131-java-testing-unit-testing
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
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/jabrena/plinth --skill 131-java-testing-unit-testing- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads 131-java-testing-unit-testing automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
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 compileormvn compilebefore applying any change - SAFETY: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition
- VERIFY: Run
./mvnw clean verifyormvn clean verifyafter 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.
FAQ
What does the 131-java-testing-unit-testing skill do?
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
How do I install 131-java-testing-unit-testing?
Run npx skills add https://github.com/jabrena/plinth --skill 131-java-testing-unit-testing in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/131-java-testing-unit-testing/ (personal) or .claude/skills/131-java-testing-unit-testing/ (project).
Which AI agents can use 131-java-testing-unit-testing?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is 131-java-testing-unit-testing safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains 131-java-testing-unit-testing?
It is published by jabrena in jabrena/plinth. It has 453 recorded installs.
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