Spring(Dependency Injection)

Spring IoC

In Spring, the Inversion of Control (IoC) principle is implemented using the Dependency Injection (DI) design pattern. Let's understand dependency injection with the help of an example. First we will see a java version of the example and later we will add spring functionalities to it. As far as the example go, its pretty simple. The QuizMater interface exposes the popQuestion() method. To keep things simple, our QuizMaster will generate only one question.

1.QuizMaster.java

2.----------------

3.package com.vaannila;

4. 

5.public interface QuizMaster {

6. 

7.public String popQuestion();

8.}

The StrutsQuizMaster and the SpringQuizMaster class implements QuizMaster interface and they generate questions related to struts and spring respectively.

01.StrutsQuizMaster.java

02.----------------------

03.package com.vaannila;

04. 

05.public class StrutsQuizMaster implements QuizMaster {

06. 

07.@Override

08.public String popQuestion() {

09.return "Are you new to Struts?";

10.}

11. 

12.}

01.SpringQuizMaster.java

02.----------------------

03.package com.vaannila;

04. 

05.public class SpringQuizMaster implements QuizMaster {

06. 

07.@Override

08.public String popQuestion() {

09.return "Are you new to Spring?";

10.}

11. 

12.}

We have a QuizMasterService class that displays the question to the user. TheQuizMasterService class holds reference to the QuizMaster.

01.QuizMasterService.java

02.-----------------------

03.package com.vaannila;

04. 

05.public class QuizMasterService {

06. 

07.private QuizMaster quizMaster = new SpringQuizMaster();

08. 

09.public void askQuestion()

10.{

11.System.out.println(quizMaster.popQuestion());

12.}

13.}

Finally we create the QuizProgram class to conduct quiz.

01.QuizProgram.java

02.----------------

03.package com.vaannila;

04. 

05.public class QuizProgram {

06. 

07.public static void main(String[] args) {

08.QuizMasterService quizMasterService = new QuizMasterService();

09.quizMasterService.askQuestion();

10.}

11. 

12.}

As you can see it is pretty simple, here we create an instance of the QuizMasterService class and call the askQuestion() method. When you run the program as expected "Are you new to Spring?" gets printed in the console.