Dependency Inversion PrincipleBy Dunith Dhanushka

Agenda

• Walkthrough of a real world scenario.• What is Inversion of Control?• What is Dependency Injection?• Multiple forms of Dependency Injection• IoC Containers

R.I.P Steve Jobs (1955 -2011)!

SOLID Principles

• Single Responsibility Principle• Open Closed Principle• Lizkov’s Substitution Principle• Interface Segregation Principle

• Dependency Inversion Principle

Meet Joe the developer!!

Joe’s contract is to build a IMDB clone!

First cut of the Movie Lister app

Problems with it

• Can’t extend the application without modifying the existing code.

• Violates Open/Closed principle!!! :S

Introduction of Interfaces

New Movie Lister App

Issues of new Movie Lister

• MovieLister class has to talk to both interface and implementation!!

• This makes MovieLister less portable.• This makes MovieLister less reusable.• Switching implementations requires a code

change! :@• Hard to unit test.

How can we fix this???

Its IT! We got remedy! ;)

• Using Setters ( Use a setter to set a Finder instance for MovieLister)

• Passing a Finder instance as constructor argument for MovieLister

• Use a Factory of Finders for MovieLister • Use a ServiceLocator

Using a setter

What really happened there?

• MovieLister depends on the Finder• Previously, MovieLister took the control of

initiating it’s dependency.• But in new version, dependency has been

instantiated by some other party for the MovieLister.

• This is called “Inversion of Control” (IoC)

This makes MovieLister

• Share across other developers.• Makes portable.• Encourages unit testing.• Preserves OCP (Movie Lister doesn’t know any

thing about its crappy implementers. Also plugging a new Finder doesn’t require a code change)

• Enables the use of Mock Objects.

Ways to achieve IoC

• Dependency Injection• Dependency Inversion Principle– A. High-level modules should not depend on low-

level modules. Both should depend on abstractions.

– B. Abstractions should not depend upon details. Details should depend upon abstractions.

• Coined by Martin Fowler

Methods of DI

• Constructor Injection• Setter Injection• Interface Injection

IoC Containers

• How does an IoC container works?

Popular IoC containers for Java

• Pico Container• Spring• JBoss Micro kernel• Google Guice• OSGi ( sort of DI, but very advanced)

Joe managed to save his head!

• Joe used Spring container.• He implemented the MovieLister as a Spring

bean.• In the beans.xml file, he wired the specific

Finder implementation to it’s interface!• Voila!

Thank you!

• We’ll meet in the next week with more Spring examples!

• Adios!