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.
Constraints
Before applying any unit test changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until resolved. After applying improvements, run full verification.
- MANDATORY: Run
./mvnw compile or mvn compile before applying any change
- SAFETY: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition
- VERIFY: Run
./mvnw clean verify or mvn clean verify after applying improvements
- BEFORE APPLYING: Read the reference for detailed examples, good/bad patterns, and constraints
When to use this skill
- Review Java code for unit tests
- Apply best practices for unit tests in Java code
- Write fast JUnit unit tests for Java code
- Improve Mockito-based Java unit tests
- Refactor Java tests to isolate collaborators
Workflow
- Compile project before unit-test changes
Run ./mvnw compile or mvn compile and stop immediately if compilation fails.
- Read unit-testing reference and evaluate coverage
Read references/131-java-testing-unit-testing.md and identify modernization and quality gaps in current tests.
- Apply unit-testing best practices
Implement or refactor tests using JUnit 5, AssertJ, Mockito, parameterization, and stronger boundary checks.
- Verify with full build
Run ./mvnw clean verify or mvn clean verify after applying improvements.
Reference
For detailed guidance, examples, and constraints, see references/131-java-testing-unit-testing.md.