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jabrenajabrena/cursor-rules-java/skills/131-java-testing-unit-testing/SKILL.md

131-java-testing-unit-testing

Skill452 starsChanged 3 months agoApache-2.0View on GitHub

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. This should trigger for requests such as 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. Part of Plinth Toolkit

We split this skill into the rules it gives an agent

10 rules. About 0 are common, 5 are distinctive, and 5 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈5

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 5

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Test independence and isolated state

    line 18
  2. Distinctive:

    Before applying any unit test changes, ensure the project compiles.

    line 31
  3. Very specific:

    MANDATORY: Run ./mvnw compile or mvn compile before applying any change

    line 33
  4. Distinctive:

    SAFETY: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition

    line 34
  5. Very specific:

    VERIFY: Run ./mvnw clean verify or mvn clean verify after applying improvements

    line 35
  6. Distinctive:

    BEFORE APPLYING: Read the reference for detailed examples, good/bad patterns, and constraints

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  7. Distinctive:

    Write fast JUnit unit tests for Java code

    line 42
  8. Very specific:

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

    line 50
  9. Very specific:

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

    line 54
  10. Very specific:

    Run ./mvnw clean verify or mvn clean verify after applying improvements.

    line 62

≈ “Common” is an estimate: at least 1 in 5 of the 308,232 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

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.

  1. Read unit-testing reference and evaluate coverage

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

  1. Apply unit-testing best practices

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

  1. 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.

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