Conditional observers

[william.drai]

Hi,

I've tried to set up a global event handler like this :

@SessionScoped
public class GlobalEventHandler {

    public void processEvent(@Observes(notifyObserver=Notify.IF_EXISTS) @Any Object event) {
       // Do something
    }
}

However it breaks at runtime by trying to handle the BeforeBeanDiscoveryEvent, because of course the session context is not active at this time and the observer processor tries to get the bean reference.
I wonder if this is a bug or not because it seems to me that IF_EXISTS should apply to both the bean instance and the underlying context.

The spec on 'conditional observers' does not seem to define a precise behaviour in this case.

[gavin.king]

Hrm. Interesting. Ordinary beans are not allowed to observe container lifecycle events. It seems like we should make that explicit and validated at startup time...

[william.drai]

In fact I don't want to observe container lifecycle events.
I just want to observe all my application events (thus the Object type) and preferably without having to implement a ApplicationEvent interface, so it would be a problem for me if what I do became forbidden by the spec.

[gavin.king]

I don't see how we could really make this work. You've explicitly specified that you want everything, by using @Any. To be honest, I really don't see your usecase for this.

[pmuir]

Gavin King wrote on Oct 28, 2009 20:24:


Hrm. Interesting. Ordinary beans are not allowed to observe container lifecycle events. It seems like we should make that explicit and validated at startup time...

This is a bug. We shouldn't be firing lifecycle events to anything that doesn't impl extension.

Please file a WELD issue with classes to reproduce.

[william.drai]

In fact this already works (almost...). This global event handler will select itself which events it needs to process, that's why I want to observe @Any Object, no problem with this, the handler can just ignore lifecycle events.
The problem is that Weld tries to trigger this observer during application startup when there is no session context. I thought that IF_EXISTS would be enough to avoid this but this is not the case because Weld still tries to get the bean reference and gets a ContextNotActiveException.

Anyway I've found a way to do what I want, hopefully this will be 'legal' in the final release :

public class GlobalObserver {
    @Inject
    private BeanManager manager;

    public void processEvent(@Observes(notifyObserver=Notify.ALWAYS) @Any Object event) {
        try {
            Bean<GlobalEventHandler> gehBean = (Bean<GlobalEventHandler>)manager.getBeans(GlobalEventHandler.class).iterator().next();
            GlobalEventHandler eventHandler = (GlobalEventHandler)manager.getReference(gehBean, GlobalEventHandler.class);
         if (eventHandler != null)
              eventHandler.processEvent(event);
         }
         catch (ContextNotActiveException e) {
             // Ignore event, no session context
         }
    }
}

@SessionScoped
public class GlobalEventHandler {

    public void processEvent(Object event) {
        ...
    }
}

The use case is relatively specific as I'm working on the integration of Flex and JCDI/Weld in the GraniteDS project. This global handler is meant to send back to the Flex client some events for which the client has registered itself as listener.
Maybe there is another way to do this with portable extensions, but I've not seen anything that would allow for intercepting any kind of event, or at least to register dynamically event observers once the application is deployed (that enough would probably fit my needs).

[gavin.king]

This is a bug. We shouldn't be firing lifecycle events to anything that doesn't impl extension.

Hrm. I agree that this sounds desirable, and I can see how you would probably read the spec to imply that, but it's not explicit in the spec. I suppose I should make that explicit. Let me think about it.

[gavin.king]

Maybe there is another way to do this with portable extensions

Yes, implement ObserverMethod and call AfterBeanDiscovery.addObserverMethod() from an Extension. If that approach doesn't work, it is definitely a bug.

[gavin.king]

Or a completely different solution is to just declare your observer method on an Extension :-)

[gavin.king]

but it's not explicit in the spec

I've updated the spec to say very explicitly that container lifecycle events aren't delivered to ordinary beans.

[william.drai]

Thanks, I'll try these solutions