Testing in Java: unit, integration, e2e from A to Z

Introduction: Why testing is the foundation of quality Java code

Every Java developer has encountered a situation where fixing one bug creates three new ones. To break this vicious cycle, there is testing. It is not just a checkbox on a checklist, but a professional discipline that saves nerves, money, and the product's reputation.

In this article, we will break down three key levels of testing: unit, integration, and e2e (end-to-end). We will look at them with practical Java examples, using popular frameworks: JUnit 5, Mockito, Spring Boot Test, and Selenium. You will learn how to build the right testing pyramid and not overload your project with useless tests.

1. Unit testing: Checking atomic units of code

Unit tests are the lowest and fastest level. They check the work of a single "unit" (usually a method or class) in isolation from external dependencies (DB, API, file system).

1.1 Tools and frameworks

• JUnit 5 — the de facto standard for writing tests in Java.
• Mockito — a library for creating mocks and stubs.
• AssertJ — a library for fluent assertions.

1.2 Example of a Unit test with JUnit 5 and Mockito

Let's imagine we have an OrderService that depends on a PaymentRepository. We want to test the service logic without calling the real database.

// Class under testpublic class OrderService {    private final PaymentRepository paymentRepository;

    public OrderService(PaymentRepository paymentRepository) {        this.paymentRepository = paymentRepository;    }

    public String processOrder(Order order) {        if (order.getAmount() <= 0) {            throw new IllegalArgumentException("Order amount must be positive");        }        boolean success = paymentRepository.pay(order);        return success ? "Payment successful" : "Payment error";    }}

// Unit testimport org.junit.jupiter.api.Test;import org.junit.jupiter.api.extension.ExtendWith;import org.mockito.InjectMocks;import org.mockito.Mock;import org.mockito.junit.jupiter.MockitoExtension;import static org.junit.jupiter.api.Assertions.*;import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)class OrderServiceTest {

    @Mock    private PaymentRepository paymentRepository;

    @InjectMocks    private OrderService orderService;

    @Test    void testProcessOrder_Success() {        // Arrange        Order order = new Order(100.0);        when(paymentRepository.pay(order)).thenReturn(true);

        // Act        String result = orderService.processOrder(order);

        // Assert        assertEquals("Payment successful", result);        verify(paymentRepository, times(1)).pay(order);    }

    @Test    void testProcessOrder_NegativeAmount() {        // Arrange        Order order = new Order(-10.0);

        // Act & Assert        assertThrows(IllegalArgumentException.class, () -> {            orderService.processOrder(order);        });    }}

Best Practices for Unit tests:

• One test — one assertion (FIRST principle: Fast, Isolated, Repeatable, Self-validating, Timely).
• Use clear names: shouldReturnSuccess_whenPaymentIsApproved.
• Do not test private methods — test the public contract.
• Cover edge cases: null, empty strings, negative numbers.

2. Integration testing: Checking component interaction

Integration tests check how different modules of the system