J2U is a conversion tool. It converts javadoc-output, which has been created with the doclet javadox, to an uml-style-model-format. This model-format is the input for another conversion tool which creates a uml-style class-diagram, which can be viewed with an svg-viewer. The XML-parsing is done thru JAXP.
The sourcecode for J2U can be downloaded here. Be warned! The application is a quick hack and not very well documented.
To create the input you need Javadox, in order to get J2U working you need a java virtual-machine (1.2 or newer should do) and the java api for XML-parsing (see above for download). To use J2U simply start it with javadox2umldiagrams javadoc.xml . If you renamed the output of javadox simply use the other filename as parameter. You get a file named javadoc.xml.aud.
Javadox is not available anymore and does not work anymore with current JDK releases (changes in API for javadoc). You can download javadox here (if you think this breaks with any copyrights, let me know).
JAXP is now integrated in JDK 1.4.x but the class XmlDocument is now in the crimson subpackage (instead of sun).