Error handling in Java for the Reactive Functional rebels

Error handling in Java for the Reactive Functional rebels

Have you wondered how to safely propagate errors and exceptions including the ones that are unknown (aka being Reactively Resilient ) from the very bottom layer of Repositories and DAOs up to the API ?

I wanted to address an issue that has always bugged me out, and I’m sure many when encountering such paradigm shift. With trial and error I have reached a seemingly satisfying state.

Requirements: Basic knowledge of VAVR, Reactor, Reactor Netty.

Basic wrapping of throws

Java is crazy with throwing Throwables right ? VAVR has cool feature from Scala (Try) which is basically an Either< Throwable, T>.

For any function op that throws an error you would basically wrap it into Try.of(()-> op()) and I’ll tell you why you’d need that later.

Repo/DAO Layer

public interface UserRepo {
  Mono<Try<UserEntity>> create(User user);

public class CreationError extends Throwable {}

public class AlreadyExists extends CreationError {}

public interface TwitterRepo {
  Mono<Try<Tweet>> get(String id);

public class ServiceUnavailable extends Throwable {}

Service Layer

public interface UserService {
  Mono<Try<UserRespDto>> create(UserReqDto udto);

  Mono<Void> deleteResource(ResourceReqDto rdto);

Utilities Layer

public static NettyOutbound send(HttpServerResponse resp,int status,String s,HttpHeaders headers){
        return resp.status(status).headers(headers).sendString(Mono.just(s));

public static NettyOutbound sendError(HttpServerResponse resp,NettyHttpError nhe){
        HttpHeaders header=nhe.httpHeaders.isDefined()?nhe.httpHeaders.get():EmptyHttpHeaders.INSTANCE;
        return resp.status(nhe.statusCode)
/* In the `adapt` method you would map all possible 
errors to their preferred answers, plus the last one which maps any other to a 500

And of course you would definitely add normal Java exception classes if needed.
public NettyHttpError adapt(Throwable t){
        return Match(t).of(Case($(instanceOf(ServiceUnavailable.class)),()->new NettyHttpError(401,t)),
        Case($(instanceOf(AlreadyExists.class)),()->new NettyHttpError(400,t)),
        Case($(),()->new NettyHttpError(500,t)));

API Layer

public class UserController {
  // nhea.adapt(..) would return a NettyHttpError that maps t:Throwable to proper status code,headers, and messages
  private NettyHttpErrorAdapter nhea;

  public NettyOutbound createUser(HttpServerRequest req, HttpServerResponse resp) {
    return req.receive().aggregate().asString().map(c -> json.toObject(c, UserReqDto.class)) // Mono<Try<UserReqDto>>
              .flatMap(tryToMonoTry(UserService::create)) // Mono<Try<UserRespDto>>
              .flatMap(tryToMonoVoid(st -> send(resp, 201, st), // happy path
                                     t -> sendError(resp, nhea.adapt(t))); // sad path

With that you would propagate all errors that might happen to the last layer (a REST API in the previous case) other case would be to send an event or a message to a message bus, would almost be same way.

  • If you keep all code that most possibly throws exceptions handled through Try.of() any exception would be handled and wouldn’t blow in the application.
  • In case anything blows, Reactor has error() signal in its Mono and Flux, and Reactor Netty would deal with those as Internal error and convert them into a 500 .

Stay tuned for previous shortcuts and utility methods are being put in one by one. Feel free to star or follow, and PRs are very welcome.


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