junit-testing

JUnit 5 Testing Patterns

Patterns for writing effective unit, integration, and slice tests in Java.

When to Use

• User is writing JUnit 5 tests
• User needs to mock dependencies with Mockito
• User asks about Spring Boot test slices (@WebMvcTest, @DataJpaTest)
• User wants integration tests with real databases (Testcontainers)
• User needs parameterized tests for multiple inputs

Core Patterns

JUnit 5 Annotations and Lifecycle

import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;

class UserServiceTest {
    private UserService userService;
    private UserRepository userRepository;

    @BeforeEach
    void setUp() {
        userRepository = new InMemoryUserRepository();
        userService = new UserService(userRepository);
    }

    @Test
    @DisplayName("creates a user with valid input")
    void createsUserWithValidInput() {
        User result = userService.createUser(new CreateUserRequest("Alice", "[email protected]"));
        assertAll(
            () -> assertNotNull(result.getId()),
            () -> assertEquals("Alice", result.getName()),
            () -> assertEquals("[email protected]", result.getEmail())
        );
    }

    @Test
    @DisplayName("throws when email is already taken")
    void throwsWhenEmailTaken() {
        userRepository.save(new User("Bob", "[email protected]"));
        assertThrows(ConflictException.class,
            () -> userService.createUser(new CreateUserRequest("Bob2", "[email protected]")));
    }

    @Nested
    @DisplayName("when user exists")
    class WhenUserExists {
        private User existingUser;

        @BeforeEach
        void setUp() {
            existingUser = userRepository.save(new User("Alice", "[email protected]"));
        }

        @Test
        void findsById() {
            Optional<User> found = userService.findById(existingUser.getId());
            assertTrue(found.isPresent());
            assertEquals("Alice", found.get().getName());
        }
    }
}