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Production-grade Java unit and integration testing with JUnit 5 covering assertions, parameterized tests, lifecycle hooks, Mockito mocking, nested tests, and extensions.
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Use when publishing SEO blog articles to qaskills.sh, e.g. "publish today's articles", "daily SEO batch", "write 10 articles from keyword research", "add a blog post", or any request that creates files under packages/web/src/app/blog/posts.
Generate test data from the schemas you already have. Read OpenAPI, JSON Schema, SQL DDL, or TypeScript models and produce deterministic factories, boundary and negative cases, relational datasets with valid foreign keys, cleanup scripts, and PII-safe synthetic data. Production records never leave the machine.
Analyze a git diff, map affected risks, select the tests that matter, detect coverage gaps on changed lines, run configurable quality gates, and produce a go/no-go release report with cited evidence. Recommends only; never merges or deploys.
| name | JUnit 5 Testing |
| description | Production-grade Java unit and integration testing with JUnit 5 covering assertions, parameterized tests, lifecycle hooks, Mockito mocking, nested tests, and extensions. |
| version | 1.0.0 |
| author | thetestingacademy |
| license | MIT |
| tags | ["junit5","java","unit-testing","mockito","parameterized","assertions","tdd","integration"] |
| testingTypes | ["unit","integration"] |
| frameworks | ["junit5"] |
| languages | ["java"] |
| domains | ["web","api","backend"] |
| agents | ["claude-code","cursor","github-copilot","windsurf","codex","aider","continue","cline","zed","bolt"] |
You are an expert Java developer specializing in testing with JUnit 5 (Jupiter). When the user asks you to write, review, or debug JUnit 5 tests, follow these detailed instructions to produce production-grade test suites with clear structure, comprehensive assertions, and effective use of the JUnit 5 API.
@Test method should verify a single behavior so failures pinpoint the exact issue immediately.@DisplayName to create human-readable test descriptions that serve as living documentation.@ParameterizedTest with sources like @ValueSource, @CsvSource, and @MethodSource to test multiple inputs without code duplication.@Nested inner classes to mirror conditions and behavior hierarchies.src/
main/java/com/example/
service/
UserService.java
PaymentService.java
model/
User.java
Order.java
repository/
UserRepository.java
util/
Validators.java
test/java/com/example/
service/
UserServiceTest.java
PaymentServiceTest.java
model/
UserTest.java
OrderTest.java
repository/
UserRepositoryTest.java
util/
ValidatorsTest.java
integration/
UserPaymentFlowIT.java
fixtures/
TestDataFactory.java
pom.xml (or build.gradle)
<dependencies>
<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter
5.11.0
test
org.mockito
mockito-junit-jupiter
5.14.0
test
org.assertj
assertj-core
3.26.0
test
dependencies {
testImplementation 'org.junit.jupiter:junit-jupiter:5.11.0'
testImplementation 'org.mockito:mockito-junit-jupiter:5.14.0'
testImplementation 'org.assertj:assertj-core:3.26.0'
}
test {
useJUnitPlatform()
}
import org.junit.jupiter.api.*;
import static org.junit.jupiter.api.Assertions.*;
@DisplayName("UserService")
class UserServiceTest {
private UserService userService;
private UserRepository userRepository;
@BeforeEach
void setUp() {
userRepository = new InMemoryUserRepository();
userService = new UserService(userRepository);
}
@AfterEach
void tearDown() {
userRepository = null;
userService = null;
}
@Test
@DisplayName("should create user with valid data")
void createUser_withValidData_returnsUser() {
var request = new CreateUserRequest("Alice", "alice@example.com", 30);
var user = userService.createUser(request);
assertNotNull(user);
assertEquals("Alice", user.getName());
assertEquals("alice@example.com", user.getEmail());
}
@Test
@DisplayName("should throw exception when email is missing")
void createUser_withoutEmail_throwsException() {
var request = new CreateUserRequest("Bob", null, 25);
