Java Unit testing guidelines

Review and improve Java unit tests using modern JUnit 5, AssertJ, and Mockito best practices.

What is covered in this Skill?

• JUnit 5 annotations: @Test, @BeforeEach, @AfterEach, @DisplayName, @Nested, @ParameterizedTest
• AssertJ fluent assertions: assertThat, assertThatThrownBy
• Given-When-Then test structure, descriptive test naming, single-responsibility tests
• Test independence and isolated state
• Parameterized tests: @ValueSource/@CsvSource/@MethodSource
• Mockito dependency mocking: @Mock, @InjectMocks, MockitoExtension
• Code coverage guidance (JaCoCo), package-private test visibility
• Testing anti-patterns: reflection, shared state, hard-coded values, testing implementation details
• Error handling: assertThatThrownBy, exception messages
• JSpecify null-safety: @NullMarked, @Nullable
• RIGHT-BICEP coverage principles, A-TRIP test quality, CORRECT boundary condition verification

Scope: The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples.