创建osgi实用程序包的用处

Question

I'm trying to develop a simple application using OSGi framework. My question involves an "utility bundle" available in the framework: let me explain with a pretty verbose example. At the moment I'm trying to build an event my bundle will send.

From what I understood, what i need is to do something like the following ( event admin felix ):

public void reportGenerated(Report report, BundleContext context)
    {
        ServiceReference ref = context.getServiceReference(EventAdmin.class.getName());
        if (ref != null)
        {
            EventAdmin eventAdmin = (EventAdmin) context.getService(ref);

            Dictionary properties = new Hashtable();
            properties.put("title", report.getTitle());
            properties.put("path" , report.getAbsolutePath());
            properties.put("time", System.currentTimeMillis());

            Event reportGeneratedEvent = new Event("com/acme/reportgenerator/GENERATED", properties);

            eventAdmin.sendEvent(reportGeneratedEvent);
        }
    }

Now, since an OSGi application may have lots of bundles, I thought to create a subclass of Event for every bundle (eg. I have a bundle named "BundleExample"? Inside it's exported classes there will be a "BundleExampleEvent"). I .

Now, the Event constructor needs a topic and a Map<String, Object> . However, to "simplify" the event constructor, I would like to have only the topic and the list of parameters to put inside the map. For example here's what might be a BundleExampleEvent class:

public class BundleExampleEvent extends Event{

    private int importantVariable;

    public BundleExampleEvent(String topic, int importantVariable) {
        super(topic, Utils.toMap("importantVariable", importantVariable));
        //here toMap is static
    }

    public int getImportantVariable() {
        return this.importantVariable;
    }   
}

Ok, please note the Utils.toMap : it's a function that allows you to convert a sequence of String, Object into a Map . Ok, now Utils is an example of a utility class (stupid, useless but a utility class nonetheless). : my thought would be to start this Utils bundle at framework boot and then whenever I need one of its utility I want to fetch a reference via @Reference annotation.

This can work greatly in any bundle interface implementation, like this:

@Component
public class BundleExampleImpl implements BundleExample {
   @Reference
   private Utils utils;

   @Override
   public String sayHello() {
      return this.utils.fetchHello();
      //another useless utility function, but hopefully it conveys what i'm trying to do
   }
}

But what about other classes (ie called by BundleExampleImpl during its work)? For example what about the BundleExampleEvent ? I need to call it from sayHello method and I want to use this utility also inside that class in order to compute the Map! In the previous example i used a static function, but I would like to use the reference of Utils OSGi gave me.

but I rather not to do it because it's pretty silly that something would depend on an "utility class"; my question are:

1. Is this the only method available if I want a "utility bundle"?

2. Or can I do something weird like adding a reference of Utils also in my BundleExampleEvent ; ie something like this:

    public class BundleExampleEvent extends Event{ @Reference private Utils utils; private int importantVariable; public BundleExampleEvent(String topic, int importantVariable) { super(topic, Utils.toMap("importantVariable", importantVariable)); //here toMap is static } public int getImportantVariable() { return this.importantVariable; } } 

3. Or maybe the whole idea of having an "utility bundle" is just pure trash?

Thanks for any reply. Hope I could convey my problem in the clearest way

Answer 1

I don't think there is any point in Utils being a service. Things should only be a service if they can conceivably have multiple implementations. In your case, the consumer of the Util functionality only ever wants a single implementation... the implementation is the contract.

I don't even think the utils code should be in a bundle. Just make it into a library that is statically linked into the bundles that need it.

Answer 2

In your case the Utils utils would be an OSGi service. Then you want to use this service inside an object that is not a service like BundleExampleEvent.

What you could do is to create a service that creates BundleExampleEvent instances and feeds it with an OSGi service. Kind of like a factory as a service. The problem with this is that services in OSGi are dynamic. If the service needed by the BundleExampleEvent instance goes away then the object would have to be discarded. So this only works for short lived objects.

In the eventadmin example a different solution would be to not use a special event class but instead create a service that has a method to send such an event. Then all the magic would happen inside this method and the result would be an event without further logic. You could also inject EventAdmin into that service using DS. This works very well in OSGI but has the disadvantage of the anemic domain model ( http://www.martinfowler.com/bliki/AnemicDomainModel.html ).

I am not sure which variant to prefer.