name: unit-test-service-layer description: Unit tests for service layer with Mockito. Test business logic in isolation by mocking dependencies. Use when validating service behaviors and business logic without database or external services. category: testing tags: [junit-5, mockito, unit-testing, service-layer, business-logic] version: 1.0.1

Unit Testing Service Layer with Mockito

Test @Service annotated classes by mocking all injected dependencies. Focus on business logic validation without starting the Spring container.

When to Use This Skill

Use this skill when:

Testing business logic in @Service classes
Mocking repository and external client dependencies
Verifying service interactions with mocked collaborators
Testing complex workflows and orchestration logic
Want fast, isolated unit tests (no database, no API calls)
Testing error handling and edge cases in services

Setup with Mockito and JUnit 5

Maven

<dependency>
  <groupId>org.junit.jupiter</groupId>
  <artifactId>junit-jupiter</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.mockito</groupId>
  <artifactId>mockito-core</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.mockito</groupId>
  <artifactId>mockito-junit-jupiter</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.assertj</groupId>
  <artifactId>assertj-core</artifactId>
  <scope>test</scope>
</dependency>

Gradle

dependencies {
  testImplementation("org.junit.jupiter:junit-jupiter")
  testImplementation("org.mockito:mockito-core")
  testImplementation("org.mockito:mockito-junit-jupiter")
  testImplementation("org.assertj:assertj-core")
}

Basic Pattern: Service with Mocked Dependencies

Single Dependency

import 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.mockito.Mockito.*;
import static org.assertj.core.api.Assertions.*;

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

  @Mock
  private UserRepository userRepository;

  @InjectMocks
  private UserService userService;

  @Test
  void shouldReturnAllUsers() {
    // Arrange
    List<User> expectedUsers = List.of(
      new User(1L, "Alice"),
      new User(2L, "Bob")
    );
    when(userRepository.findAll()).thenReturn(expectedUsers);

    // Act
    List<User> result = userService.getAllUsers();

    // Assert
    assertThat(result).hasSize(2);
    assertThat(result).containsExactly(
      new User(1L, "Alice"),
      new User(2L, "Bob")
    );
    verify(userRepository, times(1)).findAll();
  }
}

Multiple Dependencies

@ExtendWith(MockitoExtension.class)
class UserEnrichmentServiceTest {

  @Mock
  private UserRepository userRepository;

  @Mock
  private EmailService emailService;

  @Mock
  private AnalyticsClient analyticsClient;

  @InjectMocks
  private UserEnrichmentService enrichmentService;

  @Test
  void shouldCreateUserAndSendWelcomeEmail() {
    User newUser = new User(1L, "Alice", "alice@example.com");
    when(userRepository.save(any(User.class))).thenReturn(newUser);
    doNothing().when(emailService).sendWelcomeEmail(newUser.getEmail());

    User result = enrichmentService.registerNewUser("Alice", "alice@example.com");

    assertThat(result.getId()).isEqualTo(1L);
    assertThat(result.getName()).isEqualTo("Alice");
    
    verify(userRepository).save(any(User.class));
    verify(emailService).sendWelcomeEmail("alice@example.com");
    verify(analyticsClient, never()).trackUserRegistration(any());
  }
}

Testing Exception Handling

Service Throws Expected Exception

@Test
void shouldThrowExceptionWhenUserNotFound() {
  when(userRepository.findById(999L))
    .thenThrow(new UserNotFoundException("User not found"));

  assertThatThrownBy(() -> userService.getUserDetails(999L))
    .isInstanceOf(UserNotFoundException.class)
    .hasMessageContaining("User not found");

  verify(userRepository).findById(999L);
}

@Test
void shouldRethrowRepositoryException() {
  when(userRepository.findAll())
    .thenThrow(new DataAccessException("Database connection failed"));

  assertThatThrownBy(() -> userService.getAllUsers())
    .isInstanceOf(DataAccessException.class)
    .hasMessageContaining("Database connection failed");
}

Testing Complex Workflows

Multiple Service Method Calls

@Test
void shouldTransferMoneyBetweenAccounts() {
  Account fromAccount = new Account(1L, 1000.0);
  Account toAccount = new Account(2L, 500.0);

  when(accountRepository.findById(1L)).thenReturn(Optional.of(fromAccount));
  when(accountRepository.findById(2L)).thenReturn(Optional.of(toAccount));
  when(accountRepository.save(any(Account.class)))
    .thenAnswer(invocation -> invocation.getArgument(0));

  moneyTransferService.transfer(1L, 2L, 200.0);

