the ReadLine doesnt wait for input

Question

I have a socket client sending text to a socket server but the ReadLine doesnt seem to wait to receive a line before proceeding. Here is the of the server receiving the text:

public void run() {

        try {

            serveurSocket = new ServerSocket(PORT_ID);  
            connexionSocket = serveurSocket.accept();


            BufferedReader reader = new BufferedReader(new InputStreamReader(connexionSocket.getInputStream()));
            PrintWriter writer = new PrintWriter(connexionSocket.getOutputStream(), true);

            messageRecu = "";
            while (true) {
                messageRecu = reader.readLine();
                messageRecu = messageRecu.toUpperCase();
                writer.println(messageRecu);
            }

            //reader.close();
            //writer.close();


        } catch (IOException e) {
            System.out.println(e.getMessage());
            e.printStackTrace();
        }

    }

After establishing the socket between client and server, the execution halts at reader.readLine until I send manually a string thru the socket. Which is normal and wanted. Codes resumes and its fine until its loops back to reader.ReadLine() where it will read a "null" line instead of waiting for input from the socket like it did the first time... this will obviously mess up the next command to uppercase. So how can I fix this?

EDIT: I'll add the client side if that can help understand.

public class ClientSocket {

    private Socket clientSocket;

    public boolean isClosed() { return clientSocket.isClosed(); }

    public boolean connectToSocket (String ip, int port) {

        try {
            clientSocket = new Socket(ip, port);
            return true;
            } 
        catch (IOException e) { 
            System.out.println(e); 
            return false;
        }

    }

    public String sendToServer(String messageClient) {

        String messageRecu = "";

        try {

            BufferedReader reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            PrintWriter writer = new PrintWriter(clientSocket.getOutputStream(), true);

            writer.println(messageClient);
            messageRecu = reader.readLine();

            reader.close();
            writer.close();

            return messageRecu;

        } 
        catch (IOException e) {

            System.out.println(e.getMessage());
            e.printStackTrace();

            return messageRecu;

        }

    }

}

A button press will call "connectTosocket" to initiate the socket. A second button when pressed will send the content of a textfield using "sendToServer".

Server does receive the message and return it capitalized but I wish for the socket to remain open with the server and if I send an other string for the same sequence to happen. Not even sure it can be done :(

Answer 1

According to the documentation of BufferedReader#readLine, a null is returned if the end of stream has been reached.

Change your reading loop to :

while ((messageRecu = reader.readLine()) != null) {
    messageRecu = messageRecu.toUpperCase();
    writer.println(messageRecu);
}
//Get out of the loop when the end of stream is reached.

As per Reading from and Writing to a Socket chapter of the Java tutorial.

As a side note, while(true) loops are not really appreciated.

Answer 2

The "null" signals for end of connection from the client side - which is why the connection disconnects. If you want to support multiple requests, you should run a new ServerSocket.accept() each time and wait for a new client to connect.

KKMultiServer class:

import java.net.*;
import java.io.*;


public class KKMultiServer {
    public static void main(String[] args) throws IOException {

    if (args.length != 1) {
        System.err.println("Usage: java KKMultiServer <port number>");
        System.exit(1);
    }

        int portNumber = Integer.parseInt(args[0]);
        boolean listening = true;

        try (ServerSocket serverSocket = new ServerSocket(portNumber)) { 
            while (listening) {
                new KKMultiServerThread(serverSocket.accept()).start();
            }
        } catch (IOException e) {
            System.err.println("Could not listen on port " + portNumber);
            System.exit(-1);
        }
    }
}

KKMultiServerThread class:

import java.net.*;
import java.io.*;

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

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

    public void run() {

        try (
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                new InputStreamReader(
                    socket.getInputStream()));
        ) {
            String inputLine, outputLine;
            KnockKnockProtocol kkp = new KnockKnockProtocol();
            outputLine = kkp.processInput(null);
            out.println(outputLine);

            while ((inputLine = in.readLine()) != null) {
                outputLine = kkp.processInput(inputLine);
                out.println(outputLine);
                if (outputLine.equals("Bye"))
                    break;
            }
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

You can read more about sockets in Oracle tutorials