How to customize a JProgressBar?

There are a number of ways you might achieve this, one of the better ways would be to create a custom ProgressBarUI delegate which paints itself the way you want, for example…

public class FancyProgressBar extends BasicProgressBarUI {

    @Override
    protected Dimension getPreferredInnerVertical() {
        return new Dimension(20, 146);
    }

    @Override
    protected Dimension getPreferredInnerHorizontal() {
        return new Dimension(146, 20);
    }



    @Override
    protected void paintDeterminate(Graphics g, JComponent c) {

        Graphics2D g2d = (Graphics2D) g.create();

        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

        int iStrokWidth = 3;
        g2d.setStroke(new BasicStroke(iStrokWidth, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
        g2d.setColor(progressBar.getBackground());
        g2d.setBackground(progressBar.getBackground());

        int width = progressBar.getWidth();
        int height = progressBar.getHeight();

        RoundRectangle2D outline = new RoundRectangle2D.Double((iStrokWidth / 2), (iStrokWidth / 2),
                width - iStrokWidth, height - iStrokWidth,
                height, height);

        g2d.draw(outline);

        int iInnerHeight = height - (iStrokWidth * 4);
        int iInnerWidth = width - (iStrokWidth * 4);

        double dProgress = progressBar.getPercentComplete();
        if (dProgress < 0) {
            dProgress = 0;
        } else if (dProgress > 1) {
            dProgress = 1;
        }

        iInnerWidth = (int) Math.round(iInnerWidth * dProgress);

        int x = iStrokWidth * 2;
        int y = iStrokWidth * 2;

        Point2D start = new Point2D.Double(x, y);
        Point2D end = new Point2D.Double(x, y + iInnerHeight);

        float[] dist = {0.0f, 0.25f, 1.0f};
        Color[] colors = {progressBar.getBackground(), progressBar.getBackground().brighter(), progressBar.getBackground().darker()};
        LinearGradientPaint p = new LinearGradientPaint(start, end, dist, colors);

        g2d.setPaint(p);

        RoundRectangle2D fill = new RoundRectangle2D.Double(iStrokWidth * 2, iStrokWidth * 2,
                iInnerWidth, iInnerHeight, iInnerHeight, iInnerHeight);

        g2d.fill(fill);

        g2d.dispose();
    }

    @Override
    protected void paintIndeterminate(Graphics g, JComponent c) {
        super.paintIndeterminate(g, c); //To change body of generated methods, choose Tools | Templates.
    }

}

Normally you might be tempered to register this as the default look and feel delegate of all JProgressBars, but typically, I would install only on those instance of JProgressBar you really wanted it, that comes down to you.

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.LinearGradientPaint;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Point2D;
import java.awt.geom.RoundRectangle2D;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JProgressBar;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.plaf.basic.BasicProgressBarUI;

public class Test {

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

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

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

    public class TestPane extends JPanel {

        public TestPane() {
            setBackground(Color.BLACK);
            JProgressBar fancyPB = new JProgressBar();
            fancyPB.setUI(new FancyProgressBar());

            JProgressBar normalPB = new JProgressBar();

            setLayout(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;

            add(fancyPB, gbc);
            add(normalPB, gbc);

            Timer timer = new Timer(250, new ActionListener() {

                private int count = 0;

                @Override
                public void actionPerformed(ActionEvent e) {
                    fancyPB.setValue(count);
                    normalPB.setValue(count);
                    count++;
                    if (count >= 100) {
                        ((Timer)e.getSource()).stop();
                    }
                }
            });
            timer.start();
        }

    }

    public class FancyProgressBar extends BasicProgressBarUI {

        @Override
        protected Dimension getPreferredInnerVertical() {
            return new Dimension(20, 146);
        }

        @Override
        protected Dimension getPreferredInnerHorizontal() {
            return new Dimension(146, 20);
        }



        @Override
        protected void paintDeterminate(Graphics g, JComponent c) {

            Graphics2D g2d = (Graphics2D) g.create();

            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);

            int iStrokWidth = 3;
            g2d.setStroke(new BasicStroke(iStrokWidth, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
            g2d.setColor(progressBar.getBackground());
            g2d.setBackground(progressBar.getBackground());

            int width = progressBar.getWidth();
            int height = progressBar.getHeight();

            RoundRectangle2D outline = new RoundRectangle2D.Double((iStrokWidth / 2), (iStrokWidth / 2),
                    width - iStrokWidth, height - iStrokWidth,
                    height, height);

            g2d.draw(outline);

            int iInnerHeight = height - (iStrokWidth * 4);
            int iInnerWidth = width - (iStrokWidth * 4);

            double dProgress = progressBar.getPercentComplete();
            if (dProgress < 0) {
                dProgress = 0;
            } else if (dProgress > 1) {
                dProgress = 1;
            }

            iInnerWidth = (int) Math.round(iInnerWidth * dProgress);

            int x = iStrokWidth * 2;
            int y = iStrokWidth * 2;

            Point2D start = new Point2D.Double(x, y);
            Point2D end = new Point2D.Double(x, y + iInnerHeight);

            float[] dist = {0.0f, 0.25f, 1.0f};
            Color[] colors = {progressBar.getBackground(), progressBar.getBackground().brighter(), progressBar.getBackground().darker()};
            LinearGradientPaint p = new LinearGradientPaint(start, end, dist, colors);

            g2d.setPaint(p);

            RoundRectangle2D fill = new RoundRectangle2D.Double(iStrokWidth * 2, iStrokWidth * 2,
                    iInnerWidth, iInnerHeight, iInnerHeight, iInnerHeight);

            g2d.fill(fill);

            g2d.dispose();
        }

        @Override
        protected void paintIndeterminate(Graphics g, JComponent c) {
            super.paintIndeterminate(g, c); //To change body of generated methods, choose Tools | Templates.
        }

    }

}

If you want to install the look and feel delegate as the default delegate, take a look at this answer for more details (scroll down a little, it’s there)