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