JUnit 5 Testing Skill

Description

--- name: junit-5-skill description: > Generates production-grade JUnit 5 unit and integration tests in Java. Covers assertions, parameterized tests, lifecycle hooks, mocking with Mockito, and nested tests. Use when user mentions "JUnit", "JUnit 5", "@Test", "assertEquals", "Assertions", "Java unit test". Triggers on: "JUnit", "@Test", "assertEquals", "Java test", "unit test Java". languages: - Java category: unit-testing license: MIT metadata: author: TestMu AI version: "1.0" --- # JUnit 5 Testing Skill You are a senior Java developer specializing in JUnit 5 testing. ## Step 1 — Test Type ``` ├─ "unit test", "assert" → Standard unit test ├─ "parameterized", "multiple inputs" → @ParameterizedTest ├─ "mock", "Mockito" → Unit test with Mockito ├─ "integration test", "Spring" → Read reference/spring-integration.md └─ Default → Standard unit test ``` ## Core Patterns ### Basic Test ```java import org.junit.jupiter.api.*; import static org.junit.jupiter.api.Assertions.*; class CalculatorTest { private Calculator calculator; @BeforeEach void setUp() { calculator = new Calculator(); } @Test @DisplayName("Addition of two positive numbers") void addPositiveNumbers() { assertEquals(5, calculator.add(2, 3)); } @Test void divideByZero_throwsException() { assertThrows(ArithmeticException.class, () -> calculator.divide(10, 0)); } @Test void multipleAssertions() { assertAll("calculator operations", () -> assertEquals(4, calculator.add(2, 2)), () -> assertEquals(0, calculator.subtract(2, 2)), () -> assertEquals(6, calculator.multiply(2, 3)) ); } } ``` ### Assertions Reference ```java assertEquals(expected, actual); assertNotEquals(unexpected, actual); assertTrue(condition); assertFalse(condition); assertNull(object); assertNotNull(object); assertThrows(IllegalArgumentException.class, () -> service.process(null)); assertTimeout(Duration.ofSeconds(2), () -> service.longRunningOp()); assertAll("group", () -> assertNotNull(user.getName()), () -> assertTrue(user.getAge() > 0) ); assertIterableEquals(List.of(1, 2, 3), actualList); ``` ### Parameterized Tests ```java @ParameterizedTest @ValueSource(strings = {"hello", "world", "junit"}) void stringIsNotEmpty(String value) { assertFalse(value.isEmpty()); } @ParameterizedTest @CsvSource({"1,1,2", "2,3,5", "10,-5,5"}) void addNumbers(int a, int b, int expected) { assertEquals(expected, calculator.add(a, b)); } @ParameterizedTest @MethodSource("provideUsers") void validateUser(String name, int age, boolean expected) { assertEquals(expected, validator.isValid(name, age)); } static Stream<Arguments> provideUsers() { return Stream.of( Arguments.of("Alice", 25, true), Arguments.of("", 25, false), Arguments.of("Bob", -1, false) ); } @ParameterizedTest @NullAndEmptySource @ValueSource(strings = {" ", "\t"}) void blankStringsAreInvalid(String input) { assertFalse(validator.isValid(input)); } ``` ### Mocking with Mockito ```java @ExtendWith(MockitoExtension.class) class UserServiceTest { @Mock private UserRepository userRepo; @Mock private EmailService emailService; @InjectMocks private UserService userService; @Test void createUser_savesAndSendsEmail() { User user = new User("[email protected]", "Alice"); when(userRepo.save(any(User.class))).thenReturn(user); User result = userService.createUser("[email protected]", "Alice"); assertNotNull(result); verify(userRepo).save(any(User.class)); verify(emailService).sendWelcomeEmail("[email protected]"); } @Test void getUser_notFound_throwsException() { when(userRepo.findById(99L)).thenReturn(Optional.empty()); assertThrows(UserNotFoundException.class, () -> userService.getUser(99L)); } } ``` ### Nested Tests ```java @DisplayName("UserService") class UserServiceTest { @Nested @DisplayName("when creating a user") class CreateUser { @Test void withValidData_succeeds() { } @Test void withDuplicateEmail_throwsException() { } } @Nested @DisplayName("when deleting a user") class DeleteUser { @Test void existingUser_removesFromDb() { } @Test void nonExistentUser_throwsException() { } } } ``` ### Anti-Patterns | Bad | Good | Why | |-----|------|-----| | `@Test public void test1()` | `@Test void shouldCalculateSum()` | Descriptive names | | Testing private methods | Test via public API | Implementation detail | | No @DisplayName | Always add display names | Better reporting | | `assertEquals(true, x)` | `assertTrue(x)` | More readable | ## Maven Dependencies ```xml <dependency> <groupId>org.junit.jupiter</groupId> <artifactId>junit-jupiter</artifactId> <version>5.11.0</version> <scope>test</scope> </dependency> <dependency> <groupId>org.mockito</groupId> <artifactId>mockito-junit-jupiter</artifactId> <version>5.14.0</version> <scope>test</scope> </dependency> ``` ## Quick Reference | Task | Command | |------|---------| | Run all | `mvn test` or `./gradlew test` | | Run class | `mvn test -Dtest=UserServiceTest` | | Run method | `mvn test -Dtest=UserServiceTest#createUser_succeeds` | | Run tagged | `@Tag("slow")` + `mvn test -Dgroups="slow"` | | Disable | `@Disabled("Reason")` | | Conditional | `@EnabledOnOs(OS.LINUX)` | | Timeout | `@Timeout(value = 5, unit = TimeUnit.SECONDS)` | | Repeated | `@RepeatedTest(5)` | | Order | `@TestMethodOrder(MethodOrderer.OrderAnnotation.class)` | ## Deep Patterns For production-grade patterns, see `reference/playbook.md`: | Section | What's Inside | |---------|--------------| | §1 Project Setup | Maven deps, parallel config, surefire | | §2 Test Lifecycle | BeforeAll/Each, ordering, tags | | §3 Parameterized | CsvSource, MethodSource, EnumSource, ValueSource | | §4 Mockito | @Mock/@InjectMocks, captor, verify order | | §5 Nested & Dynamic | @Nested grouping, @TestFactory | | §6 AssertJ | Fluent assertions, extracting, collection checks | | §7 Conditional | @EnabledOnOs, assumptions, @EnabledIf | | §8 Custom Extensions | Timing, retry, BeforeTestExecution | | §9 CI/CD | GitHub Actions with test reporter | | §10 Debugging Table | 8 common problems with fixes | | §11 Best Practices | 12-item production checklist |

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