java-junit

SKILL.md

JUnit 5+ Best Practices

Your goal is to help me write effective unit tests with JUnit 5, covering both standard and data-driven testing approaches.

Project Setup

Use a standard Maven or Gradle project structure.

Place test source code in src/test/java.

Include dependencies for junit-jupiter-api, junit-jupiter-engine, and junit-jupiter-params for parameterized tests.

Use build tool commands to run tests: mvn test or gradle test.

Test Structure

Test classes should have a Test suffix, e.g., CalculatorTest for a Calculator class.

Use @Test for test methods.

Follow the Arrange-Act-Assert (AAA) pattern.

Name tests using a descriptive convention, like methodName_should_expectedBehavior_when_scenario.

Use @BeforeEach and @AfterEach for per-test setup and teardown.

Use @BeforeAll and @AfterAll for per-class setup and teardown (must be static methods).

Use @DisplayName to provide a human-readable name for test classes and methods.

Standard Tests

Keep tests focused on a single behavior.

Avoid testing multiple conditions in one test method.

Make tests independent and idempotent (can run in any order).

Avoid test interdependencies.

Data-Driven (Parameterized) Tests

Use @ParameterizedTest to mark a method as a parameterized test.

Use @ValueSource for simple literal values (strings, ints, etc.).

Use @MethodSource to refer to a factory method that provides test arguments as a Stream, Collection, etc.

Use @CsvSource for inline comma-separated values.

Use @CsvFileSource to use a CSV file from the classpath.

Use @EnumSource to use enum constants.

Assertions

Use the static methods from org.junit.jupiter.api.Assertions (e.g., assertEquals, assertTrue, assertNotNull).

For more fluent and readable assertions, consider using a library like AssertJ (assertThat(...).is...).

Use assertThrows or assertDoesNotThrow to test for exceptions.

Group related assertions with assertAll to ensure all assertions are checked before the test fails.

Use descriptive messages in assertions to provide clarity on failure.

Mocking and Isolation

Use a mocking framework like Mockito to create mock objects for dependencies.

Use @Mock and @InjectMocks annotations from Mockito to simplify mock creation and injection.

Use interfaces to facilitate mocking.

Test Organization

Group tests by feature or component using packages.

Use @Tag to categorize tests (e.g., @Tag("fast"), @Tag("integration")).

Use @TestMethodOrder(MethodOrderer.OrderAnnotation.class) and @Order to control test execution order when strictly necessary.

Use @Disabled to temporarily skip a test method or class, providing a reason.

Use @Nested to group tests in a nested inner class for better organization and structure.