serviceChanged handler never called

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Hi
I am trying to play with service events in a client bundle in order to be
notified when other bundles are registered.

So, I created a simple maven osgi bundler just with an activator class.

public class Activator implements BundleActivator, ServiceListener
{

    public void start(BundleContext context) throws Exception
    {
            System.out.println("Client bundle initializing on start
method...");

            context.addServiceListener(this);


    public void stop(BundleContext context) throws Exception
    {
        context.removeServiceListener(this);
    }

    public void serviceChanged(ServiceEvent event)
    {
        System.out.println("serviceChanged called");
    }

Then, in the gogo shell of felix I installed this client bundle and started
it. Then I installed other (server) bundle and started it but the
serviceChanged handler is never called. Why?

Thanks a lot

Alex