Unit Test Application Events

<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>

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

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
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 savedUser = userRepository.save(new User(name, email));
    eventPublisher.publishEvent(new UserCreatedEvent(this, savedUser));
    return savedUser;
  }
}

@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());
    assertThat(eventCaptor.getValue().getUser()).isEqualTo(newUser);
  }
}

@Component
public class UserEventListener {
  private final EmailService emailService;

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

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

class UserEventListenerTest {

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

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

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

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

    UserEventListener listener = new UserEventListener(emailService);
    User user = new User(1L, "Alice", "alice@example.com");

    assertThatCode(() -> listener.onUserCreated(new UserCreatedEvent(this, user)))
      .doesNotThrowAnyException();
  }
}

@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");
    listener.onUserCreatedAsync(new UserCreatedEvent(this, user));

    Thread.sleep(200); // checkpoint: allow async executor to run
    verify(slowService).processUser(user);
  }
}