AEM - Multiple implementation of a Sling Service

[#1]

Hi Experts,

  Per the accepted answer of this post,  "If you have 2 services implementing the same interface and you want always to get the first or second service implementation, then your design is flawed. You should use different service interfaces then". I would like to know the reason why this design is flawed. 

Kindly enlighten.

Regards,

Jai

[#2 · Accepted solution]

When you control the service interface and the implementations, it hardly makes sense to have 2 distinct implementations and annotate the @reference annotation to get explicitly implementation 1 or 2.

If you need implementation 1, why don't you reference directly reference the implementation 1? Remember, that you can do something like that:

@component(service=Service1.class)

public class Service1 {

...

}

and use 

@reference

Service1 service;

in your code to reference the Service1 component directly. You should do that for services which are not publicly available (or you need to make your service class public, which is not always what you want). 

So in this case implementing an interface with 2 implementations is overhead, when you only want implementation 1 or 2.

But of course there are usecases, where it's absolutely the right approach to provide a service interface with 2 implementations. But these are rather corner cases, and I would not expect them to happen outside of libraries or frameworks. Sorry, maybe I should have added it to that answer as well.

[#3]

@Jai1122 

What @Jörg_Hoh  meant is, service user no need to worry about which implementation reference that is begin injected. Always service with highest ranking will be injected, if ranking is same oldest service will be injected(which guarantees some stability).

Writing two service implementations at a time doesn't make sense(with in same application). It should be old or new based on ranking. 

https://helpx.adobe.com/experience-manager/using/osgi_getting_started.html

Hope this answers your question.