What is integration testing?

Integration testing is a type of software testing where individual modules are combined and tested as a group. The goal is to verify how components interact with each other and with external systems.

Main approaches:

• Top-Down: Testing starts from the top modules, with lower modules replaced by stubs.
• Bottom-Up: Testing begins with lower modules, with higher modules replaced by drivers.
• Hybrid: Combines both top-down and bottom-up approaches.

Examples of integration in Java:

• Testing service interaction with a database.
• Testing one REST API call from another.
• Testing component operation with message queue systems (e.g., Kafka).

Frameworks often used for JVM applications include Spring Boot Test, JUnit 5 with extensions (e.g., Testcontainers), Mockito (for mocking dependencies).

// Example of integration test with Spring Boot Test
@SpringBootTest
@AutoConfigureMockMvc
public class UserControllerIntegrationTest {

    @Autowired
    private MockMvc mockMvc;

    @Autowired
    private UserRepository userRepository; // Repository for DB operations

    @BeforeEach
    void setup() {
        userRepository.deleteAll(); // Clear database before test
    }

    @Test
    void testCreateUser() throws Exception {
        String userJson = "{\"username\": \"testUser\", \"email\": \"test@example.com\"}";

        mockMvc.perform(post("/users") // Call controller
                        .contentType(MediaType.APPLICATION_JSON)
                        .content(userJson))
                .andExpect(status().isCreated()) // Expect 201 status
                .andExpect(jsonPath("$.username").value("testUser")); // Check response content

        assertThat(userRepository.findByUsername("testUser")).isPresent(); // Check DB
    }
}

Advantages	Disadvantages
Detects interaction errors between modules.	More complex to write and maintain than unit tests.
Helps verify external system interactions.	Longer execution time.
Closer to real usage scenarios.	Requires environment setup (DB, services, etc.).