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java-junit

Write, review and fix Java unit tests with JUnit 5 and JUnit 6 (Jupiter), AssertJ and Mockito: assertions, parameterized and nested tests, mocking rules, deterministic time and randomness, Surefire vs Failsafe, JaCoCo coverage, optional PIT mutation testing, and guardrails for agents (never skip or delete failing tests). Use when adding, refactoring or debugging tests in Maven or Gradle projects. Framework-neutral.

skills/java-junit/SKILL.md

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# Java Testing with JUnit 5/6, AssertJ and Mockito

Framework-neutral guidance for fast, deterministic unit tests. For Spring slices use `spring-boot-testing` (if installed); for real databases/brokers in Docker use `java-testcontainers`; for failures you cannot explain, `java-debugging`.

## First: detect, don't impose

1. Read `pom.xml`/`build.gradle*`: JUnit version (`junit-jupiter`, `junit-bom`, or inherited from a Spring Boot BOM), assertion library, Mockito, Surefire/Failsafe versions, Java release.
2. Copy the conventions already in `src/test/java` (naming, assertion library, static imports, mocking style). Do not introduce AssertJ, Mockito or JaCoCo into a project that lacks them without asking.
3. Never bump the JUnit major version implicitly; upgrading is its own task (see `java-upgrade` if installed).

## JUnit 5 vs JUnit 6

| | JUnit 5 (last 5.14.x) | JUnit 6 (6.0 released; 6.1.x current) |
| --- | --- | --- |