Testing Spring Boot Applications with JUnit 5 and Mockito

Published: November 3, 2024

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Testing is a cornerstone of professional software development, ensuring your Spring Boot application works correctly, stays maintainable, and evolves confidently. In this post, we’ll explore how to leverage JUnit 5 and Mockito for effective unit and integration testing of Spring Boot applications.

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Why JUnit 5 and Mockito?

• JUnit 5 is the modern testing framework with modular architecture and improved features over JUnit 4.
• Mockito is the leading mocking framework to simulate dependencies and isolate units for testing.
• Spring Boot’s test starter integrates well with both, making testing straightforward.

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Getting Started: Setup Dependencies

In your pom.xml for Maven or build.gradle for Gradle, add:

Maven:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>

This includes JUnit 5, Mockito, and Spring Test libraries.

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Writing Unit Tests with JUnit 5 and Mockito

Example 1: Testing a Service with Mocked Repository

Assume a simple service layer:

@Service
public class UserService {

  private final UserRepository userRepository;

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

  public User findUserById(Long id) {
    return userRepository.findById(id).orElse(null);
  }
}

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Create a test class:

@ExtendWith(MockitoExtension.class)
public class UserServiceTest {

  @Mock
  private UserRepository userRepository;

  @InjectMocks
  private UserService userService;

  @Test
  void testFindUserById_UserExists() {
    User user = new User(1L, "Alice");
    Mockito.when(userRepository.findById(1L)).thenReturn(Optional.of(user));

    User result = userService.findUserById(1L);

    Assertions.assertNotNull(result);
    Assertions.assertEquals("Alice", result.getName());
  }

  @Test
  void testFindUserById_UserNotFound() {
    Mockito.when(userRepository.findById(2L)).thenReturn(Optional.empty());

    User result = userService.findUserById(2L);

    Assertions.assertNull(result);
  }
}

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Explanation:

• @Mock creates a mock of the repository.
• @InjectMocks injects the mock into the service.
• Mockito’s when(...).thenReturn(...) configures mock behavior.
• Assertions verify outcomes.

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Integration Testing with Spring Boot Test

Sometimes, you want to test components together with the Spring context.

Example 2: Testing Controller with Spring Boot

@SpringBootTest
@AutoConfigureMockMvc
public class UserControllerIntegrationTest {

  @Autowired
  private MockMvc mockMvc;

  @Test
  void testGetUser() throws Exception {
    mockMvc.perform(get("/users/1"))
      .andExpect(status().isOk())
      .andExpect(jsonPath("$.name").value("Alice"));
  }
}

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Here:

• @SpringBootTest boots the entire Spring context.
• MockMvc performs HTTP request simulations without a real server.
• jsonPath verifies JSON response structure.

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Mocking Beans in Spring Context

If you want to mock certain beans in an integration test, use:

@MockBean
private UserRepository userRepository;

This will replace the real bean with a mock in the Spring context.

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Advanced Tips

• Use parameterized tests (@ParameterizedTest) in JUnit 5 to run the same test with different inputs.
• Leverage ArgumentCaptor in Mockito to verify method arguments.
• Use @Nested tests in JUnit 5 to organize test cases.
• Consider Testcontainers for integration tests with real databases or message brokers.

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Summary

JUnit 5 and Mockito provide a powerful combo to test Spring Boot applications, from isolated units to full integration tests. Proper use of mocks, assertions, and Spring testing support ensures robust, maintainable tests.