java-junit

name	java-junit
description	Best practices for JUnit 5 unit testing, including data-driven tests and modern patterns. Trigger: When writing JUnit tests, creating unit tests, or need testing best practices.
license	MIT
metadata	{"author":"vekzz-dev","version":"1.1"}

When to Use

• Writing JUnit 5 unit tests
• Creating parameterized tests
• Setting up test infrastructure
• Need testing best practices for Java

Instructions

Project Setup

• Use a standard Maven or Gradle project structure
• Place test source code in src/test/java
• Include dependencies for junit-jupiter-api, junit-jupiter-engine, and junit-jupiter-params for parameterized tests
• Use build tool commands to run tests: mvn test or gradle test

Test Structure

• Test classes should have a Test suffix, e.g., CalculatorTest for a Calculator class
• Use @Test for test methods
• Follow the Arrange-Act-Assert (AAA) pattern
• Name tests using a descriptive convention, like methodName_should_expectedBehavior_when_scenario
• Use @BeforeEach and @AfterEach for per-test setup and teardown
• Use @BeforeAll and @AfterAll for per-class setup and teardown (must be static methods)
• Use @DisplayName to provide a human-readable name for test classes and methods

class CalculatorTest {

    private Calculator calculator;

    @BeforeEach
    void setUp() {
        calculator = new Calculator();
    }

    @Test
    @DisplayName("Should add two positive numbers")
    void add_shouldReturnSum_whenBothPositive() {
        // Arrange
        int a = 3, b = 5;

        // Act
        int result = calculator.add(a, b);

        // Assert
        assertEquals(8, result);
    }

    @Test
    @DisplayName("Should throw exception when dividing by zero")
    void divide_shouldThrowException_whenDenominatorIsZero() {
        assertThrows(ArithmeticException.class,
            () -> calculator.divide(10, 0),
            "Division by zero should throw");
    }
}

Standard Tests

• Keep tests focused on a single behavior
• Avoid testing multiple conditions in one test method
• Make tests independent and idempotent (can run in any order)
• Avoid test interdependencies

Data-Driven (Parameterized) Tests

• Use @ParameterizedTest to mark a method as a parameterized test
• Use @ValueSource for simple literal values (strings, ints, etc.)
• Use @MethodSource to refer to a factory method that provides test arguments as a Stream, Collection, etc.
• Use @CsvSource for inline comma-separated values
• Use @CsvFileSource to use a CSV file from the classpath
• Use @EnumSource to use enum constants

@ParameterizedTest
@ValueSource(ints = {1, 2, 3, 5, 8, 13})
@DisplayName("Should identify Fibonacci numbers")
void isFibonacci_shouldReturnTrue_forFibonacciNumbers(int number) {
    assertTrue(calculator.isFibonacci(number));
}

@ParameterizedTest
@CsvSource({
    "4, 2, 2",
    "10, 3, 1",
    "7, 5, 2"
})
@DisplayName("Should compute modulo for various inputs")
void modulo_shouldReturnRemainder(int dividend, int divisor, int expected) {
    assertEquals(expected, calculator.modulo(dividend, divisor));
}

@ParameterizedTest
@MethodSource("provideOrderStatuses")
@DisplayName("Should return correct display name for each status")
void getDisplayName_shouldReturnCorrectLabel(OrderStatus status, String expected) {
    assertEquals(expected, status.getDisplayName());
}

static Stream<Arguments> provideOrderStatuses() {
    return Stream.of(
        Arguments.of(OrderStatus.PENDING, "Pending"),
        Arguments.of(OrderStatus.SHIPPED, "Shipped"),
        Arguments.of(OrderStatus.DELIVERED, "Delivered")
    );
}

Assertions

• Use the static methods from org.junit.jupiter.api.Assertions (e.g., assertEquals, assertTrue, assertNotNull)
• For more fluent and readable assertions, consider using a library like AssertJ (assertThat(...).is...)
• Use assertThrows or assertDoesNotThrow to test for exceptions
• Group related assertions with assertAll to ensure all assertions are checked before the test fails
• Use descriptive messages in assertions to provide clarity on failure

