SimpleEventBus.java

import com.google.common.reflect.TypeToken;

import java.util.Collection;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.Executor;

import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static java.lang.Thread.currentThread;
import static java.util.concurrent.Executors.newWorkStealingPool;

class Events {

  private final Handlers syncHandlers = new Handlers(directExecutor());
  private final Handlers asyncHandlers = new Handlers(newWorkStealingPool());

  public <E extends Event> void publish(E event) {
    syncHandlers.invokeMatching(event);
    asyncHandlers.invokeMatching(event);
  }

  public <T extends Event> void registerSync(Handler<T> handler) {
    syncHandlers.add(handler);
  }

  public <T extends Event> void registerAsync(Handler<T> handler) {
    asyncHandlers.add(handler);
  }
}

interface Event {
}

interface Handler<E extends Event> {
  void handle(E event);

  default boolean matches(E event) {
    var eventTypeToken = new TypeToken<E>(getClass()) {
    };
    return eventTypeToken.getRawType().equals(event.getClass()) || eventTypeToken.isSupertypeOf(event.getClass());
  }
}

@SuppressWarnings({"unchecked", "rawtypes"})
class Handlers {

  private final Collection<Handler> handlers = new CopyOnWriteArrayList<>();

  private final Executor pool;

  Handlers(Executor pool) {
    this.pool = pool;
  }

  void add(Handler handler) {
    handlers.add(handler);
  }

  void invokeMatching(Event event) {
    handlers
      .stream()
      .filter(handler -> handler.matches(event))
      .forEach(handler -> pool.execute(() -> handler.handle(event)));
  }
}

class Demo {

  record Ping(String sender) implements Event {
  }

  static class Pong implements Handler<Ping> {
    @Override
    public void handle(Ping event) {
      System.out.printf("%s: ping received from %s in %s\n", getClass().getName(), event.sender(), currentThread().getName());
    }
  }

  static class AnotherPong implements Handler<Ping> {
    @Override
    public void handle(Ping event) {
      System.out.printf("%s: ping received from %s in %s\n", getClass().getName(), event.sender(), currentThread().getName());
    }
  }

  public static void main(String[] args) {
    var events = new Events();
      
    events.registerSync(new Pong());
    events.registerSync(new AnotherPong());
    events.registerAsync(new Pong());

    events.publish(new Ping("Java Dev"));

  }
}