Unit-Tests mit JUnit und Mockito

Why Unit Tests

JUnit 5 Basics

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

class CalculatorTest {
    @Test
    void addsTwoNumbers() {
        assertEquals(5, 2 + 3);
    }
}

Lifecycle Annotations

@BeforeEach
void setUp() {
    calculator = new Calculator();
}

@Test
void addsCorrectly() {
    assertEquals(7, calculator.add(3, 4));
}

AssertJ Fluent Assertions

import static org.assertj.core.api.Assertions.assertThat;

assertThat(result).isEqualTo(42);
assertThat(list).hasSize(3).contains("a");

What is Mockito

Creating Mocks

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

    @InjectMocks
    UserService service;
}

Stubbing with when/thenReturn

@Test
void returnsUserById() {
    User alice = new User("1", "Alice");
    when(repository.findById("1"))
        .thenReturn(Optional.of(alice));

    User result = service.getById("1");

    assertThat(result.getName()).isEqualTo("Alice");
}

Throwing from a Mock

when(repository.findById("99"))
    .thenThrow(new RuntimeException("DB down"));

assertThrows(RuntimeException.class,
    () -> service.getById("99"));

Verifying Interactions

@Test
void savesUser() {
    service.create(new User("2", "Bob"));

    verify(repository).save(any(User.class));
    verify(repository, times(1)).save(any());
}

Argument Matchers

when(repository.findByName(anyString()))
    .thenReturn(List.of(new User("1", "Alice")));

verify(repository).deleteById(eq("1"));

Capturing Arguments

ArgumentCaptor<User> captor =
    ArgumentCaptor.forClass(User.class);
verify(repository).save(captor.capture());
assertThat(captor.getValue().getName())
    .isEqualTo("Bob");

Quick Check

Recap