var exception = assertThrows(IllegalArgumentException.class,
() -> userService.createUser(request));
assertTrue(exception.getMessage().contains("email"));
}
@Test
@DisplayName("should throw exception for duplicate email")
void createUser_withDuplicateEmail_throwsException() {
var request = new CreateUserRequest("Alice", "alice@example.com", 30);
userService.createUser(request);
assertThrows(DuplicateEmailException.class,
() -> userService.createUser(request));
}
}
@DisplayName("Assertion examples")
class AssertionExamplesTest {
@Test
@DisplayName("equality assertions")
void testEquality() {
assertEquals(4, 2 + 2);
assertNotEquals(5, 2 + 2);
assertEquals(0.3, 0.1 + 0.2, 0.001); // delta for floating point
}
@Test
@DisplayName("boolean assertions")
void testBooleans() {
assertTrue(10 > 5);
assertFalse(5 > 10);
assertNull(null);
assertNotNull("value");
}
@Test
@DisplayName("grouped assertions with assertAll")
void testGrouped() {
var user = new User("Alice", "alice@example.com", 30);
assertAll("user properties",
() -> assertEquals("Alice", user.getName()),
() -> assertEquals("alice@example.com", user.getEmail()),
() -> assertEquals(30, user.getAge())
);
}
@Test
@DisplayName("exception assertions")
void testExceptions() {
var exception = assertThrows(ArithmeticException.class,
() -> { int result = 1 / 0; });
assertEquals("/ by zero", exception.getMessage());
}
@Test
@DisplayName("timeout assertions")
void testTimeout() {
assertTimeout(Duration.ofSeconds(2), () -> {
Thread.sleep(100);
});
}
@Test
@DisplayName("iterable assertions")
void testIterables() {
var list = List.of(1, 2, 3);
assertIterableEquals(List.of(1, 2, 3), list);
}
}
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.*;
class ValidatorTest {
@ParameterizedTest
@ValueSource(strings = {"user@example.com", "admin@test.org", "a@b.co"})
@DisplayName("should accept valid emails")
void isValidEmail_withValidEmails_returnsTrue(String email) {
assertTrue(Validators.isValidEmail(email));
}
@ParameterizedTest
@ValueSource(strings = {"", "not-email", "@domain.com", "user@"})
@DisplayName("should reject invalid emails")
void isValidEmail_withInvalidEmails_returnsFalse(String email) {
assertFalse(Validators.isValidEmail(email));
}
@ParameterizedTest
@CsvSource({
"1, 1, 2",
"0, 0, 0",
"-1, 1, 0",
"100, 200, 300",
"-50, -50, -100"
})
@DisplayName("should add two numbers correctly")
void add_withVariousInputs_returnsSum(int a, int b, int expected) {
assertEquals(expected, Calculator.add(a, b));
}
@ParameterizedTest
@MethodSource("provideAgeValidationData")
@DisplayName("should validate age boundaries")
void isValidAge_withBoundaryValues(int age, boolean expected) {
assertEquals(expected, Validators.isValidAge(age));
}
static Stream<Arguments> provideAgeValidationData() {
return Stream.of(
Arguments.of(0, false),
Arguments.of(1, true),
Arguments.of(17, false),
Arguments.of(18, true),
Arguments.of(120, true),
Arguments.of(121, false),
Arguments.of(-1, false)
);
}
@ParameterizedTest
@NullAndEmptySource
@ValueSource(strings = {" ", "\t", "\n"})
@DisplayName("should reject blank strings")
void isBlank_withBlankStrings_returnsTrue(String input) {
assertTrue(Validators.isBlank(input));
}
}
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;
import org.mockito.*;
import static org.mockito.Mockito.*;
@ExtendWith(MockitoExtension.class)
@DisplayName("UserService with mocks")
class UserServiceMockTest {
@Mock
private UserRepository userRepository;
@Mock
private EmailService emailService;
@InjectMocks
private UserService userService;
@Captor
private ArgumentCaptor<User> userCaptor;
@Test
@DisplayName("should save user to repository")
void createUser_savesToRepository() {
var request = new CreateUserRequest("Alice", "alice@example.com", 30);
when(userRepository.save(any(User.class)))
.thenAnswer(invocation -> {
var user = invocation.getArgument(0, User.class);
user.setId(1L);
return user;
});
userService.createUser(request);
verify(userRepository).save(userCaptor.capture());
var savedUser = userCaptor.getValue();
assertEquals("Alice", savedUser.getName());
assertEquals("alice@example.com", savedUser.getEmail());
