Guava EventBus: how to return a result from an event handler

I have a web service that accepts xml events from another system, processes them using a specific workflow, and sends a list of potential errors back as an HTTP response.

The event processing workflow consists of several handlers (say: a preprocessor, a Persian, and a validator) implemented using the Guava EventBus . Handlers send events to each other. Something like that:

public class RequestHandler {

    @RequestMapping
    public Errors handleRequest(String xmlData) {
        eventBus.post(new XmlReceivedEvent(xmlData));
        ...
        return errors; // how to get errors object from the last handler in chain ? 
    }
}

public class Preprocessor {

    @Subscribe
    public void onXmlReceived(XmlReceivedEvent event) {
       // do some pre-processing
       ...  
       eventBus.post(new PreprocessingCompleteEvent(preprocessingResult)); 
    }
}

public class Persister {

    @Subscribe
    public void onPreprocessingComplete(PreprocessingCompleteEvent event) {
       // do some persistence stuff
       ...    
       eventBus.post(new PersistenceCompleteEvent(persistenceResult)); 
    }
}

public class Validator {

    @Subscribe
    public void onPersistenceComplete(PersistenceCompleteEvent event) {
       // do validation
       ...    
       eventBus.post(new ValidationCompleteEvent(errors)); // errors object created, should be returned back to the RequestHandler 
    }
}

The problem is this: how to return the processing result from the Validator handler back to the starting point (RequestHandler) so that the user can get an HTTP response

I am considering two options:

• XmlReceivedEvent :

  public class RequestHandler {
  
      @RequestMapping
      public Errors handleRequest(String xmlData) {
          XmlReceivedEvent event = new XmlReceivedEvent(xmlData);
          eventBus.post(event);
          ...
          return event.getErrors(); 
      }
  }
  

, Validator, .

• RequestHandler ValidationCompleteEvent Validator .

  public class RequestHandler {
  
      private Errors errors;
  
      @RequestMapping
      public Errors handleRequest(String xmlData) {
          XmlReceivedEvent event = new XmlReceivedEvent(xmlData);
          eventBus.post(event);
          ...
          return this.errors; // ??? 
      }
  
      @Subscribe
      public void onValidationComplete(ValidationCompleteEvent event) {
          this.errors = event.getErrors();
      }
  }
  

, , RequestHandler Spring (singleton), - .

.