1.5. Inversion Of Control


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The services are not responsible for the instantiation of the components on which they depend.

This architecture provides a loosely coupled design where the implementation of dependant services can be transparently exchanged.

This pattern has several names:


Don't let the object create itself the instances of the object that it references. This job is delegated to the container (assembler in the picture).


There are two ways to inject a dependency:

  • Using a constructor:

    public ServiceA(ServiceB serviceB)
  • Using setter methods:

    public void setServiceB(ServiceB serviceB)

When a client service cannot be stored in the container, then the service locator pattern is used:

public ServiceA(){

   this.serviceB =Container.getSInstance().getService(ServiceB.class);

Side effects

  • Ease Unit test (use of Mock objects)

  • Ease Maintainability

  • Ease Refactoring

  • Component reuse ( POJOs != EJBs)

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