Use when you need to review, improve, or write Java unit tests — including migrating from JUnit 4 to JUnit 5, adopting AssertJ for fluent assertions, structuring tests with Given-When-Then, ensuring test independence, applying parameterized tests, mocking dependencies with Mockito, verifying boundar

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](references/131 java testing unit testing.md).

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