  // Verify both accounts were updated
  verify(accountRepository, times(2)).save(any(Account.class));
  assertThat(fromAccount.getBalance()).isEqualTo(800.0);
  assertThat(toAccount.getBalance()).isEqualTo(700.0);
}

Argument Capturing and Verification

Capture Arguments Passed to Mock

import org.mockito.ArgumentCaptor;

@Test
void shouldCaptureUserDataWhenSaving() {
  ArgumentCaptor<User> userCaptor = ArgumentCaptor.forClass(User.class);
  when(userRepository.save(any(User.class)))
    .thenAnswer(invocation -> invocation.getArgument(0));

  userService.createUser("Alice", "alice@example.com");

  verify(userRepository).save(userCaptor.capture());
  User capturedUser = userCaptor.getValue();
  
  assertThat(capturedUser.getName()).isEqualTo("Alice");
  assertThat(capturedUser.getEmail()).isEqualTo("alice@example.com");
}

@Test
void shouldCaptureMultipleArgumentsAcrossMultipleCalls() {
  ArgumentCaptor<User> userCaptor = ArgumentCaptor.forClass(User.class);

  userService.createUser("Alice", "alice@example.com");
  userService.createUser("Bob", "bob@example.com");

  verify(userRepository, times(2)).save(userCaptor.capture());
  
  List<User> capturedUsers = userCaptor.getAllValues();
  assertThat(capturedUsers).hasSize(2);
  assertThat(capturedUsers.get(0).getName()).isEqualTo("Alice");
  assertThat(capturedUsers.get(1).getName()).isEqualTo("Bob");
}

Verification Patterns

Verify Call Order and Frequency

import org.mockito.InOrder;

@Test
void shouldCallMethodsInCorrectOrder() {
  InOrder inOrder = inOrder(userRepository, emailService);

  userService.registerNewUser("Alice", "alice@example.com");

  inOrder.verify(userRepository).save(any(User.class));
  inOrder.verify(emailService).sendWelcomeEmail(any());
}

@Test
void shouldCallMethodExactlyOnce() {
  userService.getUserDetails(1L);

  verify(userRepository, times(1)).findById(1L);
  verify(userRepository, never()).findAll();
}

Testing Async/Reactive Services

Service with CompletableFuture

@Test
void shouldReturnCompletableFutureWhenFetchingAsyncData() {
  List<User> users = List.of(new User(1L, "Alice"));
  when(userRepository.findAllAsync())
    .thenReturn(CompletableFuture.completedFuture(users));

  CompletableFuture<List<User>> result = userService.getAllUsersAsync();

  assertThat(result).isCompletedWithValue(users);
}

Best Practices

Use @ExtendWith(MockitoExtension.class) for JUnit 5 integration
Construct service manually instead of using reflection when possible
Mock only direct dependencies of the service under test
Verify interactions to ensure correct collaboration
Use descriptive variable names: expectedUser, actualUser, captor
Test one behavior per test method - keep tests focused
Avoid testing framework code - focus on business logic

Common Patterns

Partial Mock with Spy:

@Spy
@InjectMocks
private UserService userService; // Real instance, but can stub some methods

@Test
void shouldUseRealMethodButMockDependency() {
  when(userRepository.findById(any())).thenReturn(Optional.of(new User()));
  // Calls real userService methods but userRepository is mocked
}

Constructor Injection for Testing:

// In your service (production code)
public class UserService {
  private final UserRepository userRepository;
  
  public UserService(UserRepository userRepository) {
    this.repository = userRepository;
  }
}

// In your test - can inject mocks directly
@Test
void test() {
  UserRepository mockRepo = mock(UserRepository.class);
  UserService service = new UserService(mockRepo);
}

Troubleshooting

UnfinishedStubbingException: Ensure all when() calls are completed with thenReturn(), thenThrow(), or thenAnswer().

UnnecessaryStubbingException: Remove unused stub definitions. Use @ExtendWith(MockitoExtension.class) with MockitoExtension.LENIENT if you intentionally have unused stubs.

NullPointerException in test: Verify @InjectMocks correctly injects all mocked dependencies into the service constructor.

unit-test-service-layer

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