ServiceTracker and DS combined?

Question

Let's imagine a bundle in which exists:

• A component is responsible of listen all "Device" service instances in the service registry.

• The same component needs an "adaptor factory" in order to create "Adaptors" by using the discovered devices.

• The factory is owned by another bundle.

I can solve part of the problem by using a ServiceTracker (Activator + Service Tracker): the activator instantiates the ServiceTraker and it can register all changes in "Device" services.

But i can't inject to this service tracker the DS factory created in other bundle, because it will result in two instances (one created by activator AND without the member /// another created by osgi AND with member variable ok but can't listen the "Device" service changes).

So... how can i solve this scenario? How can i have a Service Tracker (perfect for me) with a DS as a class member?

Answer 1

Use no Activator, instead use a component or service (we will call it A) with a declarative services 'activate(ComponentContext)' method. Within the activate method, you can instantiate your ServiceTracker like normal.

When you instantiate the ServiceTracker within A's activate method, you can also pass in the AdapterFactory into the ServiceTracker. You can get the AdapterFactory by pulling it out of the BundleContext taken from ComponentContext or (even better) use DS and make it a service reference to your A component.

That said: why do you need ServiceTracker for this? Unless I misunderstand, you can use DS bind and unbind to receive events on the availability of a Service.

EDIT: An (OLD) example of Bind/Unbind behavior using multiple cardinality: http://blog.tfd.co.uk/2009/11/12/declarative-optional-multiple-references-flaky-in-osgi/

EDIT: A comparision of the two approaches but doesn't go into bind/unbind so much: http://njbartlett.name/2010/08/05/when-servicetrackers-trump-ds.html

EDIT2: That said: my general policy is to not use an Activator except in super rare cases. Use DS, ipojo, etc and use the components you define with those techs in order get access to the BundleContext to build more low level objects like ServiceTrackers.