Add glow to a basic Java rectangle

I've got a very basic rectangle drawing panel, but I would like to know if there is a simple way to add some sort of glow to the rectangles.

public class Blocks extends JPanel {

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        this.setBackground(Color.BLACK);

        for (int i = 1; i <= totalSteps; i++) {
            g.setColor(Color.WHITE);
            g.fillRect(100 + i*60, 260, 50, 50);
        }
    }
}

Generating a "glow" effect is a little bit involved, depending on what you want to achieve.

I use this approach to generate glow effects for transparent/non-rectangular shapes (great for generating drop shadows for example).

This example basically creates a BufferedImage which represents the "glow", it then generates a "mask" which cuts the original BufferedImage out of it. I do it this way, as it allows me to draw the "glow" beneath transparent/translucent images. In your case, you could skip the "masking" process, but that's up to you.

You will also need a copy of the JHLabs, Image Filters, as I can't be bothered making my own blur filter

import com.jhlabs.image.GaussianFilter;
import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GraphicsConfiguration;
import java.awt.GraphicsEnvironment;
import java.awt.Transparency;
import java.awt.image.BufferedImage;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class GlowEffect {

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

    public GlowEffect() {
        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() {
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();

            int width = 50;
            int height = 50;
            int x = (getWidth() - width) / 2;
            int y = (getHeight() - height) / 2;

            BufferedImage img = generateGlow(width, height, 20, Color.YELLOW, 1f);
            g2d.drawImage(img, x - ((img.getWidth() - width) / 2), y - ((img.getHeight() - height) / 2), this);
            g2d.setColor(Color.WHITE);
            g2d.fillRect(x, y, width, height);
            g2d.dispose();
        }

    }

    public static BufferedImage generateGlow(int width, int height, int size, Color glow, float alpha) {
        BufferedImage source = createCompatibleImage(width, height);
        Graphics2D g2d = source.createGraphics();
        g2d.setColor(Color.WHITE);
        g2d.fillRect(0, 0, width, height);
        g2d.dispose();
        return generateGlow(source, size, glow, alpha);
    }

    public static BufferedImage generateGlow(BufferedImage imgSource, int size, Color color, float alpha) {

        int imgWidth = (int)Math.round(imgSource.getWidth() + (size * 2.5));
        int imgHeight = (int)Math.round(imgSource.getHeight() + (size * 2.5));

        BufferedImage imgMask = createCompatibleImage(imgWidth, imgHeight);
        Graphics2D g2 = imgMask.createGraphics();

        int x = Math.round((imgWidth - imgSource.getWidth()) / 2f);
        int y = Math.round((imgHeight - imgSource.getHeight()) / 2f);
        g2.drawImage(imgSource, x, y, null);
        g2.dispose();

        BufferedImage imgGlow = generateBlur(imgMask, size, color, alpha);

        imgGlow = applyMask(imgGlow, imgMask, AlphaComposite.DST_OUT);

        return imgGlow;

    }
    public static BufferedImage generateBlur(BufferedImage imgSource, int size, Color color, float alpha) {

        GaussianFilter filter = new GaussianFilter(size);

        int imgWidth = imgSource.getWidth();
        int imgHeight = imgSource.getHeight();

        BufferedImage imgBlur = createCompatibleImage(imgWidth, imgHeight);
        Graphics2D g2 = imgBlur.createGraphics();

        g2.drawImage(imgSource, 0, 0, null);
        g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_IN, alpha));
        g2.setColor(color);

        g2.fillRect(0, 0, imgSource.getWidth(), imgSource.getHeight());
        g2.dispose();

        imgBlur = filter.filter(imgBlur, null);

        return imgBlur;

    }

    public static BufferedImage createCompatibleImage(int width, int height) {
        return createCompatibleImage(width, height, Transparency.TRANSLUCENT);
    }

    public static BufferedImage createCompatibleImage(int width, int height, int transparency) {
        BufferedImage image = getGraphicsConfiguration().createCompatibleImage(width, height, transparency);
        image.coerceData(true);
        return image;
    }
    public static GraphicsConfiguration getGraphicsConfiguration() {
        return GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().getDefaultConfiguration();
    }

    public static BufferedImage applyMask(BufferedImage sourceImage, BufferedImage maskImage, int method) {
        BufferedImage maskedImage = null;
        if (sourceImage != null) {
            int width = maskImage.getWidth(null);
            int height = maskImage.getHeight(null);

            maskedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
            Graphics2D mg = maskedImage.createGraphics();

            int x = (width - sourceImage.getWidth(null)) / 2;
            int y = (height - sourceImage.getHeight(null)) / 2;

            mg.drawImage(sourceImage, x, y, null);
            mg.setComposite(AlphaComposite.getInstance(method));

            mg.drawImage(maskImage, 0, 0, null);

            mg.dispose();
        }
        return maskedImage;
    }

}

The basic work flow follows something like this:

Create a BufferedImage which represents the shape you want to apply a glow to (this is a opaque image)
Create a "mask" image, which is larger then the image you want to generate a glow effect around, based on the size parameter, but which has the original image painted to it in the center
Use the "mask" image to generate a "blur" image
Using the original image, mask it to the "blur" image, so that the original image is "cut out" of the "blur" image. This becomes the bases for our glow effect
Paint the "glow/blur" image, adjusting the x/y position accordingly (the glow effect is larger then the original shape, so we need to adjust it's position)
Paint the rectangle at the desired location

You'll want to have a look at Compositing Graphics for more details about how the masking process works.

I use this kind of idea to generate drop shadows for transparent/no-rectangular shapes, for example, example and example