Article 17: Testing Spring Boot Applications

1. Introduction

If code is not tested, it is not trustworthy.

Testing helps you:

• Catch bugs early
• Refactor safely
• Deploy with confidence

Spring Boot provides excellent support for unit tests and integration tests.

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2. Types of Tests in Spring Boot

2.1 Unit Tests

• Test one class at a time
• No Spring context
• Very fast

2.2 Integration Tests

• Load Spring context
• Test multiple layers together
• Slightly slower but realistic

Both are important.

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3. Testing Tools You’ll Use

Spring Boot testing stack includes:

• JUnit 5
• Mockito
• Spring Test
• MockMvc

Most are included via:

spring-boot-starter-test

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4. Unit Testing a Service (Mockito)

4.1 Example Service

@Service
public class UserService {

    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public User getUser(Long id) {
        return userRepository.findById(id)
                .orElseThrow(() -> new RuntimeException("User not found"));
    }
}

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4.2 Writing Unit Test

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

    @Test
    void shouldReturnUserWhenFound() {
        User user = new User("Bharat", "bharat@email.com");
        when(userRepository.findById(1L)).thenReturn(Optional.of(user));

        User result = userService.getUser(1L);

        assertEquals("Bharat", result.getName());
    }
}

✔ Fast
✔ No Spring startup

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5. Integration Testing with @SpringBootTest

5.1 Load Full Context

@SpringBootTest
class UserServiceIntegrationTest {

    @Autowired
    private UserService userService;

    @Test
    void contextLoads() {
        assertNotNull(userService);
    }
}

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6. Testing Controllers with MockMvc

6.1 Example Controller Test

@SpringBootTest
@AutoConfigureMockMvc
class UserControllerTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    void shouldReturnUsers() throws Exception {
        mockMvc.perform(get("/users"))
                .andExpect(status().isOk());
    }
}

MockMvc:

• Simulates HTTP calls
• No real server started

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7. Testing JSON Responses

mockMvc.perform(get("/users"))
    .andExpect(status().isOk())
    .andExpect(jsonPath("$[0].name").exists());

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8. Using Test Profiles

Create:

application-test.properties

Activate:

@ActiveProfiles("test")

Useful for:

• In-memory DB
• Isolated configs

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9. Common Testing Mistakes

❌ Not testing service layer
❌ Only integration tests
❌ Hardcoded data
❌ Ignoring edge cases

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10. Best Practices

✔ Write unit tests for logic
✔ Use integration tests for wiring
✔ Keep tests independent
✔ Tests should be readable

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11. What You Should Understand Before Moving On

You should now know:

• Difference between unit & integration tests
• How Mockito works
• How to test controllers
• How Spring Boot supports testing

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12. What’s Next?

➡ Article 18: External Configuration & Secrets

• Environment variables
• Config best practices

Type Next when you’re ready.