The "$$codebase" is a token that the JnlpDownloadServlet replaces with the codebase at the time of the request.
I don't think there is any way to echo or print variables in JNLP , but once you start web start application on your desktop. Go to Java in control panel and select application you are referring to. You can view JNLP by clicking "show jnlp" button on top panel. I think that should show the exact value of codebase derived to your local machine.
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The significant problems we face cannot be solved by the same level of thinking that created them -- Albert Einstein
subject: find out the variables' value in the jnlp file