Last time, we learned why testing matters and met our new best friends: JUnit 5 & Mockito. Now it’s time to invite them into our Spring Boot project and get everything ready for action.
⚙️ Step 1: Start with a Spring Boot Project
If you don’t already have a project, head over to Spring Initializr and create one:
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Project: Maven (or Gradle if that’s your jam)
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Language: Java
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Spring Boot: Latest stable version
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Dependencies:
Spring Web,Spring Boot DevTools
Download and unzip your project, then open it in your favorite IDE (IntelliJ, VS Code, Eclipse — I won’t judge… much).
📦 Step 2: Add JUnit 5 & Mockito Dependencies
If you created a Spring Boot 2.2+ project, JUnit 5 is already the default test engine.
But let’s make sure everything is explicit.
Maven (pom.xml):
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | <dependencies> <!-- Spring Boot Test Starter (includes JUnit 5 by default) --> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> <exclusions> <!-- Exclude JUnit 4 --> <exclusion> <groupId>junit</groupId> <artifactId>junit</artifactId> </exclusion> </exclusions> </dependency> <!-- Mockito Core --> <dependency> <groupId>org.mockito</groupId> <artifactId>mockito-core</artifactId> <version>5.11.0</version> <scope>test</scope> </dependency> </dependencies> |
(Gradle folks: you’ll need testImplementation 'org.mockito:mockito-core:5.11.0' in build.gradle.)
🛠 Step 3: Understanding the Structure
When you create a Spring Boot project, your folders will look like this:
1 2 3 4 5 6 | src ├── main │ └── java # Your main code │ └── test └── java # Your test code |
Tests for src/main/java/com.example.demo/service/MyService.java
should live in src/test/java/com.example.demo/service/MyServiceTest.java.
🧪 Step 4: Your First JUnit 5 Test
Let’s make sure our setup is working. Create a file:
src/test/java/com/example/demo/HelloWorldTest.java
1 2 3 4 5 6 7 8 9 10 11 12 13 | package com.example.demo; import org.junit.jupiter.api.Test; import static org.junit.jupiter.api.Assertions.assertEquals; class HelloWorldTest { @Test void shouldReturnHelloWorld() { String greeting = "Hello World"; assertEquals("Hello World", greeting); } } |
Run this test. If it passes ✅ — congrats! You’ve just run your first JUnit 5 test in Spring Boot.
🎭 Step 5: Adding Mockito
Let’s pretend we have a service that depends on a repository.
src/main/java/com/example/demo/service/GreetingService.java
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 | package com.example.demo.service; import org.springframework.stereotype.Service; @Service public class GreetingService { private final GreetingRepository repository; public GreetingService(GreetingRepository repository) { this.repository = repository; } public String getGreeting() { return repository.fetchGreeting(); } } |
src/main/java/com/example/demo/service/GreetingRepository.java
1 2 3 4 5 6 7 8 9 10 | package com.example.demo.service; import org.springframework.stereotype.Repository; @Repository public class GreetingRepository { public String fetchGreeting() { return "Hello from the database!"; } } |
Now, let’s test the service without touching the real repository.
src/test/java/com/example/demo/service/GreetingServiceTest.java
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | package com.example.demo.service; import org.junit.jupiter.api.Test; import org.mockito.InjectMocks; import org.mockito.Mock; import org.mockito.junit.jupiter.MockitoExtension; import static org.mockito.Mockito.when; import static org.junit.jupiter.api.Assertions.assertEquals; import org.junit.jupiter.api.extension.ExtendWith; @ExtendWith(MockitoExtension.class) class GreetingServiceTest { @Mock GreetingRepository repository; @InjectMocks GreetingService service; @Test void shouldReturnMockedGreeting() { when(repository.fetchGreeting()).thenReturn("Hello from Mockito!"); String result = service.getGreeting(); assertEquals("Hello from Mockito!", result); } } |
Run the test → green bar → happy developer.
🎨 The “Coffee Analogy” Continues
In Part 1, JUnit was our taste tester and Mockito our fake milk supplier.
Now, we’ve actually given them a kitchen to work in (Spring Boot) and stocked it with ingredients (dependencies).
We can start making mock cappuccinos at full speed.
✅ Next in Part 3
We’ll write real unit tests for a Spring Boot REST controller and see how to mock external APIs.
Remember:
Tests don’t slow you down — bugs do.
