

    while (true) {
        try {
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            out = new PrintWriter(new OutputStreamWriter(clientSocket.getOutputStream()));
            //Read The Message
            **StringBuffer buffer = new StringBuffer();
            while (true) {
                int ch = in.read();
                if ((ch < 0) || (ch == '\n')) {
                    break;
                }
                buffer.append((char) ch);
            }
            String clientRequest = buffer.toString();**

            SocketServerThread st = new SocketServerThread(clientRequest, out);
            st.start();
        } catch (IOException ioe) {
            System.out.println("Can't accept connection. Error message :" + ioe.getMessage());
        }
    }

I change the readline with read and it work, so the assumption that "\n" is the problem is correct.

The thread : a minor change in the thread (remove the reading request part since I already done that in the server)

The Client: change the readline into read just like the server one.

Thank you all for the help

Upvotes: 1

user207421

Reputation: 311054

out.write("My message");

That doesn't send a line terminator so it can never be read. Use println().

out.println(clientResponse+"\n");

That will send the clientResponse plus a newline plus a \n. The last part will probably be interpreted as a blank line. Not much point in that. Remove the \n.

do{
    response = in.readLine(); //This is where the code hang
}while (response.length()<= 0);

That's not the correct way to read lines. It will get an NPE at EOS; the response length can never be negative; and why would you send yourself blank lines? The correct way is this:

while ((response = in.readLine()) != null) {
    // if (response.length() == 0) continue; // if you must
    // process the line
}
// when you get here EOS has occurred.

Upvotes: 0

Sergei Tachenov

Reputation: 24919

Okay, I've figured this one out. It's not the client that hangs, it's the server. It tries to read a line of text from the client, but the client doesn't send the line separator:

    out.write("My message");
    out.flush();

Replace write() with println() here.

Upvotes: 9

Andreas Dolk

Reputation: 114837

According to the javaDoc, the server response actually is

"My Message:\n"+System.getProperty("line.separator")

I bet, in.readLine() works fine at least once - but you just ignore the response, because the print command is outside the loop. Move that one up, and you should see the responses on the console.

There is a (very small) chance, that the servers println() doesn't really send a \n char. So you could try this at the thread code:

 out.print(clientResponse+"\n\n");  // exchanged println with an extra \n char

Upvotes: -1

Raghuram

Reputation: 52665

Could it be because clientResponse (sent by server thread) is null and the client is waiting for response size > 0?