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DEBAL_DAS
DEBAL_DAS
New Participant
October 24, 2017
Solved

OSGi service and component

  • October 24, 2017
  • 11 replies
  • 8625 views

Hi All,

I have created 1 interface called - Interface A and I have one implementation class called: Class B

So, Class B is implementing Interface A . After deployment of OSGI bundle I could see in /system/console/bundles -

Service ID 10437 Types: Interface A

Component Name: Class B

Component ID: 3665 (Class B)

Now, I have created 1 more service called: Class C which is extending Class B , but after deployment I am not getting similar information for Class C.

Here, Component B and component C both are in active state. To understand the relation in between Service and component I have done one small changes -

Remove - Class C extends Class B

Use - Class C implements interface A and now I can see -

Service ID 10437 Types: Interface A

Component Name: Class C

Component ID: 3663 (Class C)

Is It not good idea if I follow below approach?

Class B implements Interface

Class C extends Class B

Here, I am thinking from the service design point of view.

I am using AEM 6.3 with archtype 11 and I have used  org.osgi.service.component.annotations.Component and org.apache.felix.scr.annotations.Service.

Please correct my understanding.

Thanks

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    Best answer by joerghoh

    Hi,

    the problem lies in the fact, that SCR is not designed in that way. Inheriting from a class (which is also a component) does not inherit SCR annotations / definitions.

    If you want to use inheritance to have a lot of similar services implementing the same interface, you could have an AbstractComponent  B (look the SCR documentation for this) implementing the interface A, and then making Component C and D extending B. In that case the annotations (including the references) for all references in B should be usable in C and D as well.

    On the other hand I have rarely seen this case actually implemented. I think that many very similar services implementing the same interface show a problematic software design.

    Jörg

    11 replies

    rajashekharr629
    rajashekharr629
    New Participant
    November 2, 2017

    my understanding is you want to get the service in another service ( which is like a dependency injection). please use this links to get an idea of injecting the osgi services  AEM Quick & Easy Solutions :): How to get OSGI Service reference in AEM?

    joerghoh
    joerghoh
    Employee
    November 2, 2017

    You probably know the rule "composition over inheritance" :-)

    smacdonald2008
    smacdonald2008
    New Participant
    November 2, 2017

    Looks like a much better design pattern!

    DEBAL_DAS
    DEBAL_DASAuthor
    New Participant
    November 2, 2017

    Hi,

    Thanks for your suggestion.

    I would like to say sorry that I couldn't reveal the requirement completely. But, to achieve the requirement I have created separate interface.

    So, now my design is looking as below -

    1.

    @Component

    @Service(value = { A.class })

    Class B implements Interface A

    {

    @Refereence

    Service D

    }

    2.

    @Component

    @Service(value = { D.class })

    Class C implements Interface E

    {

    @Refereence

    Service D

    }

    3.

    ClassF extends SlingAllMethodsServlet

    {

    @Refereence

    Service A

    @Refereence

    Service E

    }

    Thanks,

    Debal

    DEBAL_DAS
    DEBAL_DASAuthor
    New Participant
    October 26, 2017

    Hi,

    Thanks for your suggestion.

    I have referred following links: Apache Felix - SCR Annotations and https://searchcode.com/codesearch/view/17572101/ to get understanding of componentAbstract attribute.

    As I am using maven archtype 11 so, I have used org.osgi.service.component.annotations.Component. But, I am not getting attribute called:componentAbstract.

    I could see attribute called:componentAbstract for org.apache.felix.scr.annotations.Component.

    Have reffered:https://stackoverflow.com/questions/41280061/abstract-components-via-org-osgi-service-component-annotations to get Abstract components via org.osgi.service.component annotations.

    Do you have any other suggestion.

    Thanks,

    Debal

    joerghoh
    joerghohAccepted solution
    Employee
    October 25, 2017

    Hi,

    the problem lies in the fact, that SCR is not designed in that way. Inheriting from a class (which is also a component) does not inherit SCR annotations / definitions.

    If you want to use inheritance to have a lot of similar services implementing the same interface, you could have an AbstractComponent  B (look the SCR documentation for this) implementing the interface A, and then making Component C and D extending B. In that case the annotations (including the references) for all references in B should be usable in C and D as well.

    On the other hand I have rarely seen this case actually implemented. I think that many very similar services implementing the same interface show a problematic software design.

    Jörg

      smacdonald2008
      smacdonald2008
      New Participant
      October 25, 2017

      I have never tried this - always Class B implements Interface A. Never any issues,

      However - have you tried this:

      public class C extends B implements A

      try that

      DEBAL_DAS
      DEBAL_DASAuthor
      New Participant
      October 25, 2017

      Hi Scott,

      As you have suggested I have used the same and it's working.

      But, I don't want that Class C should implements A.

      So, I have referred http://www.computepatterns.com/76/osgi-component-vs-service-in-aem/ and

      tried to refer/call other service from component Class C

      @Component

      Class C

      {

      @Reference

      Service D d;

      }

      But still is getting "d" as null.

      Thanks,

      Debal

      smacdonald2008
      smacdonald2008
      New Participant
      October 24, 2017

      Class C should implement A. Then it will work.

      DEBAL_DAS
      DEBAL_DASAuthor
      New Participant
      October 24, 2017

      Hi Scott,

      Thanks for your suggestion.

      Let me explain you the issue in details -

      1.

      @Component

      @Service(value = { A.class })

      Class B implements Interface A

      {

      @Refereence

      Service D

      }

      2. @Component

         @Service

         Class C extends Class B

         {

         getting null for Service D

         }

        

         I have only one bundle.

      Thanks

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