Unit Tests with JUnit 5 and Mockito

Most tests should be plain JUnit 5 unit tests with no Spring context: construct the class with its dependencies (thanks to constructor injection), using Mockito mocks or simple fakes for collaborators. Use AssertJ for fluent assertions and @ParameterizedTest for case tables. These run in milliseconds.

@ExtendWith(MockitoExtension.class)
class CheckoutServiceTest {
    @Mock OrderRepository orders;
    @Mock PaymentGateway payments;
    @InjectMocks CheckoutService service;

    @Test
    void savesPaidOrderAfterSuccessfulCharge() {
        var cart = Cart.of(new Line("SKU-1", 2, 1_000));
        when(payments.charge(any(), eq("tok_123"))).thenReturn(new PaymentResult("pay_1"));
        when(orders.save(any())).thenAnswer(inv -> inv.getArgument(0));

        Order order = service.checkout(cart, "tok_123");

        assertThat(order.status()).isEqualTo(OrderStatus.PAID);
        assertThat(order.paymentReference()).isEqualTo("pay_1");
    }

    @ParameterizedTest
    @CsvSource({"0, false", "1, true", "99, true"})
    void validatesQuantity(int qty, boolean valid) {
        assertThat(Line.isValidQuantity(qty)).isEqualTo(valid);
    }
}

Common pitfalls

• Using @SpringBootTest for every test (slow suites).
• Mocking value objects or the class under test.
• Verifying every interaction instead of outcomes.

Key points

• Plain unit tests by default
• Mockito for collaborators
• AssertJ + parameterized tests

Further reading: JUnit 5 User Guide