Write JUnit 5 Tests with AI Coding Assistants

A skill for generating production-grade JUnit 5 unit and integration tests in Java, covering assertions, parameterized tests, lifecycle hooks, Mockito mocking, and nested tests. It routes on keywords - "unit test"/"assert" for a standard test, "parameterized"/"multiple inputs" for @ParameterizedTest, "mock"/"Mockito" for a test with mocking, "integration test"/"Spring" pointing to a reference/spring-integration.md doc, defaulting to a standard unit test otherwise. Core patterns show @BeforeEach setup, @Test with @DisplayName, assertThrows for exceptions, and assertAll for grouped assertions that report all failures together rather than stopping at the first. The assertions reference covers assertEquals/assertNotEquals/assertTrue/assertFalse/assertNull/assertNotNull/assertThrows/assertTimeout/assertAll/assertIterableEquals. Parameterized tests cover @ValueSource for a literal list of inputs, @CsvSource for multi-argument rows, @MethodSource pointing to a static Stream<Arguments> provider method, and @NullAndEmptySource combined with @ValueSource for blank-string edge cases. Mockito mocking uses @ExtendWith(MockitoExtension.class) with @Mock fields and @InjectMocks on the class under test, stubbing with when()/thenReturn() and verifying calls with verify(), including a not-found case returning Optional.empty() and asserting the resulting exception. Nested tests use @Nested classes with their own @DisplayName to group related test cases (e.g. "when creating a user" and "when deleting a user" as sibling nested classes). A documented anti-pattern table contrasts non-descriptive test names with descriptive ones, testing private methods versus testing through the public API, omitting @DisplayName versus always adding one, and assertEquals(true, x) versus the more readable assertTrue(x). Maven dependencies are junit-jupiter 5.11.0 and mockito-junit-jupiter 5.14.0, both test-scoped. A quick-reference table covers running all tests (mvn test / gradlew test), a single class or method, tagged tests via @Tag plus -Dgroups, @Disabled with a reason, OS-conditional execution via @EnabledOnOs, @Timeout, @RepeatedTest, and @TestMethodOrder for explicit ordering. A deeper reference/playbook.md covers project setup (Maven deps, parallel config, surefire), test lifecycle (BeforeAll/Each, ordering, tags), deeper parameterized testing (CsvSource, MethodSource, EnumSource, ValueSource), Mockito (captors, verify order), nested and dynamic tests (@TestFactory), and AssertJ fluent assertions.

Use it when the user mentions JUnit, JUnit 5, @Test, assertEquals, Assertions, or "Java unit test" - needing standard unit tests, parameterized tests, Mockito-based mocking, or Spring integration tests.

Given Java code to test, it produces a JUnit 5 test class using the appropriate pattern - standard assertions, @ParameterizedTest with the right source annotation, Mockito-mocked dependencies, or @Nested grouping - following the documented naming and anti-pattern guidance.

<dependency><groupId>org.junit.jupiter</groupId><artifactId>junit-jupiter</artifactId><version>5.11.0</version></dependency>

Uses JUnit 5 (Jupiter) as the test framework, Mockito for mocking dependencies, and integrates with Maven or Gradle for running tests via CLI commands.

Java developers writing JUnit 5 tests - unit tests, parameterized tests across multiple inputs, mocked service tests, or grouped nested test suites - who want correct annotation choice and awareness of common anti-patterns like testing private methods or vague test names.