Loading external modules written in Java.

In other languages like C++, it is possible to write libraries that can be loaded and unloaded at runtime, making it possible to add functionality to the program merely by placing third party modules or plugins in the same directory as the program. However, I don't know how to do this in Java. I have searched the internet looking for a way to accomplish this, but I could not find a straightfoward explination of how to write libraries in Java and load them at runtime. I am not looking for a prewritten framework that enables me to do this. I want to implement it myself. Is there a solution for this?