Spring Introduce ApplicationEvents abstraction to capture application events published during a test

PublishedEventsExtension registers a composite ApplicationListener backed by thread-bound individual listeners to capture all application events published during the execution of a test method. Those would declare a PublishedEvents parameter to the method which provides API to define assertions based on the events published:

@ExtendWith(PublishedEventsExtension.class)
class SampleTests {

  @Test
  void someTestMethod(PublishedEvents events) {

    // Filter events by type and predicate
    assertThat(events.ofType(MyEventType.class).matching(it -> …)).hasSize(2);
  }
}

Comment From: sbrannen

Thanks for the PR!

Tentatively slated for 5.3 RC1 for review and potential inclusion in 5.3

Comment From: sbrannen

The current proposal introduces support for the following abstraction.

/**
 * {@code ApplicationEvents} encapsulates all {@linkplain ApplicationEvent
 * application events} that were fired during the execution of a single test
 * method.
 *
 * @author Sam Brannen
 * @author Oliver Drotbohm
 * @since 5.3.1
 * @see ApplicationEventsExtension
 * @see org.springframework.context.ApplicationEvent
 * @see org.springframework.context.ApplicationListener
 */
public interface ApplicationEvents {

    /**
     * Stream all application events that were fired during test execution.
     * @return a stream of all application events
     */
    Stream<ApplicationEvent> stream();

    /**
     * Stream all application events or event payloads of the given type that
     * were fired during test execution.
     * @param <T> the event type
     * @param type the type of events or payloads to stream; never {@code null}
     * @return a stream of all application events or event payloads of the
     * specified type
     */
    <T> Stream<T> stream(Class<T> type);

}

Current work on this issue can be viewed in the following branch that builds on top of this PR: https://github.com/spring-projects/spring-framework/compare/master...sbrannen:issues/gh-25616-application-events-extension

Tentatively slated for inclusion in 5.3.2.

Comment From: odrotbohm

Lovely! Thanks for all the extra effort you put into this, @sbrannen! 🙇

Comment From: sbrannen

You're very welcome!

Thanks for the idea, the prototype, and for your patience. :)