Generate Comprehensive JUnit Test Classes

Writes comprehensive JUnit 5 test classes for Java applications following the AAA pattern (Arrange, Act, Assert), with descriptive test method names, Test-suffixed class names, @Nested classes for grouping related tests, and @DisplayName for human-readable descriptions. Tests are kept independent via @BeforeEach/@AfterEach setup and teardown, mocked external dependencies (@Mock, @InjectMocks with MockitoExtension), and explicit state reset to prevent pollution between runs. It produces concrete patterns across several JUnit 5 features: parameterized tests using @ValueSource (a list of invalid email formats) and @CsvSource (input/expected-output pairs, such as membership-tier discount calculations), nested test classes (@Nested groups like order-creation vs. order-cancellation scenarios), exception testing with assertThatThrownBy(...).isInstanceOf(...).hasMessage(...), and asynchronous-operation testing using CountDownLatch and AtomicReference to synchronize on a callback result. Assertions favor AssertJ's fluent API (assertThat(...).hasSize(...).extracting(...).containsExactlyInAnyOrder(...), and .satisfies(...) for multi-field object checks) over plain JUnit assertions.

Use it to write or extend a Java test suite that needs JUnit 5 idioms done right - parameterized cases instead of copy-pasted near-duplicate tests, @Nested grouping for a class with multiple distinct scenarios, proper mocking of collaborators via Mockito, and AssertJ assertions instead of verbose multi-line JUnit assertions. It also covers when a suite should switch to @TestInstance(TestInstance.Lifecycle.PER_CLASS) - when expensive shared setup shouldn't be recreated for every single test method - and combine multiple extensions via @ExtendWith({MockitoExtension.class, TestDatabaseExtension.class}) rather than registering mocking and infrastructure extensions separately.

Input is the Java class or behavior to be tested. Output is a JUnit 5 test class: @BeforeEach-driven setup, Mockito-mocked dependencies, @Test/@ParameterizedTest/@Nested methods with @DisplayName descriptions, AssertJ assertions, and - where the test suite itself needs configuring - a junit-platform.properties file (parallel execution mode, display-name generator) or a custom @RegisterExtension such as a database extension that loads a schema and seed data.

Built on JUnit 5 (Jupiter), Mockito (MockitoExtension, @Mock, @InjectMocks), and AssertJ for fluent assertions, with configuration support for parallel test execution and custom Extension classes registered via @ExtendWith/@RegisterExtension.

For Java developers who want tests that read as living documentation rather than a pile of boilerplate assertions - business-focused test names, one behavior verified per test, test-data builders or factory methods instead of inline object construction, @Timeout on tests that could hang, mocking only external dependencies and never the class under test, and treating meaningful coverage as the goal rather than a raw percentage target.

@ParameterizedTest
@DisplayName("Should calculate discount correctly for different membership levels")
@CsvSource({
    "BRONZE, 100.0, 95.0",
    "SILVER, 100.0, 90.0",
    "GOLD, 100.0, 80.0",
    "PLATINUM, 100.0, 70.0"
})
void shouldCalculateDiscountCorrectly(MembershipLevel level, 
                                      BigDecimal originalPrice, 
                                      BigDecimal expectedPrice) {
    // Act
    BigDecimal actualPrice = discountService.applyDiscount(originalPrice, level);
    
    // Assert
    assertThat(actualPrice).isEqualByComparingTo(expectedPrice);
}