“”Connection reset“” error after connected to second client in instant messaging application

Question

It seems like the connection is reset after the second client is connected

This is the first client, I have commented most of the line to make is clearer

    import java.awt.BorderLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.Socket;

    import javax.swing.JFrame;
    import javax.swing.JTextArea;
    import javax.swing.JTextField;
    import javax.swing.SwingUtilities;


    public class ClientOneGui extends JFrame {

        private static final long serialVersionUID = 1L;
        JTextField userInput = new JTextField();
        JTextArea chatHistory = new JTextArea();

        Socket cS; // Socket to connect to Server
        ObjectOutputStream output; // To send messages to server, server sends to clientTwo(another client)
        ObjectInputStream input; // To receive messages from server, message sent to server from clientTwo(another client)

        String message;// Variable to hold messages sent from server, which is from another client

        //Constructor
        public ClientOneGui() {
            super("ClientOne"); //Window Title
            add(userInput, BorderLayout.SOUTH); // add TextField for user input
            add(chatHistory); // add TextArea for chat history

            // Detects enter pressed by user inside the TextField 
            // then run the SendMessage method
            userInput.addActionListener(
                new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        SendMessage(e.getActionCommand());
                        userInput.setText("");
                    }
                }
            );

            //Housekeeping stuff
            this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            this.setVisible(true);
            this.setLocationRelativeTo(null);
            this.setSize(300, 400);
        }

        //This is the only method called in the main class, after the constructor
        //this method runs the other three methods inside it
        public void StartClient() {
                ConnectToServer();
                SetupStreams();
                ReceiveMessage();
        }

        //Connect to server
        public void ConnectToServer() {
            try{
                appendString("Waiting for connection"); // appendString method is created down below
                cS = new Socket("127.0.0.1",45678);
                appendString("Connected to server!");
            } catch(IOException e) {
                System.out.println("ConnectToServer method");
            }
        }

        //Setup output and input streams
        public void SetupStreams() {
            try {
                output = new ObjectOutputStream(cS.getOutputStream());
                output.flush();
                input = new ObjectInputStream(cS.getInputStream());
                appendString("Start chatting!");
            } catch (IOException e) {
                System.out.println("SetupStreams ClientOne Method");
            }
        }

        //Send message when enter is pressed
        public void SendMessage(String str) {
            try {
                output.writeObject(str); // Write message to server
                appendString(str);
            } catch (IOException e) {
                System.out.println("SendMessage ClientOne Method");
            }   
        }

        //Keep receiving message while connected
        public void ReceiveMessage() {
            do{
                try {
                    message = (String) input.readObject(); // Read the message from server
                    appendString(message);
                } catch (IOException | ClassNotFoundException e) {
                    System.out.println("Client One connection lost");
                }
            }while(cS.isConnected());
        }

        //Add text to chatHistory
        public void appendString(final String string) {
            SwingUtilities.invokeLater(new Runnable(){
                public void run() {
                    chatHistory.append(">"+string+"\n");
                }
            });
        }

    }

This is the second client, 98% identical to the first one

    import java.awt.BorderLayout;
    import java.awt.event.ActionEvent;
    import java.awt.event.ActionListener;
    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.Socket;

    import javax.swing.JFrame;
    import javax.swing.JTextArea;
    import javax.swing.JTextField;
    import javax.swing.SwingUtilities;

    public class ClientTwoGui extends JFrame {

        /**
         * 
         */
        private static final long serialVersionUID = 1L;
        JTextField userInput = new JTextField();
        JTextArea chatHistory = new JTextArea();

        Socket cS; // Socket to connect to Server
        ObjectOutputStream output; // To send messages to server, server sends to clientOne(another client)
        ObjectInputStream input; // To receive messages from server, message sent to server from clientOne(another client)

        String message; // Variable to hold messages sent from server, which is from another client

        //Constructor
        public ClientTwoGui() {
            super("ClientTwo"); //Window Title
            add(userInput, BorderLayout.SOUTH); // add TextField for user input
            add(chatHistory); // add TextArea for chat history

            // Detects enter pressed by user inside the TextField 
            // then run the SendMessage method
            userInput.addActionListener(
                new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
                        SendMessage(e.getActionCommand());
                        userInput.setText("");
                    }
                }
            );

            //Housekeeping stuff
            this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            this.setVisible(true);
            this.setLocationRelativeTo(null);
            this.setSize(300, 400);
        }

        //This is the only method called in the main class, after the constructor
        //this method runs the other three methods inside it
        public void StartClient() {
            ConnectToServer();
            SetupStreams();
            ReceiveMessage();
        }

        //Connect to server
        public void ConnectToServer() {
            try{
                appendString("Waiting for connection");
                cS = new Socket("127.0.0.1",45678);
                appendString("Connected to server!");
            } catch(IOException e) {
                e.printStackTrace();
            }
        }

