Java app can't be closed even "CLOSE ON EXIT", TCP Server

Question

I just wanna make a Server application which gets Strings and put these into a JTextArea. There are two errors I get, even no errors are showed.

1. the window can't be closed although I used this statement:

   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

2. If the client connects to the Server, the whole window turns black. What could be the error? Here the code:

Client:

public Main() {
    super("Main");
    setIconImage(Toolkit.getDefaultToolkit().getImage(Main.class.getResource("/images/ic.png")));

    panelFields = new JPanel();
    panelFields.setLayout(new BoxLayout(panelFields,BoxLayout.X_AXIS));
    panelFields2 = new JPanel();
    panelFields2.setLayout(new BoxLayout(panelFields2,BoxLayout.X_AXIS));

    scrollPane = new JScrollPane();
    panelFields.add(scrollPane);

    getContentPane().add(panelFields);
    getContentPane().add(panelFields2);
    getContentPane().setLayout(new BoxLayout(getContentPane(),BoxLayout.Y_AXIS));
    setSize(326, 264);
    setVisible(true);

    messagesArea = new JTextArea();
    scrollPane.setViewportView(messagesArea);
    messagesArea.setColumns(30);
    messagesArea.setRows(10);
    messagesArea.setEditable(false);

    startServer = new JButton("Start");
    startServer.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            socketConnection();
            startServer.setEnabled(false);
        }
    });

    panelFields.add(startServer);
}

And the Server connection:

private void socketConnection() {
    try {
        serverSocket = new ServerSocket(9090);
        System.out.println("Listening: " + serverSocket.getLocalPort());
    } catch (IOException e) {
        e.printStackTrace();
    }

    while (true) {
        try {
            socket = serverSocket.accept();
            dataInputStream = new DataInputStream(socket.getInputStream());
            System.out.println("ip: " + socket.getInetAddress());
            System.out.println("message: " + dataInputStream.readUTF());
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            if (dataInputStream != null) {
                try {
                    dataInputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

Maybe you could tell me, how I can fix those problems and also, how I can make, that the server doesn't close the socket although the client disconnects. I wanna reconnect maybe later...

Answer 1

You need to start your socket listener in its own thread, and you need to add a window close listener that shuts down that thread.

For example:

private ServerSocket serverSocket = null;
private boolean done = false;

private void startServer() {
    Thread t = new Thread(new Runnable() {
        public void Run() {
            socketConnection();
        });
    }
    t.start();
}

private void socketConnection() {
    try {
        serverSocket = new ServerSocket(9090);
        System.out.println("Listening: " + serverSocket.getLocalPort());

        while (!done) {
            try {
                final Socket socket = serverSocket.accept();
                Thread t = new Thread(new Runnable() {
                    public void Run() {
                        handle(socket);
                    }
                });
                t.start();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

private void handle(Socket socket) {
    if (socket == null) return;
    try {
        dataInputStream = new DataInputStream(socket.getInputStream());
        System.out.println("ip: " + socket.getInetAddress());
        System.out.println("message: " + dataInputStream.readUTF());
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        if (dataInputStream != null) {
            try {
                dataInputStream.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        try {
            socket.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}


public void windowClosing(WindowEvent e) { 
    done = true; 
    socketServer.close(); 
} 

Your button click listener should call startServer(), then your window close function would set done = true and call socketServer.close().

Now you have one thread for the UI, one thread for the socket server, and one thread for each connection to the server.