Inject list of objects in CDI (Weld)

Question

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 (deployment error). (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.

Answer 1

I had a look at the JSR-299 specification and it doesn't seem that you can do what you want to do easily and I do not have experience enough with Weld to provide code for this special case.

However, based on chapter 12.3 "Bean Discovery" you might be able to declare the implementations as @Alternative's (to avoid Weld complain about multiple implementations) and listen to ProcessBean events to collect when implementations of SocialNetworkService are seen.

Answer 2

Combining my attempts with an answer from the Weld forum:

@Inject @Any
private Instance<SocialNetworkService> services;

Instance implements Iterable, so it is then possible to simply use the for-each loop. The @Any qualifier is needed.

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Another way to do this is by using the event system:

• create a MessageEvent (containing all the information about the message)

• instead of injecting a list of social networks, simply inject the event:

  @Inject private Event<MessageEvent> msgEvent;
  

  and fire it: msgEvent.fire(new MessageEvent(message));

• observe the event in all services (regardless of their interface, which might be a plus):

  public void consumeMessageEvent(@Observes MessageEvent msgEvent) {..}