Dependency Injection in OSGi

The OSGi framework and its compendium services provide a whole lot of fun to build applications. Defining bundles is a cool stuff to cut the big job into pieces and enjoy the coolness of separation of concerns just like the old Romans said: Divide et Impera !

One interesting compendium specification is the Declarative Services Specification. This specification tries and IMHO succeeds very well to bring some of the cool stuff of Spring, namely Dependency Injection, into the OSGi world. Just like the application descriptors in Spring you have component descriptors in Declarative Services.

Using a component descriptor, you define the following properties of a component:


  • The name of the component and whether it is activated immediately or not

  • Whether the component is a service and the service interfaces to register the component with

  • Which other services are used by the component. These services may be injected (bound in OSGi speak) or may be looked up. There is also the notion of mandatory and optional services which provides the functionality to delay the component action until the mandatory service becomes available.

  • Configuration properties. Some properties may be injected by the descriptor itself. But at the same time, configuration properties may also be overwritten by configuration from the Configuration Admin Service. Thus the configuration of components may even be very dynamic.



The good news for the XML-haters like me: Over in the Apache Felix project we have Maven 2 plugin which takes annotations (JavaDoc or Java 5 Annotations) from your Component classes and builds the descriptors on your behalf.

So the next time, you are looking for dependency injection, you might want to consider OSGi and Declarative Services ;-)

Just for completeness, here is a list of other frameworks providing some sort of dependency injection:



In the end all work more or less the same, in that the provide some abstraction layer on top of the basic OSGi framework functionality: the Service Registry. This is really, the greatest things of all and IMHO shows the cleverness of the OSGi Framework specification: With just three basic layers (modularity, lifecycle and the service registry), you get the whole world in your hand to build flexible, modular and extensible applications.

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