JUnit 5

Introduction

JUnit 5 (Jupiter) is the standard test framework for Java. Annotations like @Test, @BeforeEach, @ParameterizedTest and @Nested structure your test suite; assertions come from AssertJ or Hamcrest.

Informative example

JUnit 5 test class:

@DisplayName("OrderService tests")
class OrderServiceTest {
    OrderService service;
    OrderRepository mockRepo;

    @BeforeEach
    void setUp() {
        mockRepo = mock(OrderRepository.class);
        service = new OrderService(mockRepo, mockGateway);
    }

    @Test
    @DisplayName("should create order with valid request")
    void createOrder() {
        var req = new OrderRequest("SKU-1", 2);
        when(mockRepo.save(any())).thenAnswer(i -> i.getArgument(0));

        Order result = service.place(req);

        assertThat(result.sku()).isEqualTo("SKU-1");
        verify(mockRepo).save(any(Order.class));
    }

    @ParameterizedTest
    @ValueSource(ints = {0, -1, 10001})
    void rejectsInvalidQuantity(int qty) {
        assertThrows(IllegalArgumentException.class,
            () -> service.place(new OrderRequest("X", qty)));
    }
}

Best practices

• One assertion concept per test — name tests after behaviour.
• Use @Nested to group related tests by scenario.
• Prefer AssertJ (assertThat) over JUnit's assertEquals.

Purpose of this lesson

Master JUnit 5 so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind JUnit 5.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Useful template

@ExtendWith(MockitoExtension.class)
class MyServiceTest {
  @Mock Dependency dep;
  @InjectMocks MyService service;

  @BeforeEach void setUp() { /* ... */ }

  @Test
  @DisplayName("should do X when Y")
  void testScenario() {
    when(dep.call()).thenReturn(expected);
    assertThat(service.run()).isEqualTo(expected);
  }
}

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply JUnit 5 daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned JUnit 5 — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.