A class that can create a circle as a png and reencode it as a dataurl, Base64 encoded, image. I used this to create markers on Google Maps in GWT.

PngToDataUrl.java

import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.awt.image.RenderedImage;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
 
import javax.imageio.ImageIO;
 
import org.apache.commons.codec.binary.Base64;
 
public class PngToDataUrl {
 
    // gets the created marker image and Base64 encodes it into a String. The
    // Base64 class is from Apache commons codec project.
    public String createEncodedMarkerIcon(int markerSize) {
        byte[] markerImage = getMarkerImage(markerSize);
        String encodedMarkerImage = Base64.encodeBase64String(markerImage);
        return encodedMarkerImage;
    }
 
    // this creates a PNG using the RenderedImage created by the createImage
    // method.
    private byte[] getMarkerImage(int size) {
        ByteArrayOutputStream byteOutputStream = new ByteArrayOutputStream();
 
        try {
            ImageIO.write(createImage(size), "PNG", byteOutputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
 
        return byteOutputStream.toByteArray();
    }
 
    // Returns a generated image using Graphics2D
    private RenderedImage createImage(int size) {
        int width = size;
        int height = size;
 
        BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
 
        // Create a graphics contents on the buffered image
        Graphics2D g2d = bufferedImage.createGraphics();
        g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER));
 
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
 
        g2d.setBackground(new Color(0, 0, 0, 0));
 
        g2d.setPaint(new Color(1.0f, 0, 0, 1.0f));
        g2d.fillOval(0, 0, width, height);
 
        // Graphics context no longer needed so dispose it
        g2d.dispose();
 
        return bufferedImage;
    }
}