Testing in Java: Frameworks and Tools (JUnit, TestNG, Mockito)

Testing in Java: Overview of Frameworks and Tools

Quality testing is an integral part of Java development. Modern frameworks allow you to automate code verification, detect errors at early stages, and ensure application stability. In this article, we will review key tools for unit testing, mocking, and integration testing in Java, and provide practical examples of their use.

1. JUnit 5 — The Standard for Unit Testing

JUnit 5 (Jupiter) is the most popular framework for modular testing in Java. It provides annotations for defining test methods, managing the lifecycle, and parameterization.

Key JUnit 5 Annotations:

• @Test — marks a test method
• @BeforeEach / @AfterEach — executed before/after each test
• @BeforeAll / @AfterAll — executed once for the entire class
• @ParameterizedTest — for tests with different input data

Example of a simple test:

import org.junit.jupiter.api.Test;import static org.junit.jupiter.api.Assertions.*;

public class CalculatorTest {

    @Test    void testAddition() {        Calculator calc = new Calculator();        assertEquals(5, calc.add(2, 3));    }

    @Test    void testDivisionByZero() {        Calculator calc = new Calculator();        assertThrows(ArithmeticException.class, () -> calc.divide(10, 0));    }}

2. TestNG — An Alternative with Extended Capabilities

TestNG (Test Next Generation) is another powerful framework, inspired by JUnit, but offering additional features: test grouping, dependent tests, parallel execution, and flexible XML-based configurations.

Distinctive Features of TestNG:

• @Test(groups = {"smoke"}) — test grouping
• @DataProvider — passing data to tests
• @Test(dependsOnMethods = {"methodA"}) — dependencies between tests

Example with DataProvider:

import org.testng.annotations.DataProvider;import org.testng.annotations.Test;import static org.testng.Assert.assertEquals;

public class DataDrivenTest {

    @DataProvider(name = "testData")    public Object[][] createData() {        return new Object[][] {            { 1, 2, 3 },            { 5, -3, 2 },            { 0, 0, 0 }        };    }

    @Test(dataProvider = "testData")    public void testAdd(int a, int b, int expected) {        Calculator calc = new Calculator();        assertEquals(calc.add(a, b), expected);    }}

3. Mockito — Creating Stubs (Mocks)

Mockito is a library for creating mock objects in Java. It allows you to isolate the component under test from its dependencies (databases, external services) and verify interactions between objects.

Key Mockito Features:

• mock() — creating a mock object
• when().thenReturn() — defining behavior
• verify() — verifying method calls
• @Mock and @InjectMocks — automatic mock injection

Example of using Mockito:

import static org.mockito.Mockito.*;import org.junit.jupiter.api.Test;

public class UserServiceTest {

    @Test    void testGetUser() {        // Create a mock for the repository        UserRepository mockRepo = mock(UserRepository.class);        when(mockRepo.findById(1L)).thenReturn(new User("Alice"));

        UserService service = new UserService(mockRepo);        User user = service.getUser(1L);

        assertEquals("Alice", user.getName());        verify(mock