Inject a List of objects in CDI (Weld)

[glammy]

Let's say I have an interface called SocialNetworkService, and three implementations - TwitterService, FacebookService and FriendFeedService.

Now I want, whenever my managed bean (or whatever web component) receives a message, to share it in all social networks. I tried:

@Inject
private List<SocialNetworkService> socialNetworkServices;

But it didn't work. (Also tried to the @Any qualifier - same result)

So, is there a way to inject a list of all (or some) implementations of an interface?

I know the rule that a given injection point should not have more than one possible bean.
I guess I can achieve that by making a producer that produces the list, and using Instance<SocialNetworkService>, but that seems like too much for this task.

[ljnelson]

Ooh!  Ooh!  One I can answer!

Look at http://download.oracle.com/javaee/6/api/javax/enterprise/inject/Instance.html and notice that it is also an Iterable (left angle bracket) T (right angle bracket).

Best,
Laird

[glammy]

I mentioned in my question that I'm aware of this option, but it seems like too much effort to me. (not that it will take me hours, but it is tedious to write a method to do that)

[ljnelson]

No need for the producer, just the injection.

So:

@Inject
private Iterable<SocialNetworkService> services;

...should do it.

Best,

Laird

[glammy]

It doesn't work.

But indeed you are right, that Instance is also Iterable. When I defined it as Instance<SocialNetworkService>, it worked. Thanks.

[glammy]

Just to note something I forgot - you need the @Any annotation in the injection point, otherwise it doesn't get filled.

[meetoblivion]

There is no way to automatically generate a list of implementations, you may be able to pull it out of the BeanManager, depending on how they're implemented, you may be able to pull them out via qualifiers.  the easiest way I can think of would be something like

public class SocialBean {
@Inject TwitterService twitter;
@Inject FacebookService facebook;
@Inject FriendFeedService friendFeed;

@Produces
public List<SocialNetworkService> produceSocialNetworkServces() {
List l = new ArrayList();
l.add(twitter;
l.add(facebook);
l.add(friendFeed);
return l;
}

}

however, it seems like you're trying to work around the framework.  It seems to me that it would be better to simply fire off events that need to be sent.  take a look at this example:

public class TwitterEvent {
private SocialNetworkStatus status;
//getter and setter, constructor with field
}

in the place where you want to notify of the event.

@Inject Event<TwitterEvent> eventHandler;
...
eventHandler.fire(new TwitterEvent(status));

Then in your TwitterService you would really just need

@Stateless
public class TwitterService {
public void handleTwitter(@Observes TwitterEvent event) {
....
}
}

[glammy]

I already got it working the way I want with Liard's suggestion. And no, I'm not working around the framework. I want to give all implementations of a given interface a chance to act on some input, with as little code as possible.

The event mechanism could be used, but I'd need a generic MessageEvent, observed by all services. But that would be again lots of code

[glammy]

Well, not that much code actually - but a redundant intermediate object - the MessageEvent