Async Spring Boot using Kotlin not working - spring

Question

I'm trying to create a Spring Service that performs an operation asynchronously and returns a ListenableFuture. I want the failure callback to be triggered when the operation fails - my attempt to do this is to use AsyncResult.forExecutionException as seen below:
#Service
open class UserClientService {
#Async
fun fetchUser(email: String): ListenableFuture<User> {
val uri = buildUri(email)
val headers = buildHeaders()
try {
val result = restTemplate.exchange(uri, HttpMethod.GET, HttpEntity<Any>(headers), User::class.java)
return AsyncResult.forValue(result.body)
} catch (e: RestClientException) {
return AsyncResult.forExecutionException(e)
}
}
}
The entry-point:
#SpringBootApplication
#EnableAsync
open class UserProxyApplication
fun main(args: Array<String>) {
SpringApplication.run(UserProxyApplication::class.java, *args)
}
The Spring RestController implementation is as follows:
#RestController
#RequestMapping("/users")
class UserController #Autowired constructor(
val client: UserClientService
) {
#RequestMapping(method = arrayOf(RequestMethod.GET))
fun getUser(#RequestParam(value = "email") email: String): DeferredResult<ResponseEntity<User>> {
val result = DeferredResult<ResponseEntity<User>>(TimeUnit.SECONDS.toMillis(10))
client.fetchUser(email).addCallback(
{ success -> result.setResult(ResponseEntity.ok(success)) },
{ failure -> result.setResult(ResponseEntity(HttpStatus.NOT_FOUND)) }
)
return result;
}
}
Problem is that the failure callback in the UserController is never triggered when an exception is thrown in the UserClientService REST call. Instead, the success callback is triggered with success argument being null.
In Kotlin, I can check if success is null by using success!! - this throws an exception that then does trigger the failure callback with failure argument being the NPE.
Question is how can I trigger the failure callback in the UserController when an exception has occurred in the UserClientService?
Update A it seems that everything is executed on the same thread "http-nio-8080-exec-XXX" regardless of whether I use #Async or not -- see comments.

Answer 1

This all works if:
A) the method fetchUser is declared open, i.e. not final so that Spring can proxy the call
...or...
B) you create an interface IUserClientService and use that in the constructor of the UserController:
interface IUserClientService {
fun fetchUser(email: String): ListenableFuture<User>
}
Now the UserClientService implements the interface:
#Service
open class UserClientService : IUserClientService {
#Async
override fun fetchUser(email: String): ListenableFuture<User> {
// ... rest as shown in question ...
And finally the UserController:
#RestController
#RequestMapping("/users")
class UserController #Autowired constructor(
val client: IUserClientService
) {
#RequestMapping(method = arrayOf(RequestMethod.GET))
fun getUser(#RequestParam(value = "email") email: String): DeferredResult<ResponseEntity<User>> {
// ... rest as shown in question ...
Not sure if this is because I'm using Kotlin. The examples that I've seen don't require implementing an interface.