How I Learned Unit Testing in Spring Boot by Building One Simple Project

When I first started learning Spring Boot, I constantly heard senior developers talking about unit testing, JUnit, Mockito, and Test-Driven Development.

The problem?

Most tutorials were either too simple or too complicated.

So instead of watching endless videos, I decided to build a small project and learn testing through actual code.

The Project

I created a simple Task Management API with the following features:

Create Task
Update Task
Delete Task
Get Task by ID
List All Tasks

Nothing fancy.

The goal wasn't to build another Todo application.

The goal was to learn testing.

What I Learned

1. Not Everything Needs a Spring Context

My first mistake was using @SpringBootTest everywhere.

Tests became slow because the entire application context was loading.

For service-layer testing, plain JUnit and Mockito were enough.

1. Mock Dependencies, Not the Class Under Test

I learned that the service should be tested while repositories are mocked.

Example:

@Mock
private TaskRepository taskRepository;

@InjectMocks
private TaskService taskService;

This allowed me to verify business logic without connecting to a database.

1. Test Behavior, Not Implementation

Initially, I focused on internal method calls.

Later, I realized it's better to test outcomes.

Instead of checking how many methods were called, I started verifying whether the correct result was produced.

1. Edge Cases Matter

Most bugs appear in unexpected scenarios.

For example:

Task not found
Null input
Empty task title
Duplicate task

Writing tests for these cases increased my confidence significantly.

1. Testing Improves Design

This was the biggest surprise.

The more I wrote tests, the more I noticed poor design decisions.

Large service classes became difficult to test.

Complex methods became obvious candidates for refactoring.

Testing didn't just improve quality.

It improved the architecture itself.

My Recommended Learning Path

If you're new to Spring Boot testing:

Learn JUnit 5 basics
Learn Mockito fundamentals
Build a small CRUD project
Write service-layer tests
Add controller tests using MockMvc
Learn integration testing afterward

Don't start with advanced topics.

Start small and write tests consistently.

Final Thoughts

Unit testing felt intimidating when I started.

Today, I see it as one of the most valuable skills a backend developer can learn.

If you're struggling with testing, stop watching tutorials for a moment.

Build a small project.

Write one test.

Then write another.

You'll learn much faster by doing than by consuming content.

Happy coding!