Recently I needed for my mapjfx project to convert an image, where I had the URL of, to a base64 encoded data-URI. The following code shows how to do the conversion (Java 8 code):

/*
 * (c) 2015 P.J. Meisch (pj.meisch@sothawo.com).
 */

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.*;
import java.util.Base64;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Test {

    private static Logger logger = Logger.getAnonymousLogger();

    public static URI getDataURIForURL(URL url) {
        URI dataUri = null;
        if (null != url) {
            try (InputStream inStreamGuess = url.openStream();
                 InputStream inStreamConvert = url.openStream();
                 ByteArrayOutputStream os = new ByteArrayOutputStream()) {
                String contentType = URLConnection.guessContentTypeFromStream(inStreamGuess);
                if (null != contentType) {
                    byte[] chunk = new byte[4096];
                    int bytesRead;
                    while ((bytesRead = inStreamConvert.read(chunk)) > 0) {
                        os.write(chunk, 0, bytesRead);
                    }
                    os.flush();
                    dataUri = new URI("data:" + contentType + ";base64," +
                            Base64.getEncoder().encodeToString(os.toByteArray()));
                } else {
                    logger.warning(() -> "could not get content type from " + url.toExternalForm());
                }
            } catch (IOException e) {
                logger.log(Level.SEVERE, "error loading data from url", e);
            } catch (URISyntaxException e) {
                logger.log(Level.SEVERE, "error building uri", e);
            }
        }
        return dataUri;
    }

    public static void main(String[] args) {
        try {
            URL url = new File("test.png").toURI().toURL();
            logger.info(() -> "url: " + url.toExternalForm());
            URI dataURI = getDataURIForURL(url);
            logger.info(() -> "dataUrl: " + dataURI.toString());
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }
    }
}

The logging output is as follows (last line shortened):

Jan 05, 2015 5:22:23 PM Test main
INFORMATION: url: file:/Users/sothawo/Test/test.png
Jan 05, 2015 5:22:23 PM Test main
INFORMATION: dataUrl: data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACgAAAAoCAYAAACM/rhtAAAAAXNSR0IArs4c6QA...

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