Use of extension in JOGL

Hi,

I would like to know how can I use an extension using JOGL.
I have found isExtensionAvailable() which returns true for the extension I would like to use.
But getExtension() for the same key returns "null".

My case is that I would like to use a geometry shader written in GLSL_120.
So I need to use glProgramParameteri() to pass the input/output geometry parameters, but this method is part of the GL4 interface.
And I want that to work on a GL 3 context.

So I'm trying to find a way to use glProgramParameteriEXT() instead, but I don't know how to get access to that.

Just use GLProfile.getMaxProgrammable() and use isFunctionAvailable() to check whether glProgramParameteri is supported. Don't ask a GL 3 context, ask the maximum core profile available to do the same thing.

Thx