Java Server Client communication seems to stuck

Question

Hello stackoverflow community,

i am stuck at a problem regarding socket communication in Java.

Here is the sample code of my Server and Client class:

Server:

public class OTPServer {


static ServerSocket serverSocket;
final static int PORT = 4242;
static Socket clientConnection;

public static void main(String[] args) {

    try {
        serverSocket = new ServerSocket(PORT);
        System.out.println("Socket initialized");
        String serverMessage = "Hello, I am the Host";
        ServerTool serverTool = new ServerTool();

        while (true) {
            clientConnection = serverSocket.accept();
            if(clientConnection.isConnected()) {
                System.out.println("Client connected");
            }

            BufferedReader clientInputReader = new BufferedReader(new InputStreamReader(clientConnection.getInputStream()));
            DataOutputStream serverOutput = new DataOutputStream(clientConnection.getOutputStream());
            System.out.println("Sending message to client: " + serverMessage);
            serverOutput.writeBytes(serverTool.encodeMessage(serverMessage));
            serverOutput.flush();
            String clientMessage = clientInputReader.readLine();
            System.out.println("Encoded answer from client: " + clientMessage);
            String decodedMessage = serverTool.decodeMessage(clientMessage);
            System.out.println("Decoded answer from client: " + decodedMessage);

            serverOutput.close();
            clientInputReader.close();

        }
    } catch (IOException e) {
        e.printStackTrace();
    }
    System.out.println("Hello, I am the OTP Server!");

}

Here is the Client:

public class OTPClient {

static Socket clientSocket;
final static int PORT = 4242;
final static String HOST = "localhost";


public static void main(String[] args) {
    System.out.println("I am the OTP Client!");
    String serverMessage;
    String clientResponse = "I am the Client";
    OTPTool otpTool = new OTPTool();

    try {

        clientSocket = new Socket(HOST, PORT);
        BufferedReader serverInput = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        DataOutputStream outputStream = new DataOutputStream(clientSocket.getOutputStream());
        System.out.println("Connection to Host established");
        serverMessage = serverInput.readLine();
        System.out.println("Encoded Message from Server: " + serverMessage);
        String decodedMessage = otpTool.decodeMessage(serverMessage);
        System.out.println("Decoded message from Server: " + decodedMessage);
        System.out.println("Answering with own message: " + clientResponse);
        outputStream.writeBytes(clientResponse);
        outputStream.flush();
        outputStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

Now where is my problem:

The connection establishes and the Server seems to send its message to the client and waits for a answer. The Client does not print the message he got from the Server. As soon as i cancel the Server the client prints the message it gets from the server as well as the information, that the answer is send end exits with exit code 0 so it seems that this part is fine it just is stuck somehow. I already tried to flush the outputStream as you see in the example code given.

Is there something obvious im missing? I know, this is really basic stuff but its my first time using sockets for communication.

Thank you in advance!

Best Regards, Ronny

Btw: i know that the server only connects to one client requesting a connection. Thats absolutely sufficient for my use.

Definitely a deadlock / concurrency issue. In which order do you run each of these classes? — vontell
– vontell, Commented Jun 19, 2016 at 13:12
You are calling readLine. Is your serverTool.encodeMessage including a line break at the end of the message? — Brett Okken
– Brett Okken, Commented Jun 19, 2016 at 13:21
@BrettOkken at the moment the serverTool.encodeMessage does only return the parameter string. @vontell at first i start the server and then the client which should be no problem as the server is waiting on a client to connect to it. — LongRon
– LongRon, Commented Jun 19, 2016 at 13:31

Brett Okken · Accepted Answer · 2016-06-19 13:44:57Z

1

It is getting stuck because serverInput.readLine(); blocks until either a line break or end of file is encountered. On the server side, you are not sending a line break, so the client blocks.

answered Jun 19, 2016 at 13:44

Brett Okken

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2 Comments

LongRon Over a year ago

Thank you! I did not know, that a regular String return value doesnt include neither of them. Works now perfektly. Is there a way to read the input without a linebreak or end of file?

Brett Okken Over a year ago

Sure. Don't call readLine. Use one of the other read methods. This requires that you define a "protocol" which defines the start/stop of messages.

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