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Jai1122
Jai1122
Level 4
January 29, 2021
Solved

AEM - Multiple implementation of a Sling Service

  • January 29, 2021
  • 2 replies
  • 3040 views

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

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Best answer by joerghoh

When you control the service interface and the implementations, it hardly makes sense to have 2 distinct implementations and annotate the @3214626 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:

 

@8220494(service=Service1.class)

public class Service1 {

...

}

 

and use 

@3214626

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.

 

2 replies

Anudeep_Garnepudi
Community Advisor
Anudeep_GarnepudiCommunity Advisor
Community Advisor
January 30, 2021

@jai1122 

What @joerghoh  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.

    joerghoh
    Adobe Employee
    joerghohAdobe EmployeeAccepted solution
    Adobe Employee
    January 30, 2021

    When you control the service interface and the implementations, it hardly makes sense to have 2 distinct implementations and annotate the @3214626 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:

     

    @8220494(service=Service1.class)

    public class Service1 {

    ...

    }

     

    and use 

    @3214626

    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.

     

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