JProgressBar not updating

问题

I have made a very simple code to show it here, i have a button that should show a JDialog to check the progress status, i am using the invoke late to go through EDT and my loop isn't in the run method, so why isn't my bar updating ? here is the code

import javax.swing.JButton;
import javax.swing.JDialog;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JProgressBar;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class JBarEx extends JFrame {
private JTextField progStatus = new JTextField("Undefined");
private JButton dialogBtn = new JButton("Show Progression dialog");
final JDialog dlg = new JDialog((JFrame) null, "prog Title", false);
final JProgressBar dpb = new JProgressBar(0, 100);

public JBarEx() {
    JPanel pan = new JPanel();
    dialogBtn.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
            showProgress();
        }
    });
    progStatus.setEditable(false);
    pan.add(progStatus);
    pan.add(dialogBtn);
    setContentPane(pan);
    this.setSize(200, 100);
    setVisible(true);
}

public void showProgress() {
    dlg.add(BorderLayout.CENTER, dpb);
    dlg.add(BorderLayout.NORTH, new JLabel("prog message"));
    dlg.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
    dlg.setSize(300, 75);
    dlg.setLocationRelativeTo(null);
    dlg.setVisible(true);

    for (int i = 0; i < 100; i++) {
        final int ii = i;
        try {
            Thread.sleep(25);
            SwingUtilities.invokeLater(new Runnable() {

                @Override
                public void run() {
                    updateBar(ii);

                }
            });
        }
        catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

public void updateBar(int newValue) {
    dpb.setValue(newValue);
}

public static void main(String[] args) {
    JBarEx jbx = new JBarEx();
}

}

回答1:

Your showProgress method is being executed within the context of the Event Dispatching Thread. The EDT is responsible for, amongst other things, processing paint requests. This means that so long as your for-loop is executing, the EDT can not process any new paint requests (or handle the invokeLater events either) as it is blocking the EDT.

While there are any number of possible ways to solve the problem, based on your code example, the simplest would be to use a SwingWorker.

It has the capacity to allow your to execute the long running task the a background thread (freeing up the EDT), but also allows you means for publishing updates (if required) so that they can be processed in the EDT and also provides handy progress notification.

For example...

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JProgressBar;
import javax.swing.SwingWorker;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.border.EmptyBorder;

public class SwingWorkerProgress {

    public static void main(String[] args) {
        new SwingWorkerProgress();
    }

    public SwingWorkerProgress() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private JProgressBar pbProgress;
        private JButton start;

        public TestPane() {

            setBorder(new EmptyBorder(10, 10, 10, 10));
            pbProgress = new JProgressBar();
            setLayout(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.insets = new Insets(4, 4, 4, 4);
            gbc.gridx = 0;
            gbc.gridy = 0;
            add(pbProgress, gbc);

            start = new JButton("Start");
            gbc.gridy++;
            add(start, gbc);

            start.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    start.setEnabled(false);
                    ProgressWorker pw = new ProgressWorker();
                    pw.addPropertyChangeListener(new PropertyChangeListener() {

                        @Override
                        public void propertyChange(PropertyChangeEvent evt) {
                            String name = evt.getPropertyName();
                            if (name.equals("progress")) {
                                int progress = (int) evt.getNewValue();
                                pbProgress.setValue(progress);
                                repaint();
                            } else if (name.equals("state")) {
                                SwingWorker.StateValue state = (SwingWorker.StateValue) evt.getNewValue();
                                switch (state) {
                                    case DONE:
                                        start.setEnabled(true);
                                        break;
                                }
                            }
                        }

                    });
                    pw.execute();
                }
            });

        }
    }

    public class ProgressWorker extends SwingWorker<Object, Object> {

        @Override
        protected Object doInBackground() throws Exception {

            for (int i = 0; i < 100; i++) {        
                setProgress(i);
                try {
                    Thread.sleep(25);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }

            return null;
        }
    }
}

Check out Concurrency in Swing for more details

回答2:

Even if you fix the loop as others have pointed out, you'd still block the event dispatch thread. The for loop is run in showProgress() which is called from an event listener. The updates are pushed to the event queue, but that does not get processed until the loop has completed.

Use a Swing Timer instead. Something like this:

Timer timer = new Timer(25, new ActionListener() {
    private int position;

    @Override
    public void actionPerformed(ActionEvent e) {
         position++;
         if (position < lastPosition) {
             updateBar(position);
         } else {
             ((Timer) e.getSource).stop();
         }
    }
});
timer.start();

where lastPosition would be the state where you want the progress bar to stop.

Unrelated to that bug, but a bug still, you should not create swing components outside the event dispatch thread. It's best to do it right from the start:

public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            JBarEx jbx = new JBarEx();
        }
    });
}