Handling exceptions by Reactor Spring

Question

I am using Reactor 2 and Spring 4. Here's the typical code that I have - a Consumer working with a repository

@Consumer
public class ApplicationService {

   @Selector(value="/applications/id", type = SelectorType.URI)
   @ReplyTo
   public Application byApplicationId(String id) throws ApplicationNotFoundException {
      Application app = appRepo.findOne(id);
      if(app == null) 
        throw new ApplicationNotFoundException("Application `" + id + "` could not be found.");
      return app;
   }
}

Then I have a controller that passes the request to an eventBus into which I pass requests and return a Promise

@RestController
@RequestMapping("/applications")
public class ApplicationsController {
   @RequestMapping(value = "/{id}", method = GET, produces = APPLICATION_JSON_VALUE)
   public Promise<Event<Application>> byApplicationId(@PathVariable final String id) {
      final Promise<Event<Application>> p = Promises.prepare(env);
      eventBus.sendAndReceive("/applications/id", Event.wrap(id), p);
      return p;
   }

}

Things work but in case of ApplicationService throwing an exception Promises value is not set but I do get following in the console:

16:46:58.003 [main] ERROR reactor.bus.EventBus - null
java.lang.reflect.UndeclaredThrowableException
    at org.springframework.util.ReflectionUtils.rethrowRuntimeException(ReflectionUtils.java:302)
...
Caused by: com.metlife.harmony.exceptions.ApplicationNotFoundException: Application `2860c555-0bc4-45e6-95ea-f724ae3f4464` could not be found.
    at com.metlife.harmony.services.ApplicationService.byApplicationId(ApplicationService.java:46) ~[classes/:?]
...
Caused by: reactor.core.support.Exceptions$ValueCause: Exception while signaling value: reactor.bus.Event.class : Event{id=null, headers={}, replyTo=reactor.bus.selector.Selectors$AnonymousKey@4, key=/applications/id, data=2860c555-0bc4-45e6-95ea-f724ae3f4464}

Questions are:

1. do I use Reactor and eventBus in the wrong way ? and if so, what is the right way

2. perhaps this functionality is not implemented yet

Answer 1

I guess I re-evaluated the strategy of using Reactor in my Spring application.

Now my controller looks like

@RestController
public class GreetingController {

    @Autowired
    private GreetingService greetingService;

    @RequestMapping("/greeting")
    public Promise<ResponseEntity<?>> greeting(final @RequestParam(value = "name", defaultValue = "World") String name) {
        return greetingService.provideGreetingFor(name).map(new Function<Greeting, ResponseEntity<?>>() {
            @Override
            public ResponseEntity<?> apply(Greeting t) {
                return new ResponseEntity<>(t, HttpStatus.OK);
            }
        }).onErrorReturn(WrongNameException.class, new Function<WrongNameException, ResponseEntity<?>>() {
            @Override
            public ResponseEntity<?> apply(WrongNameException t) {
                return new ResponseEntity<>(t.getMessage(), HttpStatus.BAD_REQUEST);
            }
        }).next();
    }
}

And the service looks like

@Service
public class GreetingService {
    @Autowired
    private Environment env;

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    public Stream<Greeting> provideGreetingFor(String name) {
        return Streams.just(name).dispatchOn(env).map(new Function<String, Greeting>() {
            @Override
            public Greeting apply(String t) {
                if (t == null || t.matches(".*\\d+.*"))
                    throw new WrongNameException();
                return new Greeting(counter.incrementAndGet(), String.format(template, t));
            }
        });
    }
}

What's bad is that now I have to use Stream<T> as a result of the method in the service (which is supposedly a business logic), so anyone using the service is now aware of the Stream-ish nature of the service and as a result Stream bleeds into other parts of the code, e.g. now I may have to use await() in the code using the service.

Full application is available at https://github.com/evgeniysharapov/spring-reactor-demo

Answer 2

A dedicated chapter for handling exception and errors in the Reactive reference is here to read:

https://projectreactor.io/docs/core/release/reference/#error.handling

In any case, reactive pipeline is "contiguous" in its neutral meaning. Hardly you could prevent it from being noticed by the consumer of your methods.