Within the JNLP file that's used to start the Webstart Swing application, you can specify parameters. The JNLP file can be dynamic as well, so you can have a JSP file that outputs a JNLP file (instead of HTML file which is the default).
If you start your webapplication by a JNLP file that is generated using a JSP file, you can add parameters dynamically.
Alternatively, you can use an
applet. The applet tag can contain parameters. The applet tag can be put into a JSP file, and then you can add parameters from within the html code. The HTML code can be generated by a JSP file, so that code can be dynamic as well.