Where can I download JNLP.jar?
C:\Program Files\Java\jdk1.6.0_27\sample\jnlp\servlet
In other words, in the sample\jnlp\servlet
directory of your JDK.
You can also find the javax.jnlp.*
classes in lib/javaws.jar
under your JRE installation.
The classes shown at the link are primarily of the JNLP API. It is automatically on the run-time class-path of apps launched using Java Web Start.
As to how to add them to the compile-time class-path, see the tips in (the build files of) these demos.