Mastering Unit Testing in Spring Boot – Part 1: Why Test, and Meet JUnit 5 & Mockito

 By the end of this series, you’ll not only write tests — you’ll write them so good, future you will high-five past you.

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🚀 Why Should You Care About Unit Testing?

Let’s start with a confession:
I once worked on a Spring Boot project where no tests existed. Everything seemed fine… until it wasn’t. A small “harmless” change broke the login feature. The fix took hours, and the team lost trust in every new commit.

Unit tests are your safety net — they let you change code with confidence. Think of them as:

• 🛡 Shields protecting your app from bugs.

• 📜 Documentation that never lies (unlike that dusty wiki).

• 🕵 Detectives catching bad behavior before it reaches production.

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🧪 What is Unit Testing Anyway?

In simple terms:

Unit testing checks the smallest parts of your code (units) in isolation to ensure they behave as expected.

With Spring Boot, that “unit” could be:

• A service method

• A controller endpoint

• A utility class

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🎭 Enter JUnit 5

📜 The Backstory

JUnit was born in 1997, created by Kent Beck (yes, the father of Extreme Programming) and Erich Gamma (one of the “Gang of Four” design pattern legends).
Their goal? Give Java developers a simple framework to write repeatable automated tests.

💡 What’s New in JUnit 5?

JUnit has evolved over decades — now at version 5, it’s modular, powerful, and annotation-rich:

• JUnit Platform – Runs the tests.

• JUnit Jupiter – New programming model & annotations.

• JUnit Vintage – Runs old JUnit 3 & 4 tests.

Common annotations you’ll meet:

1 2 3 4 5

@Test // Marks a test method @BeforeEach // Runs before each test @AfterEach // Runs after each test @BeforeAll // Runs before all tests @AfterAll // Runs after all tests

🕵 Meet Mockito

📜 The Origin Story

Mockito appeared in 2008, created by Szczepan Faber (then at ThoughtWorks).
Why? Developers were struggling with brittle test doubles (fakes, stubs, mocks). Mockito’s mission was to make mocking:

• Readable

• Type-safe

• Fluent

💡 Key Features

• Create mocks easily:

1	MyService mockService = mock(MyService.class);

• Stub behavior:

1	when(mockService.getData()).thenReturn("Hello World");

• Verify interactions:

1	verify(mockService).getData();

• Zero boilerplate for dependency injection in Spring Boot with @Mock and @InjectMocks.

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🎨 An Illustrated Analogy

Imagine you’re testing a coffee machine:

• JUnit 5 is the taste tester — takes a sip and says “Yep, this tastes like cappuccino.”

• Mockito is the fake milk supplier — you don’t want to rely on real milk deliveries during testing, so you mock it.

(Insert cute illustration of a coffee machine, a happy tester, and a milk carton labeled “mock”)

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🌱 What’s Next?

This is just the warm-up. In Part 2, we’ll:

• Set up JUnit 5 & Mockito in a Spring Boot project.

• Write our first simple unit test.

• Mock dependencies like a pro.

Until then, remember:

Untested code is like an unchecked parachute — you can jump… but you probably shouldn’t.

📌 Series Outline:

1. Why Unit Testing? Meet JUnit 5 & Mockito (You’re here)

2. Setting Up JUnit 5 & Mockito in Spring Boot

3. Writing Your First Unit Test

4. Advanced Mockito Tricks

5. Best Practices for Test Readability & Maintenance