Java simple TCP client/server test freezes

Question

I'm trying to get how to make a java server/client program but I don't understand why it doesn't work, my server's interface isn't displayed even though I run the loop in a separate thread and I managed to get port errors at some point, I don't even know what to try anymore, so here's the code: (I executed both programs on one single computer)

what am I doing wrong?

client:

    import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.Socket;
import java.net.UnknownHostException;

import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;


public class Client extends JFrame {

String sentence;
String modifiedSentence;
int port = 6791;

JTextField tf = new JTextField();
JButton valider = new JButton("Send");
JLabel retour = new JLabel("nothing");

Socket clientSocket;
DataOutputStream outToServer;
BufferedReader inFromServer;


public Client() throws UnknownHostException, IOException{   

    this.setSize(200, 130);
    this.setDefaultCloseOperation(EXIT_ON_CLOSE);
    this.setTitle("Client");
    this.setResizable(false);




    JPanel zoneclient = (JPanel)getContentPane();

    Dimension d = new Dimension(300 , 25);
    tf.setPreferredSize(d);
    tf.setMaximumSize(d);
    retour.setPreferredSize(d);
    valider.setPreferredSize(d);

    zoneclient.setLayout(new BoxLayout(zoneclient, BoxLayout.Y_AXIS));
    zoneclient.add(new JLabel("Client"));
    zoneclient.add(tf); 
    zoneclient.add(retour);
    zoneclient.add(valider);



    valider.addActionListener(new appActionListener());






}

class appActionListener implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub
        sentence = tf.getText();

        try {

            BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));



            clientSocket = new Socket("localhost", port);
            clientSocket.setReuseAddress(true);

            outToServer = new DataOutputStream(clientSocket.getOutputStream());
            inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        } catch (Exception f) {
            // TODO: handle exception
            System.out.println("connexion failed");
        }

        try {
            outToServer.writeBytes(sentence + '\n');
            modifiedSentence = inFromServer.readLine();
            retour.setText(modifiedSentence);
            clientSocket.close();
        } catch (IOException e1) {
            // TODO Auto-generated catch block
                e1.printStackTrace();
                tf.setText("error");
            }

        }

}

}

and here's the server:

    package server;

import java.awt.Dimension;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;

import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

class MyServer extends JFrame{

    ServerSocket welcomeSocket;

    String clientSentence;
    String capitalizedSentence;
    JLabel retour = new JLabel("nothing");

    public MyServer() throws IOException{

        System.out.println("server started");
        this.setSize(200, 130);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setTitle("Server");
        this.setResizable(false);

        JPanel zoneclient = (JPanel)getContentPane();

        Dimension d = new Dimension(300 , 25);

        retour.setPreferredSize(d);

        zoneclient.setLayout(new BoxLayout(zoneclient, BoxLayout.Y_AXIS));
        zoneclient.add(new JLabel("Server"));
        zoneclient.add(retour);


        welcomeSocket = new ServerSocket(6791);
        welcomeSocket.setReuseAddress(true);
        MyThread th = new MyThread();
        th.run();
    }


    class MyThread extends Thread{
        public void run(){
            while (true) {
                Socket connectionSocket;
                try {
                    connectionSocket = welcomeSocket.accept();
                    BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
                    DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
                    clientSentence = inFromClient.readLine();
                    retour.setText("Received: " + clientSentence);
                    capitalizedSentence = clientSentence.toUpperCase();
                    outToClient.writeBytes(capitalizedSentence);
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                    System.out.println("???");
                }
            }
        }
    }
}

Answer 1

The problem is the Client code wait for the new line character \n from server. Since you are calling readLine of the stream.

 modifiedSentence = inFromServer.readLine();

Solution:

Change your server code to write \n too as below.

outToClient.writeBytes(capitalizedSentence+'\n');

Note: I think you don't need the new BufferedReader(new nputStreamReader(System.in)) inside actionPerformed()