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

1. Compile project before unit-test changes

Run ./mvnw compile or mvn compile and stop immediately if compilation fails.

2. Read unit-testing reference and evaluate coverage

Read references/131-java-testing-unit-testing.md and identify modernization and quality gaps in current tests.

3. Apply unit-testing best practices

Implement or refactor tests using JUnit 5, AssertJ, Mockito, parameterization, and stronger boundary checks.

4. 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.