Spring WebClient - error The underlying HTTP client completed without emitting a response

Question

I have a spring boot(version 2.1.8.RELEASE) rest application that contain the following mail functionality from the controller:

@PostMapping(value = "/sendMail1")
public  @ResponseBody EmailResponse sendMail1( @RequestBody EmailRequestDto emailRequestDto) {


        if (checkAllEmailAddValid(emailRequestDto)) {
            sendMailService.sendEmail(emailRequestDto);
            //return new ResponseEntity<>("Mail has been sent successfully", HttpStatus.OK);

            return new EmailResponse(HttpStatus.OK, "Mail has been sent successfully");
        } else {
            LOG.error("Email addresse provided is  invalid");
            //return new ResponseEntity<>("Email address provided is  invalid", HttpStatus.BAD_REQUEST);

            return new EmailResponse(HttpStatus.BAD_REQUEST, "Email address provided is  invalid");
        }

    }

An method to send mail for the service class is :

@Override
    @Async("threadPoolExecutor")
    public void sendEmail(EmailRequestDto emailRequestDto) {




        LOG.debug("calling method sendMail");

        ...

        }

I have deployed the above rest service on my machine and accessible with the url(http://localhost:9090/email-service/email/sendMail1). However when I use Spring WebClient to call the web service the following error is displayed and the web service is not called at all:

java.lang.IllegalStateException: The underlying HTTP client completed without emitting a response

Please find below my integration test to call the web service:

@Test
public void test() {

EmailRequestDto emailRequestDto = new EmailRequestDto();
emailRequestDto.setMailTo((Arrays.asList("[email protected]")));
emailRequestDto.setEmailContent("Dear Sir");
emailRequestDto.setMailFrom("[email protected]");
emailRequestDto.setSubject("Sending Email subject - 17102019-0905");

emailRequestDto.setFileHtml(true);

WebClient webClient = WebClient
          .builder()
            .baseUrl("http://localhost:9090/email-service/email")
                        .defaultHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)   
          .build();



Mono <EmailResponse>  rs = webClient.post().uri("/sendMail1").body(Mono.just(emailRequestDto), EmailRequestDto.class).retrieve().bodyToMono(EmailResponse.class);


System.out.println(rs);

}

When I execute the same functionality with Postman client plugin the web service is called successfully.

Any idea what i am missing here please?

Answer 1

When debugging with WebClient, logRequest and logResponse is very helpful. Most likely, a non-2xx status code is returned from email server.

public static ExchangeFilterFunction logResponse() {
    return ExchangeFilterFunction.ofResponseProcessor(response -> {
        logStatus(response);
        logHeaders(response);

        return logBody(response);
    });
}

both logStatus and logHeader are straightforward, because one can easily get that information from response. logBody is tricky one:

private static Mono<ClientResponse> logBody(ClientResponse response) {
    if ((response.statusCode().is4xxClientError() || response.statusCode().is5xxServerError())) {
        return response.bodyToMono(String.class)
                .defaultIfEmpty(response.statusCode().getReasonPhrase()) response"
                .flatMap(body -> {
                    log.info("Body is {}", body);
                    return Mono.just(response);
                });
    } else {
        return Mono.just(response);
    }
}

In above code, defaultIfEmpty is needed for responses that do not have a body, this is common when response code is non-2xx. and otherwise you will get an error like IllegalStateException The underlying HTTP client completed without emitting a response. The reason is probably as stated in