OSGi: What happens when an imported service is stopped while the service is still running

Question

Say I am using a service A which is imported in another service B. While B is running normally(ofcourse A is Active), what will happen is service A is uninstalled while service B is still running? Service A -> Service B

What will be the different scenarios in case I am using ServiceReference, ServiceTracker & DS?

Answer 1

When a service is unpublished in OSGi, an event is sent to all bundles currently using that service to tell them that they should stop using it.

If you are using DS, your unbind method will be called. When it is called, you should make best efforts to stop using the service as soon as possible. But ultimately OSGi is a cooperative system, it cannot force you to release the service. However if you don't then you can cause problems, for example the service publisher will not be fully garbage-collected. You end up sabotaging the dynamics of the OSGi platform, possibly creating memory leaks and so on.

If you are using ServiceTracker then the removedService method will be called, and you need to respond in the same way. But didn't I tell you in the other question not to use ServiceTracker?? ;-)

If you are using ServiceReference then you need to explicitly register a ServiceListener in order to receive these events. This is why you really really shouldn't use this low-level API until you have gained a lot more experience (and once you do have that experience, you won't want to use it anyway!).

Answer 2

First of all: one of the advantages of OSGi is that the behaviour of the framework and standard services are clearly specified. Those specifications can be downloaded from the OSGi Alliance web site, or, if you don't like reading PDFs, ordered for print. The question you are asking is perfectly answered in those specifications.

That said, in summary: when a service is unregistered:

• The ServiceReference object remains as it is. However, a call to ServiceReference.getService() will return null. Note that when using ServiceReferences you should release any references to the actual service object as retrieved via getService(), this normally requires some kind of tracking of the service.
• For ServiceTracker ServiceTracker.remove is called. This normally results in a call to removedService() on the ServiceTracker or the defined ServiceTrackerCustomizer.
• For DS, the defined unbind method for the referenced service is called (if specified). Furthermore, if the cardinality for the used service indicates that the service is mandatory, the using service may also be unregistered, even possibly deactivated or a new instance activated depending on the availability of alternative services and the policy defined for the service.