Creating a Plugin System in Java

I am attempting to make a game using java, and I need a plugin system for the server...

How might I make it so that there is a folder named plugins that the users can simply drop jar files in there and I can call functions within those jar files all at once?

This is implying that a community would make plugins that I don't know about (so i need to have it run all plugins even if I don't know the plugins name)

(I would appreciate it if I didn't need to use someone else's framework like jspf)