        //Setup output and input streams
        public void SetupStreams() {
            try {
                output = new ObjectOutputStream(cS.getOutputStream());
                output.flush();
                input = new ObjectInputStream(cS.getInputStream());
                appendString("Start chatting!");
            } catch (IOException e) {
                System.out.println("SetupStreams ClientTwo Method");
            }
        }

        //Send message when enter is pressed
        public void SendMessage(String str) {
            try {
                output.writeObject(str); // Write message to server
                appendString(str);
            } catch (IOException e) {
                System.out.println("SendMessage ClientTwo Method");
            }
        }

        //Keep receiving message while connected
        public void ReceiveMessage() {
            do{
                try {
                    message = (String) input.readObject(); // Read the message from server
                    appendString(message);
                } catch (IOException | ClassNotFoundException e) {
//=====================================================================================
                    System.out.println("Client Two connection lost"); // <<<<<Error
//=====================================================================================             
                }
            }while(cS.isConnected());
        }

        //Append string to chatHistory
        public void appendString(final String string) {
            SwingUtilities.invokeLater(new Runnable(){
                public void run() {
                    chatHistory.append(">"+string+"\n");
                }
            });
        }
    }

This is the server side, which sits in between the two clients, sends and receive messages

    import java.io.IOException;
    import java.io.ObjectInputStream;
    import java.io.ObjectOutputStream;
    import java.net.ServerSocket;
    import java.net.Socket;

    public class TheServer {

        //One server socket, two client sockets
        ServerSocket sS;
        Socket cSOne;
        Socket cSTwo;

        //The streams
        ObjectOutputStream outputOne; //input Stream for first client
        ObjectInputStream inputOne; //output Stream for first client
        ObjectOutputStream outputTwo; //input Stream for second client
        ObjectInputStream inputTwo;//output Stream for second client

        //The messages
        String messageOne; // messages from client one
        String messageTwo; // messages from client two

        //This is the only method called
        public void StartServer() {
            System.out.println("Server Started");
            try {
                        sS = new ServerSocket(45678);
                        WaitForClient();
                        SetupStreams();
            } catch (IOException e) {
                System.out.println("StartServer Method " + e);
            }
        }

        //Connect to client
        public void WaitForClient() {
            try{
                cSOne = sS.accept();
                System.out.println("Connected to client one");
                cSTwo = sS.accept();
                System.out.println("Connected to client Two");
            } catch(IOException e) {
                System.out.println("WaitForClient Method");
            }
        }

        //Setup output and input streams
        public void SetupStreams() {
            try {
                outputOne = new ObjectOutputStream(cSOne.getOutputStream());
                outputOne.flush();
                inputOne = new ObjectInputStream(cSOne.getInputStream());
                System.out.println("Client One streams setup");

                outputTwo = new ObjectOutputStream(cSTwo.getOutputStream());
                outputTwo.flush();
                inputTwo = new ObjectInputStream(cSTwo.getInputStream());

                System.out.println("Client Two streams setup");
            } catch (IOException e) {
                System.out.println("SetupStreams Method");
            }
        }

        //Keep receiving messages while both clients are connected
        public void ReceiveMessage() {
            do{
                try {
                    messageOne = (String)inputOne.readObject();
                    messageTwo = (String)inputTwo.readObject();
                    outputTwo.writeObject(messageOne);
                    System.out.println(messageOne);
                    outputOne.writeObject(messageTwo);
                    System.out.println(messageTwo);
                } catch (ClassNotFoundException | IOException e) {
                    System.out.println("SendReceiveMessage Method " + e);
                }
            }while(!messageOne.equals(null)||!messageTwo.equals(null));
        }

        //Keep sending messages while both clients are connected
        public void SendMessage() {
            do{
                try {
                    outputTwo.writeObject(messageOne);
                    System.out.println(messageOne);
                    outputOne.writeObject(messageTwo);
                    System.out.println(messageTwo);
                } catch (IOException e) {
                    System.out.println("SendReceiveMessage Method " + e);
                }
            }while(!messageOne.equals(null)||!messageTwo.equals(null));
        }

    }

From what I understand, the connection didn't kept alive after both clients are connected, I have read that the connections aren't suppose to be closed until they are told to. I don't know what makes it closed by itself.

The error I received is at line 101 in client two codes, meaning the connection is closed after the streams are created.

public class ServerMain {
    public static void main(String[] args) {
        TheServer sc = new TheServer);
        sc.StartServer();
    }
}

Answer 1

Server executes StartServer() and terminates, closing the sockets:

public void StartServer() {
        System.out.println("Server Started");
        try {
                    sS = new ServerSocket(45678);
                    WaitForClient();
                    SetupStreams();
        } catch (IOException e) {
            System.out.println("StartServer Method " + e);
        }
    }

I think you are missing to call ReceiveMessage() method, perhaps it should be done in StartServer() after calling to SetupStreams() :

public void StartServer() {
        System.out.println("Server Started");
        try {
                    sS = new ServerSocket(45678);
                    WaitForClient();
                    SetupStreams();
                    ReceiveMessage();
        } catch (IOException e) {
            System.out.println("StartServer Method " + e);
        }
    }