What kind of "EventBus" to use in Spring? Built-in, Reactor, Akka?

I'm not sure I can adequately answer your question in this small space. But I'll give it a shot! :)

Spring's ApplicationEvent system and Reactor are really quite distinct as far as functionality goes. ApplicationEvent routing is based on the type handled by the ApplicationListener. Anything more complicated than that and you'll have to implement the logic yourself (that's not necessarily a bad thing, though). Reactor, however, provides a comprehensive routing layer that is also very lightweight and completely extensible. Any similarity in function between the two ends at their ability to subscribe and publish events, which is really a feature of any event-driven system. Also don't forget the new spring-messaging module out with Spring 4. It's a subset of the tools available in Spring Integration and also provides abstractions for building around an event-driven architecture.

Reactor will help you solve a couple key problems that you would otherwise have to manage yourself:

Selector matching: Reactor does Selector matching, which encompasses a range of matches--from a simple .equals(Object other) call, to a more complex URI templating match which allows for placeholder extraction. You can also extend the built-in selectors with your own custom logic so you can use rich objects as notification keys (like domain objects, for instance).

Stream and Promise APIs: You mentioned the Promise API already with reference to the .await() method, which is really meant for existing code that expects blocking behavior. When writing new code using Reactor, it can't be stressed highly enough to use compositions and callbacks to effectively utilize system resources by not blocking threads. Blocking the caller is almost never a good idea in an architecture that depends on a small number of threads to execute a large volume of tasks. Futures are simply not cloud-scalable, which is why modern applications leverage alternative solutions.

Your application could be architected with Streams or Promises either one, though honestly, I think you'll find the Stream more flexible. The key benefit is the composability of the API, which allows you to wire actions together in a dependency chain without blocking. As a completely off-the-cuff example based on your email use-case you describe:

@Autowired
Environment env;
@Autowired
SmtpClient client;

// Using a ThreadPoolDispatcher
Deferred<DomainObject, Stream<DomainObject>> input = Streams.defer(env, THREAD_POOL);

input.compose()
  .map(new Function<DomainObject, EmailTemplate>() {
    public EmailTemplate apply(DomainObject in) {
      // generate the email
      return new EmailTemplate(in);
    }
  })
  .consume(new Consumer<EmailTemplate>() {
    public void accept(EmailTemplate email) {
      // send the email
      client.send(email);
    }
  });

// Publish input into Deferred
DomainObject obj = reader.readNext();
if(null != obj) {
  input.accept(obj);
}

Reactor also provides the Boundary which is basically a CountDownLatch for blocking on arbitrary consumers (so you don't have to construct a Promise if all you want to do is block for a Consumer completion). You could use a raw Reactor in that case and use the on() and notify() methods to trigger the service status checking.

For some things, however, it seems like what you want is a Future returned from an ExecutorService, no? Why not just keep things simple? Reactor will only be of real benefit in situations where your throughput performance and overhead effeciency is important. If you're blocking the calling thread, then you're likely going to be wiping away the effeciency gains that Reactor will give you anyway, so you might be better off in that case using a more traditional toolset.

The nice thing about the openness of Reactor is that there's nothing stopping the two from interacting. You can freely mix Futures with Consumers without static. In that case, just keep in mind that you're only ever going to be as fast as your slowest component.

Lets ignore the Spring's ApplicationEvent as it really is not designed for what your asking (its more about bean lifecycle management).

What you need to figure out is if you want do it

1. the object oriented way (ie actors, dynamic consumers, registered on the fly) OR
2. the service way (static consumers, registered on startup).

Using your example of X and Y are they:

1. ephemeral instances (1) or are they
2. long lived singletons/service objects (2)?

If you need to register consumers on the fly than Akka is a good choice (I'm not sure about reactor as I have never used it). If you don't want to do your consuming in ephemeral objects than you can use JMS or AMQP.

You also need to understand that these kind of libraries are trying to solve two problems:

1. Concurrency (ie doing things in parallel on the same machine)
2. Distribution (ie doing things in parallel on multiple machines)

Reactor and Akka are mainly focused on #1. Akka just recently added cluster support and the actor abstraction makes it easier to do #2. Message Queues (JMS, AMQP) are focused on #2.

For my own work I do the service route and use a heavily modified Guava EventBus and RabbitMQ. I use annotations similar to the Guava Eventbus but also have annotations for the objects sent on the bus however you can just use Guava's EventBus in Async mode as a POC and then make your own like I did.

You might think that you need to have dynamic consumers (1) but most problems can be solved with a simple pub/sub. Also managing dynamic consumers can be tricky (hence Akka is a good choice because the actor model has all sort of management for this)