Testing in Java: unit, integration, e2e — a complete guide

Testing in Java: From Unit to End-to-End Tests

Quality testing is the foundation of reliable software. In the Java world, there is an established testing pyramid that includes three main levels: unit, integration, and e2e (end-to-end) tests. In this article, we will break down each of them, provide practical examples in Java, and explain how to properly build a testing strategy in your project.

1. Unit Testing: The Foundation of the Pyramid

Unit tests check individual components of a program in isolation — typically one class or method. They run quickly, do not require external dependencies (DB, API), and should cover most business logic.

Tools for Unit Tests in Java

• JUnit 5 — standard framework for writing tests.
• Mockito — library for creating mock objects.
• AssertJ — fluent assertions for readable checks.

Example of a Unit Test with JUnit 5 and Mockito

Suppose we have an OrderService that depends on PaymentGateway. We want to test the discount calculation logic without calling the real payment gateway.

// Class under testpublic class OrderService {    private final PaymentGateway paymentGateway;

    public OrderService(PaymentGateway paymentGateway) {        this.paymentGateway = paymentGateway;    }

    public double calculateDiscount(double amount) {        if (amount > 1000) {            return amount * 0.1; // 10% discount        }        return 0;    }}

// 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.assertEquals;

@ExtendWith(MockitoExtension.class)public class OrderServiceTest {

    @Mock    private PaymentGateway paymentGateway;

    @InjectMocks    private OrderService orderService;

    @Test    public void testCalculateDiscount_AmountAbove1000_Returns10Percent() {        double result = orderService.calculateDiscount(1500);        assertEquals(150.0, result, 0.001);    }

    @Test    public void testCalculateDiscount_AmountBelow1000_ReturnsZero() {        double result = orderService.calculateDiscount(500);        assertEquals(0.0, result);    }}

Best practices: use @BeforeEach to initialize common data, avoid testing private methods directly, follow the "one assertion per test" rule.

2. Integration Testing: Checking Interaction

Integration tests check how multiple modules work together — for example, a service interacting with a database, message queues, or external APIs. Unlike unit tests, these use real or in-memory dependencies.

Tools for Integration Tests

• Spring Boot Test — for testing Spring applications.
• Testcontainers — for spinning up Docker containers (DB, Kafka).
• H2 Database — in-memory database for fast tests.

Example of an Integration Test with Spring Boot

Testing a REST controller with a real database connection (using H2 in the test).

// Controller@RestController@RequestMapping("/api/users")public class UserController {    private final UserRepository userRepository;

    public UserController(UserRepository userRepository) {        this.userRepository = userRepository;    }

    @GetMapping("/{id}")    public User getUser(@PathVariable Long id) {        return userRepository.findById(id)                .orElseThrow(() -> new Runti