How should HTML for Java components reference reso

Resources for HTML (style sheets, images etc.) might not be loaded correctly when:

A Swing app. is distributed, or when it is put into a Jar file.
The HTML is generated at run-time.

How can such resources be accessed reliably in the HTML?

HTML from a Jar file that links resources (e.g. CSS or images) by relative references will work just fine.

E.G.

This example loads HTML (that has a relative reference to an image) from a Jar.

import javax.swing.*;
import java.net.URL;

class ShowHtml {

    public static void main(String[] args) {
        final String address =
            "jar:http://pscode.org/jh/hs/object.jar!/popup_contents.html";
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                try {
                    URL url = new URL(address);
                    JEditorPane jep = new JEditorPane(url);
                    JFrame f = new JFrame("Show HTML in Jar");
                    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                    f.add(new JScrollPane(jep));
                    f.pack();
                    f.setSize(400,300);
                    f.setLocationByPlatform(true);
                    f.setVisible(true);
                } catch(Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
}

Screenshot

HTML

The HTML that is being loaded.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<!--       
 *        Copyright (C) 1997  Sun Microsystems, Inc
 *                    All rights reserved.
 *          Notice of copyright on this source code 
 *          product does not indicate publication. 
 * 
 * RESTRICTED RIGHTS LEGEND: Use, duplication, or disclosure by 
 * the U.S. Government is subject to restrictions as set forth 
 * in subparagraph (c)(1)(ii) of the Rights in Technical Data
 * and Computer Software Clause at DFARS 252.227-7013 (Oct. 1988) 
 * and FAR 52.227-19 (c) (June 1987).
 *
 *    Sun Microsystems, Inc., 2550 Garcia Avenue,
 *    Mountain View, California 94043.
 *
-->
<HTML>
<HEAD>
<TITLE>
Editing Project Attributes
</TITLE>
</HEAD>
<BODY BGCOLOR="#ffffff">
<IMG SRC="images/popup_icon.gif" width="24" height="24"> <b>Popup Window</b>
<p>
Popup windows appear near the location from which they are
activated.  They are not contained in frames and thus
cannot be resized or moved by the user.  Popups are
dismissed by clicking anywhere in the help viewer.
<p>
Popup windows can be activated by clicking on a text object, 
graphic object, or JComponent button.  All three examples are
included in this demo.
<p>
<A HREF="popup_contents2.html">More...</A>
</body>
</html>

E.G. 2

For dynamically created HTML, the JRE will probably use the class file's location as the presumed location of the HTML. But to remove all doubt, we can specify the base element in the head.

import javax.swing.*;

class HtmlUsingBase {

    public static void main(String[] args) {
        final String htmlContent =
            "<html>" +
            "<head>" +
            "<base href='http://www.gravatar.com/'>" +
            "</head>" +
            "<body>" +
            "<h1>Image path from BASE</h1>" +
            "<img src='avatar/a1ab0af4997654345d7a9" +
            "49877f8037e?s=128&d=identicon&r=PG'" +
            " width='128' height='128'>" +
            "</body>" +
            "</html>";
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JLabel label = new JLabel(htmlContent);
                JOptionPane.showMessageDialog(null, label);
            }
        });
    }
}