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)