Switching between EJBs implementations using Weld

[mylos78]

Hi all,
I have a standard java Enterprise 6 project where several @Singleton beans are defined.
I'd like to switch between the EJB implementation at runtime, just like I would do in Spring with Spring beans.

I'm not sure where to start: should I use an @Alternative annotation, or it can be done by simply using Qualifiers.
Any help is highly appreciated.
Mylos

[guidbona.guidbona.yahoo.com]

Hi Mylos!

I would approach your problem in this way:

Create a producer bean which contains the logic upon which you want to choose the proper service implementation; something like this:

@RequestScoped
public class MyServiceProvider {
    private @Inject MyServiceImpl1 impl1;
    private @Inject MyServiceImpl2 impl2;
    ...

    @Produces @SmartlyChoosen
    MyService getMyService() {
        if (...)
            return impl1;
        else
            return impl2;
    }
}

Here I assume that MyServiceImpl1 etc. are the @Singleton EJBs and MyService is the common interface.
I also assume that the implementation  choice logic is per request, otherwise you can change the scope of the producer bean.

You have to create the @SmartlyChoosen qualifier, and of course use it in the injection point:

    ...
    @Inject @SmartlyChoosen MyService myService;
    ...

otherwise CDI would have multiple beans satisfying MyService, each actual EJB implementation plus the one from the producer method.

Another approach would be this:

First of all, create a different qualifier for each implementation of your service, and of course annotate each implementation of your service with a different qualifier.
Than, when you need to use the service, ask for it to the BeanManager, using the getBeans method.

The BeanManager can be injected, the getBeans method returns a Set of beans but you can safely assume you have only one unless you messed up with annotations :P

But I think the first approach is far more elegant (that is actually to say that the second is really ugly).

If someone has a better idea, please explain, I'm very interested!

[mylos78]

Hi Guido,
thank you very much for your sueful suggestion. This does make sense to get started with Weld. Anyway, should I use just a configuration file to switch between implementations, my only option is using @Alternatives right ?
thanks
Mylos

[guidbona.guidbona.yahoo.com]

Glad to be useful Mylos :)

If the choice of your service's implementation is known at deployment time (well, actually at packaging time), @Alternative is the basic way to go. For some more advanced use cases you will find Seam Config useful.
It allows fine-grained bean configuration too. I've never had a chance to use it yet, so I cannot provide a simple example, but the documentation will do.

In some cases, Seam Solder's @Default is enough, as it allows to define a default implementation bean that comes in when no other implementation is found.

If, on the other hand, the configuration file is provided at runtime, you can use a mix of the two techniques I described: create a producer bean that reads the configuration file, and let the @Produces method find the right implementation getting it from the BeanManager, perhaps by class name. Just beware to provide the configuration file before the producer is invoked...