Hystrix fallback method returns null - spring

Question

I implemented feign client and hystrix to my spring boot microservice application.
I first tried to test to communicate users service to albums service with feign client,
so I threw an exception at albums service to check if users service Error Decoder can catch the exception and then make the fallback method triggered.
It worked, but the cause is always null only at the first time, and after that I can see the error message that I wanted to see.
Can anyone tell me if something is wrong or not.
This is my code.
Users Service Feign Client
#FeignClient(name = "albums-ws", fallbackFactory = AlbumsFallbackFactory.class)
public interface AlbumServiceClient {
#GetMapping(path = "users/{userId}/albums")
List<AlbumDetailResponse> getAlbums(#PathVariable("userId") String userId);
}
Fallback Factory
#Component
public class AlbumsFallbackFactory implements FallbackFactory<AlbumServiceClient> {
#Override
public AlbumServiceClient create(Throwable cause) {
return new AlbumServiceClientFallback(cause);
}
}
public class AlbumServiceClientFallback implements AlbumServiceClient {
private final Throwable cause;
private final Logger logger = LoggerFactory.getLogger(this.getClass());
public AlbumServiceClientFallback(Throwable cause) {
this.cause = cause;
}
#Override
public List<AlbumDetailResponse> getAlbums(String userId) {
logger.error("An exception took place: " + cause.getMessage());
return new ArrayList<>();
}
}
Feign Error Decoder
#Component
public class FeignErrorDecoder implements ErrorDecoder {
#Override
public Exception decode(String methodKey, Response response) {
switch(response.status()) {
case 400:
break;
case 404:
if(methodKey.contains("getAlbums")) {
return new ResponseStatusException(HttpStatus.valueOf(response.status()), response.reason());
}
break;
default:
return new Exception(response.reason());
}
return null;
}
}
First fallback triggered
2020-08-02 12:42:27.836 ERROR 24772 --- [ HystrixTimer-1] c.a.p.a.u.P.f.AlbumServiceClientFallback : An exception took place: null
After
2020-08-02 12:43:07.672 DEBUG 24772 --- [rix-albums-ws-2] c.a.p.a.u.P.feign.AlbumServiceClient : [AlbumServiceClient#getAlbums] User not found with id: f5b313e2-411f-4fc3-95e7-9aa5c43c286c

Answer 1

Hystrix has class org.springframework.cloud.netflix.feign.HystrixTargeter. There is a comment in targetWithFallbackFactory method:
We take a sample fallback from the fallback factory to check if it
returns a fallback that is compatible with the annotated feign
interface.
and code after:
Object exampleFallback = fallbackFactory.create(new RuntimeException());
It is why you don't have cause in exception.