@Test
@DisplayName("Should create order with all required fields")
void createOrder_shouldSetAllFields() {
    Order order = orderService.create("customer-1", List.of("item-a"), 2990);

    assertAll("order fields",
        () -> assertEquals("customer-1", order.getCustomerId(), "customer id"),
        () -> assertEquals(2, order.getItemCount(), "item count"),
        () -> assertNotNull(order.getCreatedAt(), "creation timestamp"),
        () -> assertEquals(OrderStatus.PENDING, order.getStatus(), "initial status")
    );
}

// AssertJ example
@Test
@DisplayName("Should return orders sorted by date")
void findRecentOrders_shouldReturnSortedResults() {
    List<Order> orders = orderService.findRecent(7);

    assertThat(orders)
        .isNotEmpty()
        .hasSizeLessThanOrEqualTo(50)
        .extracting(Order::getStatus)
        .contains(OrderStatus.SHIPPED);
}

Mocking and Isolation

• Use a mocking framework like Mockito to create mock objects for dependencies
• Use @Mock and @InjectMocks annotations from Mockito to simplify mock creation and injection
• Use interfaces to facilitate mocking

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {

    @Mock
    private PaymentGateway paymentGateway;

    @Mock
    private InventoryClient inventoryClient;

    @InjectMocks
    private OrderService orderService;

    @Test
    @DisplayName("Should process payment when placing order")
    void placeOrder_shouldProcessPayment() {
        // Arrange
        when(inventoryClient.isAvailable("item-1")).thenReturn(true);
        when(paymentGateway.charge(any(Payment.class)))
            .thenReturn(new PaymentResult("tx-123", PaymentStatus.SUCCESS));

        // Act
        Order order = orderService.placeOrder("customer-1", "item-1");

        // Assert
        assertThat(order.getTransactionId()).isEqualTo("tx-123");
        verify(paymentGateway).charge(any(Payment.class));
    }

    @Test
    @DisplayName("Should throw when inventory is insufficient")
    void placeOrder_shouldThrow_whenItemUnavailable() {
        when(inventoryClient.isAvailable("item-1")).thenReturn(false);

        assertThrows(InsufficientInventoryException.class,
            () -> orderService.placeOrder("customer-1", "item-1"));

        verify(paymentGateway, never()).charge(any());
    }
}

Test Organization

• Group tests by feature or component using packages
• Use @Tag to categorize tests (e.g., @Tag("fast"), @Tag("integration"))
• Use @TestMethodOrder(MethodOrderer.OrderAnnotation.class) and @Order to control test execution order when strictly necessary
• Use @Disabled to temporarily skip a test method or class, providing a reason
• Use @Nested to group tests in a nested inner class for better organization and structure

@DisplayName("OrderService")
class OrderServiceTest {

    @Nested
    @DisplayName("placeOrder")
    class PlaceOrder {

        @Test
        @DisplayName("Should create order successfully")
        void shouldCreateOrder() { }

        @Test
        @DisplayName("Should reject duplicate orders")
        void shouldRejectDuplicate() { }
    }

    @Nested
    @DisplayName("cancelOrder")
    class CancelOrder {

        @Test
        @DisplayName("Should cancel pending order")
        void shouldCancelPending() { }

        @Test
        @DisplayName("Should throw when order is already shipped")
        void shouldThrowWhenShipped() { }
    }
}

Commands

# Run all tests
mvn test

# Run specific test class
mvn test -Dtest=CalculatorTest

# Run tests with specific tag
mvn test -Dgroups=fast

# Run tests with Gradle
gradle test

# Run specific test with Gradle
gradle test --tests CalculatorTest

Resources

• JUnit 5 User Guide
• AssertJ Documentation
• Mockito Documentation

Context7 Integration

For up-to-date information on JUnit 5 and AssertJ:

1. Resolve the libraryId - Use context7_resolve-library-id:

   • libraryName: "junit 5", "assertj", or "mockito"
   • query: what you need (e.g., "parameterized tests", "assertions")

2. Query the docs - Use context7_query-docs:

   • libraryId: "/junit/junit5" or "/assertj/assertj"
   • query: your specific question

Before writing tests, consult Context7 to get updated examples.