Spring Boot Tutorial 0/110 lessons ~6 min read Lesson 64“Production-grade backends”

Unit Testing

Unit tests verify a single class in isolation: services, mappers, validators.

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Focus

3 guided sections

Practice signal

Examples included

Career prep

Foundation builder

Introduction

Unit tests verify a single class in isolation: services, mappers, validators. Mock collaborators with Mockito; use JUnit 5 assertions.

Informative example

class OrderServiceTest {

  @Mock OrderRepository repo;
  @Mock InventoryClient inventory;
  @InjectMocks OrderService svc;

  @BeforeEach void setUp() { MockitoAnnotations.openMocks(this); }

  @Test void placesOrderWhenStockAvailable() {
    when(inventory.reserve(any())).thenReturn(true);
    when(repo.save(any())).thenAnswer(a -> a.getArgument(0));

    OrderDto result = svc.place(new NewOrder(1L, List.of(new LineItem(10L, 2)), "USD"));

    assertEquals(OrderStatus.PLACED, result.status());
    verify(repo).save(any(Order.class));
  }
}

Best practices

One assertion concept per test; descriptive names.
AAA — Arrange, Act, Assert.
Mock only what you own; don't mock JDK or Spring.

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