OSGI service vs. Singleton? - osgi

Question

I am a beginner to OSGI and I am wondering if someone can enlighten me about the difference between creating OSGI service vs singleton pattern. For example, suppose I have a bundle core which provides IService, and multiple bundles that needs to access this. I can:
register a service in the core-bundle, in which the plugins can access
provide a singleton class, which provides the service
Using OSGI service seems to be quite cumbersome; and since the plugins have to depend on Core anyways (to get the interface), what's the advantage of using OSGI service?

Answer 1

Services are the connections between independent modules. Having modules depend on services (with their specification packages) can significantly reduce coupling between modules and thus provide much of the benefits of modularity.
I think the singleton pattern is used in two different ways: you just want a single object be shared between a set of users (e.g. a Log Service) or you can really only have one instance (e.g. there is only one piece of hardware). In general, I see that most people in the enterprise software world talk about the former. However, experience shows that when projects grow, singletons become less singleton but more a shared object, or at least an appearing to be shared object.
The nice thing in OSGi is that you can model both and the clients of the "singleton" are oblivious of it, nor does it require some central configuration. The reason is that OSGi relies on modules in charge, registering a service is a local decision as is listening to a service.
The power of services are not in its dynamics (they are cool though, especially during development), the essence of service is that they provide full local control inside the module without central configuration. Once you understand how powerful this is, there is no way back :-)
Last, OSGi services are not cumbersome, not since we have DS with annotations. Registering a service is now much simpler than creating a Spring bean, no xml, no central configuration:
// A component registered as a ISingleton service
#Component
public class MyImpl implements ISingleton {
void doSingle() { ... }
}
// A component that uses the ISingleton component
#Component
public class MyConsumer {
#Reference
void setISingleton(ISingleton is) { ... }
}
... And the dynamics come largely for free ...

Answer 2

Short answer: if you don't -- and won't -- need the benefit of an OSGi service (e.g., dynamically-managed service implementations and service searches), then you don't need an OSGi service.
But there is more to consider here than whether or not the service would be cumbersome. Heck, OSGi itself can be considered cumbersome. Will another bundle need to provide an implementation of that class? Maybe not. Will the Core bundle ever shut down or otherwise be unable to provide an implementation on demand? Maybe.
To determine if a service is right for the class in question, read the run-down of the specific benefits of a service on the OSGi Alliance's What Is OSGi page. They have a very good explanation of how your singleton class may become more cumbersome than a service.
Good luck.

Answer 3

My OSGi Threading Model 's poc is resulted into believing me that, every service is a singleton for a service consumer. As the only one service object get registered into the osgi service registry. (but you can override this behavior also). So as far as programming is considered, the behavior of a singleton class and an OSGi service is the same. Your class level variables are shared among the various service consumer calls.
I will say OSGI Service is Singleton++
But there are also differences.
OSGi gives you a separate class-loader for each service which is not possible in a singleton. All {singleton} classes are loaded by a single classloader. We can't have two classes with the same name (fully qualified name) in a singleton but this is possible in OSGi.
In certain situations we must be confirmed that a class should be loaded only once (making hibernate session factory, hdfc service initialization, POJO creations which are heavy initializations required only once). Now if you are living in a Java EE scenario some times your singleton class gets loaded twice by two different classloaders. So this results into two times the execution of a static block; an unnecessary job.
Such classloader problems are easily handled by OSGi (as you are a beginner I feel classloading itself is a problem for you in the next few days).
Another great feature provided by OSGi is updating a bundle.
Consider you changed the code in your singleton class. Now you need to deploy this updated class in your running application. You essentially need to restart the system, so that every singleton class loader updates the new instance of the singleton. This is not required in OSGi, just update the bundle.
I will say if you're going to design for larger applications (enterprise scale), or if you need to design code for a limited hardware capacity (low memory constraints, low computing power) then go for OSGi, it is best for the extreme ends. For all others your normal java coding will work perfectly.

Answer 4

You can manage the life cycle (deploy new version of the service, concurrently run multiple versions etc) of a service but you can't manage the life cycle of singleton without restarting the JVM (even with restart you can just have 1 version available at any point of time).