How to handle non-free parts of a free software project

[Manuel Faux]

NetBeans 7.4 requires a file called jnlp-servlet.jar which is part of
the Oracle JDK and itself non-free licensed. The file is not required
for building, but for specific functions of the software.

More concrete, the file is required if "one wants to build a packaged
war file of JNLP version of a suite".

It seems like only the class jnlp.sample.servlet.JnlpDownloadServlet is
required, but I could not find it in the OpenJKD.

How is this normally handled? Should we add a file to the docs which
describes that for this specific functionality that file is required?

Manuel