java applet: JProgressBar updating/refreshing only at completion

Question

I'm just trying to iterate in for loop and updating values of JProgressBar inside the loop, initiating SwingUtilities.invokeLater.. but it's only updating after completion. Below is my simple code:

import java.awt.Color;
import java.awt.Rectangle;
import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.JProgressBar;
import javax.swing.SwingUtilities;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class ProgressBar extends JApplet {
    JProgressBar pg = null;
    JPanel panel = null;
    JButton start = null;
    int i;

    public void init() {
        pg = new JProgressBar(0, 100);
        pg.setStringPainted(true);
        this.setSize(1200, 600);
        this.setLayout(null);
        panel = new JPanel();
        panel.setLayout(null);
        panel.setBackground(Color.white);
        this.setContentPane(panel);
        this.setVisible(true);
        pg.setBounds(new Rectangle(630, 20, 150, 30));
        add(pg);


        start = new JButton("start");
        start.setBounds(new Rectangle(330, 20, 200, 30));
        start.setBackground(Color.lightGray);
        panel.add(start);

        start.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                pg.setValue(0);
                pg.setStringPainted(true);

                for (i = 0; i <= 100; i = i + 1) {
                    SwingUtilities.invokeLater(new Runnable() {
                        public void run() {
                            try {
                                pg.setValue(i);
                                java.lang.Thread.sleep(100);
                            } catch (Exception e) {}
                        }
                    });
                }
            }
        });
    }
}

How to reflect the updation of progress bar in incrementing way as it's updating only 100% after for loop completion.

`}});}}});}}` There is no need for that level of 'shortening source'. Readability is better. Please use a consistent and logical indent for code blocks. — Andrew Thompson, Jun 29 '12 at 11:43

score 6 · Accepted Answer · answered Jun 29 '12 at 11:40

You have to process any calculation/handling stuff in a separate thread and update UI within the Event Dispatch Thread (EDT):

    start.addActionListener ( new ActionListener ()
    {
        public void actionPerformed ( ActionEvent e )
        {
            pg.setValue ( 0 );
            pg.setStringPainted ( true );
            new Thread ( new Runnable ()
            {
                public void run ()
                {
                    for ( i = 0; i <= 100; i = i + 1 )
                    {
                        SwingUtilities.invokeLater ( new Runnable ()
                        {
                            public void run ()
                            {
                                pg.setValue ( i );
                            }
                        } );
                        try
                        {
                            java.lang.Thread.sleep ( 100 );
                        }
                        catch ( Exception e )
                        {
                            //
                        }
                    }
                }
            } ).start ();

        }
    } );

Any action/mouse/key and other listeners methods are always executed within the EDT thread, so you have to move your code into a separate thread. Otherwise you will do all the calculations/handling inside the EDT thread (and that causes the UI stuck).

@MikleGarin Thanks a lot for explanation and code..it worked very well! Thanks again .. — Ashish Kataria, Jun 30 '12 at 09:30

score 4 · Answer 2 · answered Jun 29 '12 at 11:43

You are blocking the EDT (Event Dispatching Thread) which is why the progress bar cannot update itself. Your for-loop will push 100 InvocationEvent's on the EventQueue. These 100 events will occur in the order you put them on the queue. When you update the value, a repaint event is pushed on the EventQueue, but that repaint event will only occur after all your InvocationEvent's have been performed.

Never put a Thread.sleep inside the EDT (it blocks the UI and gives a poor user experience)
Put your long-running operations in a SwingWorker
Or use a Swing timer for repetitive tasks

java applet: JProgressBar updating/refreshing only at completion

2 Answers2