INTRODUCTION TO TEST

Testing is the process of checking whether your code works as expected.

In Spring, tests help verify:

The correctness of business logic
Integration between components (e.g., controllers, services, repositories)
Application behavior in different environments

Why Write Tests?

Benefit	Description
Catch bugs early	Prevent errors before production
Save time	Easier debugging, faster fixes
Improve design	Testable code is often cleaner and modular
Confidence	Safer refactoring and deployment
Automation	Continuous Integration (CI) tools rely on test automation

Unit Tests

Unit tests focus on testing individual methods or classes in isolation.

Tools:

JUnit 5 (JUnit Jupiter)
Mockito (for mocking dependencies)

Example:

java
@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

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

        User result = userService.getUserById(1L);

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

Unit tests do not load the Spring context.

Integration Tests

Integration tests check how multiple components work together using real Spring beans.