fmeschbe's blog

In the Apache Sling project we have an interesting problem : Knowing when the application has finished its startup. Coming from a background of a traditional application, you know when the system has finished its startup. For example, a servlet container knows it has finished the startup, when all web applications have been started. In Apache Sling, the situation is a bit different: Apache Sling is an extensible system, where extensions may simply be added by adding more bundles. "Easy", you say, "just wait for all bundles to have been started and you know when the application is ready". True, but there is a catch. To extend Apache Sling, you register services with the OSGi registry. "Still easy", you might say. Right, if the services are all started by bundle activators, we still can depend on having all bundles started for the system to be ready. Again, this is only part of the story: Some services depend on other services. So the dependent services may

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 mandator