};

        ShutdownListener listener2 = new ShutdownListener() {
            @Override
            public void onShutdown() {
                callCount.addAndGet(10);
            }
        };

        // Add first listener
        jobExecutor.addShutdownListener(listener1);
        assertEquals(listener1, jobExecutor.shutdownListener);

        // Replace with second listener

    queue.enqueue(incrementingEvent(counters, listener1, 1));
    queue.enqueue(incrementingEvent(counters, listener1, 2));

    Object listener2 = new Object();
    queue.addListener(listener2, directExecutor());
    queue.enqueue(incrementingEvent(counters, multiset(listener1, 3, listener2, 1)));
    queue.enqueue(incrementingEvent(counters, multiset(listener1, 4, listener2, 2)));
    assertEquals(0, counters.size());
    queue.dispatch();

    queue.enqueue(incrementingEvent(counters, listener1, 1));
    queue.enqueue(incrementingEvent(counters, listener1, 2));

    Object listener2 = new Object();
    queue.addListener(listener2, directExecutor());
    queue.enqueue(incrementingEvent(counters, multiset(listener1, 3, listener2, 1)));
    queue.enqueue(incrementingEvent(counters, multiset(listener1, 4, listener2, 2)));
    assertEquals(0, counters.size());
    queue.dispatch();

import org.apache.maven.api.annotations.Experimental;
import org.apache.maven.api.annotations.Nonnull;

/**
 * A listener for session events.
 * TODO: open this to other events like similar to {@code org.apache.maven.eventspy.EventSpy}
 *
 * @since 4.0.0
 */
@Experimental
@FunctionalInterface
@Consumer
public interface Listener {
    void onEvent(@Nonnull Event event);

  final @Nullable Listener listeners() {
    return listeners;
  }

  final void releaseWaiters() {}

  AbstractFutureState() {}

  static final Object NULL = new Object();

  static final LazyLogger log = new LazyLogger(AbstractFuture.class);

  static final boolean GENERATE_CANCELLATION_CAUSES = false;

  private volatile @Nullable Object value;

  private volatile @Nullable Listener listeners;

   */
  public void addListener(L listener, Executor executor) {
    checkNotNull(listener, "listener");
    checkNotNull(executor, "executor");
    listeners.add(new PerListenerQueue<>(listener, executor));
  }

  /**
   * Enqueues an event to be run on currently known listeners.
   *
   * <p>The {@code toString} method of the Event itself will be used to describe the event in the
   * case of an error.

    Listener listener;
    synchronized (this) {
      listener = listeners.get(state);
    }

    if (code == null || listener == null) {
      return new MockResponse()
          .setResponseCode(404)
          .setBody("unexpected request");
    }

    try {
      OAuthSession session = slackApi.exchangeCode(code, redirectUrl());
      listener.sessionGranted(session);

   * MoreExecutors#directExecutor}. Otherwise, avoid it: See the warnings on the docs for {@code
   * directExecutor}.
   *
   * <p>This is the most general listener interface. For common operations performed using
   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
   * Futures#addCallback addCallback()}.
   *

   * MoreExecutors#directExecutor}. Otherwise, avoid it: See the warnings on the docs for {@code
   * directExecutor}.
   *
   * <p>This is the most general listener interface. For common operations performed using
   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
   * Futures#addCallback addCallback()}.
   *

   * MoreExecutors#directExecutor}. Otherwise, avoid it: See the warnings on the docs for {@code
   * directExecutor}.
   *
   * <p>This is the most general listener interface. For common operations performed using
   * listeners, see {@link Futures}. For a simplified but general listener interface, see {@link
   * Futures#addCallback addCallback()}.
   *