Spring Basics - What Is A Dependency?



Most important feature of Spring Framework is Dependency Injection. To understand Dependency Injection, you need to understand the concept of a Dependency.

You will learn

  • What is a Dependency?
  • How are applications built? How is one layer dependent on another?
  • How is a class dependent on another?
  • How does the Spring Framework do Dependency Injection?

In this article, we have a look at what a dependency is in general terms, and then in the context of the Spring framework.

Free Courses - Learn in 10 Steps

Spring Framework

This is the second article in a series of articles on Spring Framework:

Dependencies At A High Level

We build enterprise applications in multiple layers: image info

A typical Java application will have three layers in its architecture: web, business and data.

  • The web layer
  • The business layer
  • The data layer

In the above scenario:

  • Web Layer depends on Business Layer. The business layer is a dependency for the web layer.
  • Business layer depends on Data Layer. The data layer is a dependency for the business layer.

Dependencies At Class Level

Let’s look at an example:

	public class ClientBOImpl implemented ClientBO {
		ProductDO productDO;
		ClientDO clientDo;

		public Amount getClientProductsSum(long cliendId) {

		public void saveChangedProducts(long clientId,
										List<Product> userEnteredProducts) {


ClientBOImpl is the business class, and it makes use of two data layer classes - ProductDO and ClientDO.

Let’s now have a look at the business logic within ````ClientBOImpl```:

  • getClientProductsSum() : This returns the sum of all products for a given client.
  • saveChangedProducts() : When products are modified on the application page, this method is called.

Both methods in ClientBOImpl need either ProductDO or ClientDO. ProductDO and ClientDO are dependencies of ClientBOImpl.

Inputs/Outputs Are Not Dependencies

If you look at public Amount getClientProductsSum(long clientId), clientId is merely an input, not a dependency. Similarly, the total calculated amount returned by getClientProductsSum is an output, not a dependency.

A Few More Examples Of Dependencies


Have a look at the following code:

	public class ComplexAlgorithmImpl {
		private SortAlgorithm sortAlgorithm;	

	public interface SortAlgorithm {
		public int[] sort(int[] numbers);

	public class QuickSortAlgorithm implements SortAlogrithm {

ComplexAlgorithmImpl performs a lot of complex logic, and sorting is one of the steps.

The SortAlgorithm is a dependency of ComplexAlgorithmImpl.

Since SortAlgorithm is an interface, you can easily change the actual sort algorithm used by ComplexAlgorithmImpl, without changing its code.


Consider the following code:

	import java.sql.ResultSet;

	public class PersonJdbcDao {
		JdbcTemplate jdbcTemplate;

		class PersonRowMapper implements RowMapper<Person> {
			public Person mapRow(ResultSet rs, int rowNum) throws SQLException {

To execute a query on the database, PersonJdbcDao needs JdbcTemplate. Therefore, JdbcTemplate is a dependency of PersonJdbcDao.

Let’s look at a simple method:

	public Person findById(int id) {
		return jdbcTemplate.queryForObject(//...);

id is the input for this method, and the output returned is of type Person.

In the above method, we are making use of a dependeny jdbcTemplate . The inputs and outputs are not dependencies.

Do check out our video on the same topic:

image info


In this article, we focused on the most important concept in Spring Framework - a dependency.

8 AWS Certification Courses

We are releasing courses for 3 Certifications under 3 categories - STEP BY STEP, EXAM REVIEW and PRACTICE TESTS:

  • Step by Step courses are for you if you are getting started with AWS
  • Exam Review courses help you to quickly review for the exam in a few hours
  • Practice Tests help you to test your preparation - How ready are you for the exam?


EXAM REVIEW COURSES: You prepare for the exam for 1-6 months. How do you remember everything you studied? Review for the exam in under 6 HOURS using this crash courses!

PRACTICE TESTS: Test yourselves for the certification exam with our practice tests

Best Selling Udemy Courses

Image Image Image Image Image Image Image Image Image Image Image

Join 450,000 Learners and 30+ Amazing Courses

350,000 Learners are learning everyday with our Best Selling Courses : Spring Boot Microservices, Spring, Spring Boot, Web Services, Hibernate, Full Stack React, Full Stack Angular, Python, Spring Interview Guide, Java Interview, Java Functional Programming, AWS, Docker, Kubernetes, PCF, AWS Fargate and Azure

Do not know where to start your learning journey? Check out our amazing learning paths:
Learning Path 01 - Spring and Spring Boot Web Applications and API Developer,
Learning Path 02 - Full Stack Developer with Spring Boot, React & Angular,
Learning Path 03 - Cloud Microservices Developer with Docker and Kubernetes,
Learning Path 04 - Learn Cloud with Spring Boot, AWS, Azure and PCF and
Learning Path 05 - Learn AWS with Microservices, Docker and Kubernetes



Related Posts

Understanding jwt token

Let us understand the json web tokens

Single sign-on workflow

Let us understand the single sign-on workflow

Oauth2.0 - Resource Owner Password Credentials grant workflow

Let us understand the Oauth2.0 Resource Owner Password Credentials grant workflow

Oauth2.0 - Implicit grant workflow

Let us understand the Oauth2.0 implicit grant workflow

Oauth2.0 - Client Credentials grant workflow

Let us understand the Oauth2.0 client credentials grant workflow

Oauth2.0 - Authorization grant workflow

Let us understand the Oauth2.0 authorization grant workflow

Writing Integration Tests for Rest Services with Spring Boot

Setting up a basic REST Service with Spring Boot is a cake walk. We will go one step further and add great integration tests!

Integrating Spring Boot and Spring JDBC with H2 and Starter JDBC

Learn using Spring Boot Starter JDBC to connect Spring Boot to H2 (in memory database) using Spring JDBC. You will create a simple project with Spring Boot. You will add code to the project to connect to a database using Spring JDBC. You will learn to implement the basic CRUD methods.

JUnit Tutorial for Beginners in 5 Steps

JUnit Tutorial for Beginners in 5 Steps. Setting up a basic JUnit example and understanding the basics of junit.

JPA and Hibernate Tutorial For Beginners - 10 Steps with Spring Boot and H2

JPA and Hibernate in 10 Steps with H2 - Setting up a basic project example with Spring Boot and in memory database H2. Its a cake walk.