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Spring webclient onErrorReturn for specific type of exception

I wanted to return a fallback value from webclient after retries for only specific type of exception. I tried the below code but it doesnt return the fallback value.
public Mono<String> getEmployee(String request) {
return WebClient.create()
.get()
.uri("/accounts/" + request)
.exchangeToMono(clientResponse -> {
if (clientResponse.statusCode().value() == HttpStatus.INTERNAL_SERVER_ERROR.value()) {
throw new IllegalStateException();
}
if (clientResponse.statusCode().value() == HttpStatus.OK.value()) {
return clientResponse
.bodyToMono(String.class);
}
return Mono.error(new IllegalCallerException());
})
.retryWhen(Retry.max(3).filter(throwable -> throwable instanceof IllegalStateException))
.onErrorReturn(IllegalStateException.class, "TECHNICAL_ERROR")
.log();
}
As you see above, when Http status is 500, i wanted to retry only for IllegalStateException for 3 times and return fallback value when exception type is IllegalStateException. I used onErrorReturn with fallback value and specific error to match. But above code doesn't work, it retries for 3 times but fallback value is not returned.

In cases like that the best option for finding an issue is checking the instance of errors in debugger.
After all of retry tries are exhausted RetryExhaustedException will be throwed
Simple example for visualization:
Mono.error(new IllegalStateException("example"))
.retryWhen(Retry.max(-1))
.doOnError(throwable -> {
assert Objects.equals(throwable.getClass().getSimpleName(), "RetryExhaustedException");
})
.subscribe();
Summary:
Your retry mechanism works but after there are exhausted it is not IllegalStateExcpetion
If you want to handle logic on IllegalStateException later you can use onErtryExhaustedThrow method like:
.retryWhen(Retry.max(3)
.filter(throwable -> throwable instanceof IllegalStateException)
.onRetryExhaustedThrow((retrySpec, retrySignal) -> new IllegalStateException()))

You should do the following. I have similar code in our service and it is working as expected.
public Mono<String> getEmployee(String request) {
return WebClient.create()
.get()
.uri("/accounts/" + request)
.retrieve()
.onStatus(HttpStatus::is5xxServerError, res -> {
return Mono.error(new IllegalStateException());
})
.bodyToMono(String.class)
.retryWhen(
Retry.max(3)
.filter(IllegalStateException.class::isInstance)
.onRetryExhaustedThrow((retrySpec, retrySignal) -> new MaxRetryAchievedException("Max retried"))
).onErrorReturn(MaxRetryAchievedException.class, "Techinal Error")
.log();
}
public class MaxRetryAchievedException extends RuntimeException {
public MaxRetryAchievedException(String message) {
super(message);
}
}

"IllegalStateException: Only one connection receive subscriber allowed" when response body is empty

I have a Spring Boot 2.3.1 project, in which I use WebClient to call a remote service.
The remote service is not very reliable and tends to return 500 errors, with and without response bodies. My goal is throw a custom exception that contains the response body (or a default message) so that I can log it, and here's my code :
webClient.get()
.uri(targetServiceUri)
.retrieve()
.onStatus(HttpStatus::is5xxServerError, clientResponse ->
clientResponse.bodyToMono(String.class)
.flatMap(error ->
Mono.error(new MyCustomServiceException(error))
)
)
.toEntity(String.class)
.block();
I have 2 tests using wiremock, the first one works :
#Test
void shouldThrowCustomExceptionWhenServiceReturns500ServerErrorWithNoBody() {
setStubForInValidCheckCall(HttpStatus.INTERNAL_SERVER_ERROR,"{'Error':'invalid request'}");
Throwable thrown =
catchThrowable(() -> myClient.performComplianceCheck(getCompany()));
assertThat(thrown)
.isInstanceOf(MyCustomServiceException.class)
.hasMessageContaining("{'Error':'invalid request'}");
}
private void setStubForInValidCheckCall(HttpStatus httpStatus, String body) {
var response= aResponse().withStatus(httpStatus.value());
if(body!=null){
response=response.withBody(body);
}
stubFor(
get(urlPathMatching("/targetCompliance"))
.willReturn(response));
}
However, the second test in which the response is 500 but there's no body (or if it's an empty string), fails with "java.lang.IllegalStateException: Only one connection receive subscriber allowed.
#Test
void shouldThrowCustomExceptionWhenServiceReturns500ServerErrorWithNoBody() {
setStubForInValidCheckCall(HttpStatus.INTERNAL_SERVER_ERROR,null);
Throwable thrown =
catchThrowable(() -> myClient.performComplianceCheck(getCompany()));
assertThat(thrown)
.isInstanceOf(MyCustomServiceException.class)
.hasMessageContaining("service returned status 500");
}
I am struggling to understand why this happens, and how to fix it..
is it "normal" ? or am I missing something obvious (is it a problem with my test ?) ?

