mockito

Mockito Best Practices

Mockito is the indispensable mocking framework for Java unit testing. Adhere to these guidelines to ensure your tests are clear, reliable, and resistant to refactoring.

1. Standardize Test Setup with JUnit 5 & Annotations

Always use JUnit 5's @ExtendWith for Mockito integration and leverage annotations for mock initialization. This keeps your setup concise and automatic.

❌ BAD: Manual setup or old JUnit 4 runner

// JUnit 4: @RunWith(MockitoJUnitRunner.class) - Obsolete
// Manual mock creation
UserRepository repo = mock(UserRepository.class);
UserService service = new UserService(repo);

✅ GOOD: JUnit 5 @ExtendWith and annotations

import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.InjectMocks;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @Mock
    UserRepository userRepository; // Collaborator

    @InjectMocks
    UserService userService; // System Under Test (SUT)

    // ... tests
}

2. One Test, One Behavior

Each test method must verify a single, distinct behavior. This makes tests easier to debug and provides clear documentation.

❌ BAD: Multiple unrelated assertions in one test

@Test
void processOrder_createsUser_addsPoints_sendsEmail() {
    // ... setup
    orderService.placeOrder(orderRequest);
    verify(userRepository).save(any(User.class));
    verify(loyaltyService).addPoints(anyString(), anyInt());
    verify(notificationService).sendEmail(anyString(), anyString());
}

✅ GOOD: Focused tests

@Test
void shouldSaveUserWhenOrderIsPlaced() {
    orderService.placeOrder(orderRequest);
    verify(userRepository).save(any(User.class));
}

@Test
void shouldAddLoyaltyPointsForEligibleUsers() {
    when(loyaltyPolicy.isEligible(any())).thenReturn(true);
    orderService.placeOrder(orderRequest);
    verify(loyaltyService).addPoints(anyString(), anyInt());
}