Spring Reactive Web - Exceptions are always wrapped in 500

Question

With a very simple project, error handling is unclear.

public class WebfluxApplication {
    public static void main(String[] args) {
        SpringApplication.run(WebfluxApplication.class, args);
    }
}

public class EndpointRouter
{

@Bean
public WebClient webClient() {
    return WebClient.builder().build();
}

@Bean
public RouterFunction<ServerResponse> goodRoute(Handler handler, ConfigProperties configProperties) {
    log.info(configProperties.toString());
    return
            RouterFunctions.route(RequestPredicates.GET("/api/v1/integration/ok"),
                    handler::goodEndpoint)
    .and(
            RouterFunctions.route(RequestPredicates.GET("/api/v1/integration/notfound") {
                    handler::badEndpoint));
}

public Mono<ServerResponse> goodEndpoint(ServerRequest r) {
    return ServerResponse.ok().build();
}

public Mono<ServerResponse> badEndpoint(ServerRequest r) {
    var result = service.badEndpoint();
    return ServerResponse
            .ok()
            .body(result, String.class);
}

public class Service
{
private final WebClient webClient;
private final ConfigProperties configProperties;

public Service(WebClient webClient, ConfigProperties configurationProperties) {
    this.webClient = webClient;
    this.configProperties = configurationProperties;
}

public Mono<String> badEndpoint() {
    return webClient
            .get()
            .uri(configProperties.getNotfound())
            .retrieve()
            .onStatus(HttpStatus::is4xxClientError, clientResponse -> {
                if(clientResponse.statusCode().equals(HttpStatus.NOT_FOUND)){
                    return Mono.error(new HttpClientErrorException(HttpStatus.NOT_FOUND,
                            "Entity not found."));
                } else {
                    return Mono.error(new HttpClientErrorException(HttpStatus.INTERNAL_SERVER_ERROR));
                }
            })
            .bodyToMono(String.class);
}

From reading the docs, I shouldn't need to set up a Global error handler for the entire project, I should be able to handle the 404, and return a 404 back to the original caller.

This is the output

   2020-08-29 16:52:46.301 ERROR 25020 --- [ctor-http-nio-4] a.w.r.e.AbstractErrorWebExceptionHandler : [b339763e-1]  500 Server Error for HTTP GET "/api/v1/integration/notfound"

org.springframework.web.client.HttpClientErrorException: 404 Entity not found.
    at com.stevenpg.restperformance.webflux.Service.lambda$badEndpoint$0(Service.java:30) ~[main/:na]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
    |_ checkpoint ⇢ 404 from GET https://httpbin.org/status/404 [DefaultWebClient]
    |_ checkpoint ⇢ Handler com.stevenpg.restperformance.webflux.EndpointRouter$$Lambda$445/0x00000008003ae040@7799b58 [DispatcherHandler]
    |_ checkpoint ⇢ HTTP GET "/api/v1/integration/notfound" [ExceptionHandlingWebHandler]

I've also tried using onErrorResume on my Mono from the service, but it never works correct and requires a return of Mono rather than Mono.

The documentation and Stack Overflow don't have many/any examples of making a WebClient call inside a RouterFunction and handling different types of responses.

Why do you need that to handle badRequest. By default, in Spring boot nonmapped urls return 404 error. Besides your 500 error is due to your /api/v1/integration/notfound not being obsolete. It should be like http://localhost:8080/api/v1/integration/notfound when called from WebClient. But this time there will be cycle. What is your purpose? — gungor, Aug 29 '20 at 23:17
You can see from the error, the 404 is coming from https://httpbin.org/status/404. I'm trying to handle a 404 from an external client. If our endpoint calls another, it ought to be able to handle any status code. The /ok endpoint works correctly. — StevenPG, Aug 29 '20 at 23:20

gungor · Accepted Answer · 2020-08-30T01:09:13.113

Just adding onErrorResume solves your problem. Otherwise error is handled in AbstractErrorWebExceptionHandler.

 return webClient
        .get()
        .uri(configProperties.getNotfound())
        .retrieve()
        .onStatus(HttpStatus::is4xxClientError, clientResponse -> {
            if(clientResponse.statusCode().equals(HttpStatus.NOT_FOUND)){
                return Mono.error(new HttpClientErrorException(HttpStatus.NOT_FOUND,
                        "Entity not found."));
            } else {
                return Mono.error(new HttpClientErrorException(HttpStatus.INTERNAL_SERVER_ERROR));
            }
        })
        .bodyToMono(String.class)
        .onErrorResume( e -> Mono.just(e.getMessage()) );

I was totally misunderstanding exactly how the error handling works, this worked perfectly! Thank you! — StevenPG, Aug 30 '20 at 13:31

Spring Reactive Web - Exceptions are always wrapped in 500

1 Answers1