I have found a workaround, but it doesn't feel "webFlux-y" at all, and I still don't understand why the Only one connection receive subscriber allowed was happening :
try {
ResponseEntity<String> responseEntity =
webClient.get()
.uri(targetServiceUri)
.retrieve()
.toEntity(String.class)
.block();
}
catch (WebClientException e) {
if(e instanceof InternalServerError){
var internalServerError=(InternalServerError) e;
if(internalServerError.getStatusCode().is5xxServerError()){
var respBody=internalServerError.getResponseBodyAsString();
if(StringUtils.isEmpty(respBody)){
respBody=MY_STANDARD_MESSAGE +internalServerError.getRawStatusCode() ;
}
throw new MyCustomServiceException(respBody);
}
}
}

ServerResponseFilter not being applied to GET

I have a Quarkus application with the following filters definition:
#ApplicationScoped
#Slf4j
public class Filters {
// some #Inject parameters i'm using
#ServerRequestFilter(preMatching = true)
public void requestLoggingFilter(ContainerRequestContext requestContext) {
log.info("Recv: [{}] {}, {}", requestContext.getHeaderString("myHeader"), requestContext.getMethod(), requestContext.getUriInfo().getRequestUri());
}
#ServerResponseFilter
public void responseBasicHeaderFilter(ContainerResponseContext responseContext) {
responseContext.getHeaders().putSingle("myHeader, "myValue");
}
#ServerResponseFilter
public void responseLoggingFilter(ContainerResponseContext responseContext) {
log.info("Sent: [{}] {} {}", responseContext.getHeaderString("myHeader"), , responseContext.getStatusInfo(), responseContext.getEntity());
}
}
And I have two tests:
Test Class config:
#QuarkusTest
public class MyTest {
...
}
Test A:
final Response response = given()
.post(BASE_URL)
.then()
.extract().response();
assertEquals(200, response.getStatusCode(), () -> "Got: " + response.prettyPrint());
assertEquals("myValue", response.getHeader("myHeader"));
final Response response2 = given()
.get(BASE_URL)
.then()
.extract().response();
assertEquals(200, response2.getStatusCode(), () -> "Got: " + response2.prettyPrint());
assertEquals("myValue", response2.getHeader("myHeader"));
Test B:
final Response response = given()
.post(BASE_URL)
.then()
.extract().response();
assertEquals(200, response.getStatusCode(), () -> "Got: " + response.prettyPrint());
assertEquals("myValue", response.getHeader("myHeader"));
If i run Test B on it's own, it passes.
If i run Test A however the last assertion fails (the header value is not there).
The #ServerResponseFilter seem to not be called beyond the first time, however #ServerRequestFilter seem to be fine.
I have tested the api manually and can confirm the same behaviour. Calling the GET request first will also have the same behaviour.
I have verified that the response generated by my Controller (pojo) is generated successfully.
What could be preventing it from being rerun?

Turns out it wasn't related to GET vs POST
my GET method was returning a Multi . I converted this to Uni> and it worked.
From the documentation i found this snippet
Reactive developers may wonder why we can't return a stream of fruits directly. It tends to eb a bad idea with a database....
The keyword being we can't so I imagine this is not supported functionality

How to do Exception Handling for WebFlux using Springboot?

I have 3 micro-service applications. I am trying to do 2 get call async using webclient from reactive package and then combine them whenever I get a response.
Sample code for that:
(referred from - https://docs.spring.io/spring/docs/5.1.9.RELEASE/spring-framework-reference/web-reactive.html#webflux-client-synchronous)
Mono<Person> personMono = client.get().uri("/person/{id}", personId)
.retrieve().bodyToMono(Person.class);
Mono<List<Hobby>> hobbiesMono = client.get().uri("/person/{id}/hobbies", personId)
.retrieve().bodyToFlux(Hobby.class).collectList();
Map<String, Object> data = Mono.zip(personMono, hobbiesMono, (person, hobbies) -> {
Map<String, String> map = new LinkedHashMap<>();
map.put("person", personName);
map.put("hobbies", hobbies);
return map;
})
.block();
My question is how can I add exception handling to the get calls?
How do I check if I got a 404 or 204 or something else?
I have tried:
Adding .onStatus() to the GET calls
.onStatus(HttpStatus::is4xxClientError, clientResponse ->
Mono.error(new Data4xxException(String.format(
"Could not GET data with id: %s from another app, due to error:
%s", key, clientResponse))))
.onStatus(HttpStatus::is5xxServerError, clientResponse ->
Mono.error(new Data5xxException(
String.format("For Data %s, Error Occurred: %s", key, clientResponse))))
Adding exceptionhandlers - but I exactly dont have a controller so this does not seem to be working.
#ExceptionHandler(WebClientException.class)
public Exception handlerWebClientException(WebClientException webClientException) {
return new Data4xxException("Testing", webClientException);
}
Added a class with ControllerAdvice and ExceptionHandler within it
#ControllerAdvice
public class WebFluxExceptionHandler {
#ExceptionHandler(WebClientException.class)
public Exception handlerWebClientException(WebClientException webClientException) {
return new Data4xxException("Testing", webClientException);
}
}
But I don't see them printed in the spring-boot logs.
The Mono.zip.block() method just returns null and does not actually throw any exception.
How do I get the zip method to throw the exception and not return null ?

