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DEBAL_DAS
DEBAL_DAS
New Member
October 24, 2017
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OSGi service and component

  • October 24, 2017
  • 11 replies
  • 8635 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

    smacdonald2008
    smacdonald2008
    Level 10
    October 24, 2017

    See this Stack-over flow thread for more information on OSGi and inheritance.

    java - OSGI: bundles, class inheritance and maven scope - Stack Overflow

    DEBAL_DAS
    DEBAL_DASAuthor
    New Member
    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

    Debal Das, Senior AEM Consultant
    smacdonald2008
    smacdonald2008
    Level 10
    October 24, 2017

    Class C should implement A. Then it will work.

    DEBAL_DAS
    DEBAL_DASAuthor
    New Member
    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

    Debal Das, Senior AEM Consultant
    smacdonald2008
    smacdonald2008
    Level 10
    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

    joerghoh
    Adobe Employee
    joerghohAdobe EmployeeAccepted solution
    Adobe 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

      DEBAL_DAS
      DEBAL_DASAuthor
      New Member
      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

      Debal Das, Senior AEM Consultant
      DEBAL_DAS
      DEBAL_DASAuthor
      New Member
      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, Senior AEM Consultant
      smacdonald2008
      smacdonald2008
      Level 10
      November 2, 2017

      Looks like a much better design pattern!

      joerghoh
      Adobe Employee
      joerghohAdobe Employee
      Adobe Employee
      November 2, 2017

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

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