The document explains the Dependency Inversion Principle and Inversion of Control (IoC) through a real-world scenario, focusing on a movie lister application developed by 'Joe'. It highlights issues with the initial design violating principles like Open/Closed and demonstrates how to improve the application using Dependency Injection methods such as setter and constructor injection. The use of IoC containers, particularly the Spring container, allows for better modular design, making the application more portable and easier to test.

1. Dependency Inversion PrincipleBy DunithDhanushka

2. AgendaWalkthrough of a real world scenario.What is Inversion of Control?What is Dependency Injection?Multiple forms of Dependency InjectionIoC ContainersR.I.P Steve Jobs (1955 -2011)!

3. SOLID PrinciplesSingle Responsibility PrincipleOpen Closed PrincipleLizkov’s Substitution PrincipleInterface Segregation PrincipleDependency Inversion Principle

4. Meet Joe the developer!!

5. Joe’s contract is to build a IMDB clone!

6. First cut of the Movie Lister app

7. Problems with itCan’t extend the application without modifying the existing code.Violates Open/Closed principle!!! :S

8. Introduction of Interfaces

10. New Movie Lister App

11. Issues of new Movie ListerMovieLister 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.

12. How can we fix this???

13. 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 MovieListerUse a Factory of Finders for MovieListerUse a ServiceLocator

14. Using a setter

15. What really happened there?MovieLister depends on the FinderPreviously, 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)

16. This makes MovieListerShare 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.

17. Ways to achieve IoCDependency InjectionDependency Inversion PrincipleA. 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

18. Methods of DIConstructor InjectionSetter InjectionInterface Injection

19. IoC ContainersHow does an IoC container works?

20. Popular IoC containers for JavaPico ContainerSpringJBoss Micro kernelGoogle GuiceOSGi ( sort of DI, but very advanced)

21. 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!

22. Thank you!We’ll meet in the next week with more Spring examples!Adios!