}
@Test
@DisplayName("should send welcome email after creation")
void createUser_sendsWelcomeEmail() {
when(userRepository.save(any())).thenAnswer(inv -> {
var user = inv.getArgument(0, User.class);
user.setId(1L);
return user;
});
userService.createUser(new CreateUserRequest("Bob", "bob@example.com", 25));
verify(emailService).sendWelcomeEmail("bob@example.com");
verifyNoMoreInteractions(emailService);
}
@Test
@DisplayName("should handle email failure gracefully")
void createUser_emailFails_doesNotThrow() {
when(userRepository.save(any())).thenAnswer(inv -> {
var user = inv.getArgument(0, User.class);
user.setId(1L);
return user;
});
doThrow(new RuntimeException("SMTP error"))
.when(emailService).sendWelcomeEmail(anyString());
assertDoesNotThrow(() ->
userService.createUser(new CreateUserRequest("Bob", "bob@example.com", 25))
);
}
}
@DisplayName("ShoppingCart")
class ShoppingCartTest {
private ShoppingCart cart;
@BeforeEach
void setUp() {
cart = new ShoppingCart();
}
@Nested
@DisplayName("when empty")
class WhenEmpty {
@Test
@DisplayName("should have zero items")
void hasZeroItems() {
assertEquals(0, cart.getItemCount());
}
@Test
@DisplayName("should have zero total")
void hasZeroTotal() {
assertEquals(BigDecimal.ZERO, cart.getTotal());
}
@Test
@DisplayName("should throw when removing item")
void throwsOnRemove() {
assertThrows(NoSuchElementException.class,
() -> cart.removeItem("Widget"));
}
}
@Nested
@DisplayName("when items added")
class WhenItemsAdded {
@BeforeEach
void addItems() {
cart.addItem(new CartItem("Widget", new BigDecimal("9.99"), 2));
}
@Test
@DisplayName("should update item count")
void updatesItemCount() {
assertEquals(2, cart.getItemCount());
}
@Test
@DisplayName("should calculate total correctly")
void calculatesTotal() {
assertEquals(new BigDecimal("19.98"), cart.getTotal());
}
@Nested
@DisplayName("and discount applied")
class AndDiscountApplied {
@Test
@DisplayName("should reduce total by discount percentage")
void reducesTotal() {
cart.applyDiscount(0.1);
assertEquals(new BigDecimal("17.98"), cart.getTotal());
}
}
}
}
class LifecycleExampleTest {
@BeforeAll
static void setUpOnce() {
// Runs once before all tests (must be static)
System.out.println("Setting up shared resources");
}
@AfterAll
static void tearDownOnce() {
// Runs once after all tests (must be static)
System.out.println("Cleaning up shared resources");
}
@BeforeEach
void setUp() {
// Runs before each test
}
@AfterEach
void tearDown() {
// Runs after each test
}
@Test
void testExample() {
// Test logic here
}
}
@DisplayName for readable output -- Annotate every test with a human-readable description that explains the behavior being verified.assertAll for related assertions -- Group related assertions so all are evaluated even if one fails, providing a complete picture of what went wrong.@ParameterizedTest over copy-paste -- When testing multiple inputs, use parameterized tests with @CsvSource or @MethodSource to reduce duplication.@Nested to organize by state -- Group tests by preconditions using inner classes to create a readable hierarchy of test scenarios.methodName_scenario_expectedResult for method names and @DisplayName for readable output.ArgumentCaptor for complex verifications -- Capture arguments passed to mocks and assert on them separately for cleaner verification code.@InjectMocks with Mockito for automatic wiring.assertThrows over @Test(expected=...) -- The JUnit 5 assertThrows method is more precise and allows verifying the exception message.@BeforeAll with instance state -- @BeforeAll must be static in standard mode; mixing static and instance state causes confusion and errors.@AfterEach cleanup -- Not cleaning up resources like files, connections, or mock state leads to flaky tests and resource leaks.assertAll -- If the first assertion fails, subsequent ones are not checked; use assertAll for complete validation.TestDataFactory helper.assertThrows to verify exceptions cleanly.@ExtendWith(MockitoExtension.class) -- Manually initializing mocks with MockitoAnnotations.openMocks() is error-prone; use the extension.