GUI is not responding to button clicks

Question

I'm trying to write a simple port scanner in Java. It works, but the entire window stops responding to clicks once the scanning begins. This won't allow me to use the stop button to pause the operation or exit the window. Here is my code:

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;

import javax.swing.*;
import java.awt.event.*;
public class GUIScanner {
    JFrame frame = new JFrame("Port Scanner");
    JTextField textField = new JTextField("",20);
    JPanel panel = new JPanel();
    JButton button = new JButton("Scan");
    JButton button2 = new JButton("Stop");
    JLabel label = new JLabel("");
    boolean stopped = false;
public GUIScanner() {
    initialize();
}

public static void main(String[] args) {
    GUIScanner scanner = new GUIScanner();
}

public void initialize() {
    panel.add(textField);
    panel.add(button);
    panel.add(button2);
    frame.add(panel);
    frame.setSize(500, 500);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    button.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            String address = textField.getText().toString();
            scanHost(address, 200);
        }
    });
    button2.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            if(!stopped) {
                stopped = true;
                button2.setText("Resume");
            } else {
                stopped = false;
                button2.setText("Stop");
            }
        }
    });
}
public void scanHost(String ip, int timeout) {
    while(!stopped) {
        button2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if(!stopped) {
                    stopped = true;
                    button2.setText("Resume");
                } else {
                    stopped = false;
                    button2.setText("Stop");
                }
            }
        });
        for(int port = 0; port <= 65535; port++) {
            frame.setTitle("Scanning port " + port + " of 65535");
            Socket socket = new Socket();
            try {
                socket.connect(new InetSocketAddress(ip, port), timeout);
                socket.close();
                System.out.println(port);
            } catch (IOException e) {

            }
    }
    }
}
}

I'm not sure if the issue is a threading issue or if it is something else in my code. I'm fairly inexperienced with Java. I'm using Eclipse Oxygen if that helps.

Answer 1

Use a SwingWorker to run another thread in the background. This prevents Swing from being blocked when you pause the background thread. Here is the corrected version of the port scanner code:

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Socket;

import javax.swing.*;
import java.awt.event.*;

public class GUIScanner {
    JFrame frame = new JFrame("Port Scanner");
    JTextField textField = new JTextField("", 20);
    JPanel panel = new JPanel();
    JButton button = new JButton("Scan");
    JButton button2 = new JButton("Stop");
    JLabel label = new JLabel("");
    boolean stopped = false;

    PausableSwingWorker<Void, String> scanningWorker;

    abstract class PausableSwingWorker<K, V> extends SwingWorker<K, V> {

        private volatile boolean isPaused;
        final Object lock = new Object();

        public final void pause() {

            if (!isPaused() && !isDone()) {
                isPaused = true;
            }
        }

        public final void resume() {
            if (isPaused() && !isDone()) {
                isPaused = false;
                synchronized(lock) {
                    lock.notify();
                }
            }
        }
        public final boolean isPaused() {
            return isPaused;
        }
    }

    public GUIScanner() {
        initialize();
    }

    public static void main(String[] args) {
        GUIScanner scanner = new GUIScanner();
    }

    public void initialize() {
        panel.add(textField);
        panel.add(button);
        panel.add(button2);
        frame.add(panel);
        frame.setSize(500, 500);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                String address = textField.getText().toString();
                scanHost(address, 200);
            }
        });
        button2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                if (!stopped) {
                    stopped = true;
                    button2.setText("Resume");
                    scanningWorker.pause();
                } else {
                    stopped = false;
                    button2.setText("Stop");
                    scanningWorker.resume();
                }
            }
        });
    }

    public void scanHost(String ip, int timeout) {
        scanningWorker = new PausableSwingWorker<Void, String>() {
            @Override
            public Void doInBackground() {

                for (int port = 0; port <= 65535; port++) {
                    if (!isPaused()) {
                        frame.setTitle("Scanning port " + port + " of 65535");
                        Socket socket = new Socket();
                        try {
                            socket.connect(new InetSocketAddress(ip, port), timeout);
                            socket.close();
                            System.out.println("Port " + port + " is open");
                        } catch (IOException e) {
                        }
                    }
                    else {
                        synchronized(lock) {
                            try {
                                lock.wait();
                            } catch (InterruptedException e) {
                                // TODO Auto-generated catch block
                                e.printStackTrace();
                            }
                        }
                    }
                }
                return null;
            }
            @Override
            public void done() {
            }
        };
    scanningWorker.execute();
    }
}