131-java-testing-unit-testing

name	131-java-testing-unit-testing
description	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 boundary conditions (RIGHT-BICEP, CORRECT, A-TRIP), leveraging JSpecify null-safety annotations, or eliminating testing anti-patterns such as reflection-based tests or shared mutable state. Part of the skills-for-java project
license	Apache-2.0
metadata	{"author":"Juan Antonio Breña Moral","version":"0.13.0-SNAPSHOT"}

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

Reference

For detailed guidance, examples, and constraints, see references/131-java-testing-unit-testing.md.