eXo Platform 3.5

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## Chapter 54. Inversion Of Control

## Overview

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 :

- Hollywood principle : "don't call me, I will call you"

- Inversion of Control

- Dependency injection

## How

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).

## 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);

    }

## Side effects

- Ease Unit test (use of Mock objects)

- Ease Maintainability

- Ease Refactoring

- Component reuse ( POJOs != EJBs)

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