Have you already asked yourself, how it comes, that you just need to implement an interface, mark the implementation as service, and — oh magic — your code is called at the right spot. Without any registration.
For example when you wrote your first sling servlet (oh sure, you did it!) it looked like this:
@Service
@Component
@Property (name=“sling.servlet.paths”, value=“/bin/myservlet”)
public class MyServlet extends SlingSafeMethodServlet {
...
}
and that’s it. How does Sling (or OSGI or whoever) knows that there is a new servlet in the container and calls it, when you visit $Host/bin/myservlet. Or how can you build such a mechanism yourself?
Basically you just use the power of OSGI, which is capable to notify you about any change of the status of an existing bundle and service.
If you want to keep track of all Sling servlets registered in the system you just need to write some more annotations:
@Service
@Component(value=ServletTracker.class)
@Reference (name=“servlets”, policy=ReferencePolicy.DYNAMIC, cardinality=ReferenceCardinality.OPTIONAL_MULTIPLE, interfaceReference=Servlet.class)
public class ServletTracker {
List<Servlet> allServlets = new ArrayList<Servlet>();
protected bindServlets (Servlet servlet, final Map<String, Object> properties) {
allServlets.add (servlet);
}
protected unbindServlets(Servlet servlet, final Map<String,Object> properties) {
allServlets.remove(servlet);
}
public doSomethingUseful() {
for (Servlet servlet : allServlets) {
// do something useful with them ...
}
}
(Of course you can track any other interface through which services are offered. But be aware, that in many cases only a single instance of a service exists.)
The magic mostly is in the @Reference annotation, which defines that there is optional reference, which takes zero till unlimited references of services implementing the class/interface “Servlet”. By default methods are called, for which the names are constructed using the “name” statement, resulting in “bindServlets” when a new servlet is registered, and “unbindServlets” when the servlet is unregistered. You can use these methods to whatever you want to do, for example storing the references locally and calling them whenever appropriate. And that’s it.
If you use this approach, your code is called whenever some service implementing a certain interface is being activated/deactivated. With the SCR annotations it’s all possible without having too much trouble and the best of all: Nearly all just by declaration.
If you like to have some more control over (or just want to code) you can use a ServiceTracker (a nice small example for it is the Apache Aries JMX Whiteboard) to keep manually track of services.
And as recommended reading right now: Learn all the other cool stuff at the Felix SCR page.
And if you need to have more code which uses this approach, you might want to have a look at the SlingPostServlet, which is a excellent example of using this pattern. Oh, and by the way: This pattern is called OSGI whiteboard pattern.
Things on a content management system 
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