Inversion Of Control
1 Overview
The services are not responsible for the instantiation of the components they depend on.
This architecture provides a loosely coupled design where the implementation of dependant services can be transparently exchanged.
This pattern has several names :
- Hollywood principle : "don't call me, I will call you"
- Inversion of Control
- Dependency injection
2 How
Not to let the object create itself the instances of the object it references. This job is delegated to the container (assembler in the picture).
3 Injection
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);
}4 Side effects
- Ease Unit test (use of Mock objects)
- Ease Maintainability
- Ease Refactoring
- Component reuse ( POJOs != EJBs)
