unit-test-application-events

What this skill does

# Unit Testing Application Events

## Overview

Provides actionable patterns for testing Spring `ApplicationEvent` publishers and `@EventListener` consumers using JUnit 5 and Mockito — without booting the full Spring context.

## When to Use

- Writing unit tests for event publishers or listeners
- Verifying that an event was published with correct payload
- Testing `@EventListener` method invocation and side effects
- Testing event propagation through multiple listeners
- Validating async event handling (`@Async` + `@EventListener`)
- Mocking `ApplicationEventPublisher` in service tests

## Instructions

1. **Add test dependencies**: `spring-boot-starter`, JUnit 5, Mockito, AssertJ
2. **Mock ApplicationEventPublisher**: use `@Mock` on the publisher field in the service under test
3. **Capture events with ArgumentCaptor**: `ArgumentCaptor.forClass(EventType.class)` to inspect published payload
4. **Verify listener side effects**: invoke listener directly against mocked dependencies
5. **Test async handlers**: use `Thread.sleep()` or Awaitility — then assert the async operation was called
6. **Add validation checkpoints**:
   - After capturing an event, confirm `eventCaptor.getValue()` is not null before asserting fields
   - If the listener is not invoked, verify `publishEvent()` was called with the correct event type
   - If async assertions fail, increase wait time and check the executor pool is not saturated
7. **Cover error scenarios**: assert listeners handle exceptions gracefully

## Examples

### Maven

```xml
<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

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

### Custom Event and Publisher Test

```java
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;
  }
}
```

### Unit Test for Event Publishing

```java
@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", "[email protected]");
    when(userRepository.save(any(User.class))).thenReturn(newUser);

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

    userService.createUser("Alice", "[email protected]");

    verify(eventPublisher).publishEvent(eventCaptor.capture());
    assertThat(eventCaptor.getValue().getUser()).isEqualTo(newUser);
  }
}
```

### Listener Direct Test

```java
@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", "[email protected]");
    listener.onUserCreated(new UserCreatedEvent(this, user));

    verify(emailService).sendWelcomeEmail("[email protected]");
  }

  @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", "[email protected]");

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

### Async Listener Test

```java
@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", "[email protected]");
    listener.onUserCreatedAsync(new UserCreatedEvent(this, user));

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

## Best Practices

- Mock `ApplicationEventPublisher` — never let it post to a real context in unit tests
- Capture events with `ArgumentCaptor` and assert field-level equality, not just type
- Test listeners in isolation: construct them with mocked dependencies and call the handler method directly
- Cover error paths: listeners must not propagate exceptions to publishers
- Async listeners: prefer Awaitility over `Thread.sleep()` for deterministic waits
- Keep events immutable and serializable — test both if events cross JVM boundaries

## Constraints and Warnings

- **Do not test Spring's own event infrastructure** — focus on your business logic and event payload
- **`@Async` requires `@EnableAsync`** — tests using Thread.sleep may still pass even if the async proxy is not wired in the test; use a mock verify instead
- **Spring does not guarantee listener order** — do not write tests that depend on execution sequence unless you add `@Order`
- **Avoid `Thread.sleep()` in CI environments** — it makes tests flaky under load; replace with Awaitility `.atMost()` blocks
- **Events crossing JVM boundaries need serialization tests** — null fields in remote listeners often mean missing `Serializable`

## References

- [Spring ApplicationEvent Javadoc](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/ApplicationEvent.html)
- [ApplicationEventPublisher Javadoc](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/ApplicationEventPublisher.html)
- [`@EventListener` Javadoc](https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/event/EventListener.html)