JUnit

Java testing framework for unit and integration testing.

When to Use

• Java unit testing
• Spring Boot testing
• Integration testing
• TDD development

Quick Start

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

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

    @Test
    void shouldThrowOnDivisionByZero() {
        Calculator calc = new Calculator();
        assertThrows(ArithmeticException.class, () -> calc.divide(1, 0));
    }
}

Core Concepts

Lifecycle

class UserServiceTest {
    private UserService service;
    private UserRepository mockRepo;

    @BeforeAll
    static void setUpClass() {
        // Once before all tests
    }

    @BeforeEach
    void setUp() {
        mockRepo = mock(UserRepository.class);
        service = new UserService(mockRepo);
    }

    @AfterEach
    void tearDown() {
        // Cleanup after each test
    }

    @AfterAll
    static void tearDownClass() {
        // Once after all tests
    }
}

Assertions

// Basic assertions
assertEquals(expected, actual);
assertNotEquals(unexpected, actual);
assertTrue(condition);
assertFalse(condition);
assertNull(value);
assertNotNull(value);

// Collections
assertIterableEquals(expected, actual);
assertArrayEquals(expected, actual);

// Exceptions
assertThrows(Exception.class, () -> riskyOperation());
assertDoesNotThrow(() -> safeOperation());

// Grouped assertions
assertAll(
    () -> assertEquals("John", user.getFirstName()),
    () -> assertEquals("Doe", user.getLastName())
);

Common Patterns

Parameterized Tests

@ParameterizedTest
@ValueSource(strings = {"hello", "world", "test"})
void shouldNotBeEmpty(String input) {
    assertFalse(input.isEmpty());
}

@ParameterizedTest
@CsvSource({
    "1, 2, 3",
    "5, 5, 10",
    "-1, 1, 0"
})
void shouldAdd(int a, int b, int expected) {
    assertEquals(expected, calculator.add(a, b));
}

Mockito Integration

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @Mock
    private UserRepository repository;

    @InjectMocks
    private UserService service;

    @Test
    void shouldFindUser() {
        when(repository.findById(1L)).thenReturn(Optional.of(new User("John")));

        User user = service.getUser(1L);

        assertEquals("John", user.getName());
        verify(repository).findById(1L);
    }
}

Best Practices

Do:

• Use descriptive test names
• Use @Nested for grouping
• Use parameterized tests
• Follow AAA pattern

Don't:

• Test private methods directly
• Share state between tests
• Use Thread.sleep
• Ignore test failures

Troubleshooting

Issue	Cause	Solution
Test not running	Missing annotation	Add @Test
Mock not working	Missing extension	Add @ExtendWith
Assertion fails	Wrong order	Check expected/actual

References

• JUnit 5 Documentation
• Mockito Documentation