How to implement a single instance Java application?

Question

Sometime I see many application such as msn, windows media player etc that are single instance applications (when user executes while application is running a new applicatio

Answer 1

public class SingleInstance {
    public static final String LOCK = System.getProperty("user.home") + File.separator + "test.lock";
    public static final String PIPE = System.getProperty("user.home") + File.separator + "test.pipe";
    private static JFrame frame = null;

    public static void main(String[] args) {
        try {
            FileChannel lockChannel = new RandomAccessFile(LOCK, "rw").getChannel();
            FileLock flk = null; 
            try {
                flk = lockChannel.tryLock();
            } catch(Throwable t) {
                t.printStackTrace();
            }
            if (flk == null || !flk.isValid()) {
                System.out.println("alread running, leaving a message to pipe and quitting...");
                FileChannel pipeChannel = null;
                try {
                    pipeChannel = new RandomAccessFile(PIPE, "rw").getChannel();
                    MappedByteBuffer bb = pipeChannel.map(FileChannel.MapMode.READ_WRITE, 0, 1);
                    bb.put(0, (byte)1);
                    bb.force();
                } catch (Throwable t) {
                    t.printStackTrace();
                } finally {
                    if (pipeChannel != null) {
                        try {
                            pipeChannel.close();
                        } catch (Throwable t) {
                            t.printStackTrace();
                        }
                    } 
                }
                System.exit(0);
            }
            //We do not release the lock and close the channel here, 
            //  which will be done after the application crashes or closes normally. 
            SwingUtilities.invokeLater(
                new Runnable() {
                    public void run() {
                        createAndShowGUI();
                    }
                }
            );

            FileChannel pipeChannel = null;
            try {
                pipeChannel = new RandomAccessFile(PIPE, "rw").getChannel();
                MappedByteBuffer bb = pipeChannel.map(FileChannel.MapMode.READ_WRITE, 0, 1);
                while (true) {
                    byte b = bb.get(0);
                    if (b > 0) {
                        bb.put(0, (byte)0);
                        bb.force();
                        SwingUtilities.invokeLater(
                            new Runnable() {
                                public void run() {
                                    frame.setExtendedState(JFrame.NORMAL);
                                    frame.setAlwaysOnTop(true);
                                    frame.toFront();
                                    frame.setAlwaysOnTop(false);
                                }
                            }
                        );
                    }
                    Thread.sleep(1000);
                }
            } catch (Throwable t) {
                t.printStackTrace();
            } finally {
                if (pipeChannel != null) {
                    try {
                        pipeChannel.close();
                    } catch (Throwable t) {
                        t.printStackTrace();
                    } 
                } 
            }
        } catch(Throwable t) {
            t.printStackTrace();
        } 
    }

    public static void createAndShowGUI() {

        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(800, 650);
        frame.getContentPane().add(new JLabel("MAIN WINDOW", 
                    SwingConstants.CENTER), BorderLayout.CENTER);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}