Testing with JUnit 5, Mockito, AssertJ

Practical testing patterns for Java services and APIs.
Prefer readable tests over clever tests.

Selective Reading Rule

Read ONLY files relevant to the request! Use the content map to focus.

Content Map

Core Patterns

1. JUnit 5 Structure

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

    @Test
    void findById_whenExists_returnsUser() {
        var user = User.create("u1", "[email protected]");
        when(userRepository.findById("u1")).thenReturn(Optional.of(user));

        var result = userService.findById("u1");

        assertThat(result).isPresent();
        assertThat(result.get().getEmail()).isEqualTo("[email protected]");
    }
}

2. Parameterized Tests

@ParameterizedTest
@ValueSource(strings = {"[email protected]", "[email protected]"})
void validateEmail_acceptsValid(String email) {
    var result = EmailValidator.isValid(email);
    assertThat(result).isTrue();
}

@ParameterizedTest
@CsvSource({
    "PENDING, true",
    "ACTIVE, false"
})
void isInactive_flagsStatus(String status, boolean expected) {
    assertThat(UserStatus.valueOf(status).isInactive()).isEqualTo(expected);
}

3. Mockito Stubbing and Verification

@Test
void createUser_savesEntity() {
    var request = new CreateUserRequest("[email protected]", "Dev");
    when(userRepository.existsByEmail(request.email())).thenReturn(false);

    userService.create(request);

    verify(userRepository).save(any(User.class));
}

@Test
void updateUser_throwsWhenMissing() {
    when(userRepository.findById("missing")).thenReturn(Optional.empty());

    assertThatThrownBy(() -> userService.update("missing", new UpdateUserRequest("name")))
        .isInstanceOf(ResourceNotFoundException.class)
        .hasMessageContaining("User");
}

4. AssertJ for Readability

assertThat(users)
    .hasSize(3)
    .extracting(User::getEmail)
    .contains("[email protected]");

assertThat(response)
    .returns(HttpStatus.CREATED, ResponseEntity::getStatusCode)
    .extracting(ResponseEntity::getBody)
    .isNotNull();

5. Test Lifecycle and Organization

@Nested
class CreateUserTests {

    @Test
    void rejectsDuplicateEmail() { }

    @Test
    void hashesPassword() { }
}

@BeforeEach
void setup() {
    // Use only for shared, simple setup
}

Decision Checklist

• Test name explains behavior? (when/then or should format)
• Only one reason to fail?
• Mocks only for external dependencies?
• Assertions are readable?
• Parameterized test when data-driven?

Anti-Patterns

Related Skills