OSGi
OSGi (Open Services Gateway Initiative) still is an important technologys for building extendable server software composed of components and services. OSGi aims to the middleware layer needed for such applications. Programming is done in Java.
It's important to understand concepts of OSGi, but for that in turn you have to be familiar with classloading. It's not much to read and I promise that you'll be surprised how easy OSGi can be.