1.5. Inversion Of Control

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 can not 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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