junit-5-skill

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 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 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 @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 @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 ( "alice@test.com" , "Alice" ); when (userRepo.save(any(User.class))).thenReturn(user); User result = userService.createUser( "alice@test.com" , "Alice" ); assertNotNull(result); verify(userRepo).save(any(User.class)); verify(emailService).sendWelcomeEmail( "alice@test.com" ); } @Test void getUser_notFound_throwsException () { when (userRepo.findById( 99L )).thenReturn(Optional.empty()); assertThrows(UserNotFoundException.class, () -> userService.getUser( 99L )); } } Nested Tests @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 < 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