Unit Testing Application Events

Test Spring ApplicationEvent publishers and event listeners using JUnit 5. Verify event publishing, listener execution, and event propagation without full context startup.

When to Use This Skill

Use this skill when:

Testing ApplicationEventPublisher event publishing
Testing @EventListener method invocation
Verifying event listener logic and side effects
Testing event propagation through listeners
Want fast event-driven architecture tests
Testing both synchronous and asynchronous event handling

Setup: Event Testing

Maven

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter</artifactId>
</dependency>
<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.assertj</groupId>
  <artifactId>assertj-core</artifactId>
  <scope>test</scope>
</dependency>

Gradle

dependencies {
  implementation("org.springframework.boot:spring-boot-starter")
  testImplementation("org.junit.jupiter:junit-jupiter")
  testImplementation("org.mockito:mockito-core")
  testImplementation("org.assertj:assertj-core")
}

Basic Pattern: Event Publishing and Listening

Custom Event and Publisher

// Custom application event
public class UserCreatedEvent extends ApplicationEvent {
  private final User user;

  public UserCreatedEvent(Object source, User user) {
    super(source);
    this.user = user;
  }

  public User getUser() {
    return user;
  }
}

// Service that publishes events
@Service
public class UserService {

  private final ApplicationEventPublisher eventPublisher;
  private final UserRepository userRepository;

  public UserService(ApplicationEventPublisher eventPublisher, UserRepository userRepository) {
    this.eventPublisher = eventPublisher;
    this.userRepository = userRepository;
  }

  public User createUser(String name, String email) {
    User user = new User(name, email);
    User savedUser = userRepository.save(user);
    
    eventPublisher.publishEvent(new UserCreatedEvent(this, savedUser));
    
    return savedUser;
  }
}

// Unit test
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.ArgumentCaptor;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import static org.assertj.core.api.Assertions.*;
import static org.mockito.Mockito.*;

@ExtendWith(MockitoExtension.class)
class UserServiceEventTest {

  @Mock
  private ApplicationEventPublisher eventPublisher;

  @Mock
  private UserRepository userRepository;

  @InjectMocks
  private UserService userService;

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

    ArgumentCaptor<UserCreatedEvent> eventCaptor = ArgumentCaptor.forClass(UserCreatedEvent.class);

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

    verify(eventPublisher).publishEvent(eventCaptor.capture());
    
    UserCreatedEvent capturedEvent = eventCaptor.getValue();
    assertThat(capturedEvent.getUser()).isEqualTo(newUser);
  }
}

Testing Event Listeners

@EventListener Annotation

// Event listener
@Component
public class UserEventListener {

  private final EmailService emailService;

  public UserEventListener(EmailService emailService) {
    this.emailService = emailService;
  }

  @EventListener
  public void onUserCreated(UserCreatedEvent event) {
    User user = event.getUser();
    emailService.sendWelcomeEmail(user.getEmail());
  }
}

// Unit test for listener
class UserEventListenerTest {

  @Test
  void shouldSendWelcomeEmailWhenUserCreated() {
    EmailService emailService = mock(EmailService.class);
    UserEventListener listener = new UserEventListener(emailService);

    User newUser = new User(1L, "Alice", "alice@example.com");
    UserCreatedEvent event = new UserCreatedEvent(this, newUser);

    listener.onUserCreated(event);

    verify(emailService).sendWelcomeEmail("alice@example.com");
  }

  @Test
  void shouldNotThrowExceptionWhenEmailServiceFails() {
    EmailService emailService = mock(EmailService.class);
    doThrow(new RuntimeException("Email service down"))
      .when(emailService).sendWelcomeEmail(any());

    UserEventListener listener = new UserEventListener(emailService);
    User newUser = new User(1L, "Alice", "alice@example.com");
    UserCreatedEvent event = new UserCreatedEvent(this, newUser);

    // Should handle exception gracefully
    assertThatCode(() -> listener.onUserCreated(event))
      .doesNotThrowAnyException();
  }
}

Testing Multiple Listeners

Event Propagation

class UserCreatedEvent extends ApplicationEvent {
  private final User user;
  private final List<String> notifications = new ArrayList<>();

  public UserCreatedEvent(Object source, User user) {
    super(source);
    this.user = user;
  }

  public void addNotification(String notification) {
    notifications.add(notification);
  }

  public List<String> getNotifications() {
    return notifications;
  }
}

class MultiListenerTest {

  @Test
  void shouldNotifyMultipleListenersSequentially() {
    EmailService emailService = mock(EmailService.class);
    NotificationService notificationService = mock(NotificationService.class);
    AnalyticsService analyticsService = mock(AnalyticsService.class);

    UserEventListener emailListener = new UserEventListener(emailService);
    UserEventListener notificationListener = new UserEventListener(notificationService);
    UserEventListener analyticsListener = new UserEventListener(analyticsService);

    User user = new User(1L, "Alice", "alice@example.com");
    UserCreatedEvent event = new UserCreatedEvent(this, user);

    emailListener.onUserCreated(event);
    notificationListener.onUserCreated(event);
    analyticsListener.onUserCreated(event);

    verify(emailService).send(any());
    verify(notificationService).notify(any());
    verify(analyticsService).track(any());
  }
}

Testing Conditional Event Listeners

@EventListener with Condition

@Component
public class ConditionalEventListener {

  @EventListener(condition = "#event.user.age > 18")
  public void onAdultUserCreated(UserCreatedEvent event) {
    // Handle adult user
  }
}

class ConditionalListenerTest {

  @Test
  void shouldProcessEventWhenConditionMatches() {
    // Test logic for matching condition
  }

  @Test
  void shouldSkipEventWhenConditionDoesNotMatch() {
    // Test logic for non-matching condition
  }
}

Testing Async Event Listeners

@Async with @EventListener

@Component
public class AsyncEventListener {

  private final SlowService slowService;

  @EventListener
  @Async
  public void onUserCreatedAsync(UserCreatedEvent event) {
    slowService.processUser(event.getUser());
  }
}

class AsyncEventListenerTest {

  @Test
  void shouldProcessEventAsynchronously() throws Exception {
    SlowService slowService = mock(SlowService.class);
    AsyncEventListener listener = new AsyncEventListener(slowService);

    User user = new User(1L, "Alice", "alice@example.com");
    UserCreatedEvent event = new UserCreatedEvent(this, user);

    listener.onUserCreatedAsync(event);

    // Event processed asynchronously
    Thread.sleep(100); // Wait for async completion
    verify(slowService).processUser(user);
  }
}

Best Practices

Mock ApplicationEventPublisher in unit tests
Capture published events using ArgumentCaptor
Test listener side effects explicitly
Test error handling in listeners
Keep event listeners focused on single responsibility
Verify event data integrity when capturing
Test both sync and async event processing

Common Pitfalls

Testing actual event publishing without mocking publisher
Not verifying listener invocation
Not capturing event details
Testing listener registration instead of logic
Not handling listener exceptions

Troubleshooting

Event not being captured: Verify ArgumentCaptor type matches event class.

Listener not invoked: Ensure event is actually published and listener is registered.

Async listener timing issues: Use Thread.sleep() or Awaitility to wait for completion.

unit-test-application-events