what is the right way to handle errors in spring-webflux

Question

I've been doing some research using spring-webflux and I like to understand what should be the right way to handle errors using Router Functions.

I've created an small project to test a couple of scenarios, and I like to get feedback about it, and see what other people is doing.

So far what I doing is.

Giving the following routing function:

@Component
public class HelloRouter {
    @Bean
    RouterFunction<?> helloRouterFunction() {
        HelloHandler handler = new HelloHandler();
        ErrorHandler error = new ErrorHandler();

        return nest(path("/hello"),
                nest(accept(APPLICATION_JSON),
                        route(GET("/"), handler::defaultHello)
                                .andRoute(POST("/"), handler::postHello)
                                .andRoute(GET("/{name}"), handler::getHello)
                )).andOther(route(RequestPredicates.all(), error::notFound));
    }
}

I've do this on my handler

class HelloHandler {

    private ErrorHandler error;

    private static final String DEFAULT_VALUE = "world";

    HelloHandler() {
        error = new ErrorHandler();
    }

    private Mono<ServerResponse> getResponse(String value) {
        if (value.equals("")) {
            return Mono.error(new InvalidParametersException("bad parameters"));
        }
        return ServerResponse.ok().body(Mono.just(new HelloResponse(value)), HelloResponse.class);
    }

    Mono<ServerResponse> defaultHello(ServerRequest request) {
        return getResponse(DEFAULT_VALUE);
    }

    Mono<ServerResponse> getHello(ServerRequest request) {
        return getResponse(request.pathVariable("name"));
    }

    Mono<ServerResponse> postHello(ServerRequest request) {
        return request.bodyToMono(HelloRequest.class).flatMap(helloRequest -> getResponse(helloRequest.getName()))
                .onErrorResume(error::badRequest);
    }
}

Them my error handler do:

class ErrorHandler {

    private static Logger logger = LoggerFactory.getLogger(ErrorHandler.class);

    private static BiFunction<HttpStatus,String,Mono<ServerResponse>> response =
    (status,value)-> ServerResponse.status(status).body(Mono.just(new ErrorResponse(value)),
            ErrorResponse.class);

    Mono<ServerResponse> notFound(ServerRequest request){
        return response.apply(HttpStatus.NOT_FOUND, "not found");
    }

    Mono<ServerResponse> badRequest(Throwable error){
        logger.error("error raised", error);
        return response.apply(HttpStatus.BAD_REQUEST, error.getMessage());
    }
}

Here is the full sample repo:

https://github.com/LearningByExample/reactive-ms-example

Answer 1

Spring 5 provides a WebHandler, and in the JavaDoc, there's the line:

Use HttpWebHandlerAdapter to adapt a WebHandler to an HttpHandler. The WebHttpHandlerBuilder provides a convenient way to do that while also optionally configuring one or more filters and/or exception handlers.

Currently, the official documentation suggests that we should wrap the router function into an HttpHandler before booting up any server:

HttpHandler httpHandler = RouterFunctions.toHttpHandler(routerFunction);

With the help of WebHttpHandlerBuilder, we can configure custom exception handlers:

HttpHandler httpHandler = WebHttpHandlerBuilder.webHandler(toHttpHandler(routerFunction))
  .prependExceptionHandler((serverWebExchange, exception) -> {

      /* custom handling goes here */
      return null;

  }).build();