Introduces what is Inversion of Control, Dependency Injection and How they relates to the Dependency Inversion Principle

1. Dependency Inversion Principle By DunithDhanushka

2. 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)!

3. SOLID Principles Single Responsibility Principle Open Closed Principle Lizkov’s Substitution Principle Interface Segregation Principle Dependency 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 it Can’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 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.

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 MovieLister Use a Factory of Finders for MovieLister Use a ServiceLocator

14. Using a setter

15. 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)

16. 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.

17. 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

18. Methods of DI Constructor Injection Setter Injection Interface Injection

19. IoC Containers How does an IoC container works?

20. Popular IoC containers for Java Pico Container Spring JBoss Micro kernel Google Guice OSGi ( 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!