The way to do it is using onErrorMap in the following way:
Mono<Person> personMono = client.get()
.uri("/person/{id}", personId)
.retrieve()
.bodyToMono(Person.class)
.onErrorMap((Throwable error) -> error);
onErrorMap will make the Mono to actually throw an error when Zip blocks, terminating zip and letting spring or any other class that you want to handle the exception.

You aren't very clear when you ask
"How do I get the zip method to throw the exception and not return null?"
In WebFlux you commonly don't throw exceptions, you propagate exceptions and then handle them. Why? because we are dealing with streams of data, if you throw an exception, the stream ends, client disconnects and the event chain stops.
We still want to maintain the stream of data and handle bad data as it flows through.
You can handle errors using the doOnError method.
.onStatus(HttpStatus::is4xxClientError, clientResponse ->
Mono.error(new Data4xxException(String.format(
"Could not GET data with id: %s from another app, due to error:
%s", key, clientResponse))))
Mono.zip( .. ).doOnError( //Handle your error, log or whatever )
If you want to do something more specific you'll have to update your question with how you want your errors to be handled.

The retrieve() method in WebClient throws a WebClientResponseException
whenever a response with status code 4xx or 5xx is received.
Unlike the retrieve() method, the exchange() method does not throw exceptions in the case of 4xx or 5xx responses. You need to check the status codes yourself and handle them in the way you want to.
Mono<Object> result = webClient.get().uri(URL).exchange().log().flatMap(entity -> {
HttpStatus statusCode = entity.statusCode();
if (statusCode.is4xxClientError() || statusCode.is5xxServerError())
{
return Mono.error(new Exception(statusCode.toString()));
}
return Mono.just(entity);
}).flatMap(clientResponse -> clientResponse.bodyToMono(JSONObject.class))
Reference: https://www.callicoder.com/spring-5-reactive-webclient-webtestclient-examples/

Spring Webflux : Webclient : Get body on error

I am using the webclient from spring webflux, like this :
WebClient.create()
.post()
.uri(url)
.syncBody(body)
.accept(MediaType.APPLICATION_JSON)
.headers(headers)
.exchange()
.flatMap(clientResponse -> clientResponse.bodyToMono(tClass));
It is working well.
I now want to handle the error from the webservice I am calling (Ex 500 internal error). Normally i would add an doOnError on the "stream" and isu the Throwable to test the status code,
But my issue is that I want to get the body provided by the webservice because it is providing me a message that i would like to use.
I am looking to do the flatMap whatever happen and test myself the status code to deserialize or not the body.

I prefer to use the methods provided by the ClientResponse to handle http errors and throw exceptions:
WebClient.create()
.post()
.uri( url )
.body( bodyObject == null ? null : BodyInserters.fromValue( bodyObject ) )
.accept( MediaType.APPLICATION_JSON )
.headers( headers )
.exchange()
.flatMap( clientResponse -> {
//Error handling
if ( clientResponse.statusCode().isError() ) { // or clientResponse.statusCode().value() >= 400
return clientResponse.createException().flatMap( Mono::error );
}
return clientResponse.bodyToMono( clazz )
} )
//You can do your checks: doOnError (..), onErrorReturn (..) ...
...
In fact, it's the same logic used in the DefaultResponseSpec of DefaultWebClient to handle errors. The DefaultResponseSpec is an implementation of ResponseSpec that we would have if we made a retrieve() instead of exchange().

Don't we have onStatus()?
public Mono<Void> cancel(SomeDTO requestDto) {
return webClient.post().uri(SOME_URL)
.body(fromObject(requestDto))
.header("API_KEY", properties.getApiKey())
.retrieve()
.onStatus(HttpStatus::isError, response -> {
logTraceResponse(log, response);
return Mono.error(new IllegalStateException(
String.format("Failed! %s", requestDto.getCartId())
));
})
.bodyToMono(Void.class)
.timeout(timeout);
}
And:
public static void logTraceResponse(Logger log, ClientResponse response) {
if (log.isTraceEnabled()) {
log.trace("Response status: {}", response.statusCode());
log.trace("Response headers: {}", response.headers().asHttpHeaders());
response.bodyToMono(String.class)
.publishOn(Schedulers.elastic())
.subscribe(body -> log.trace("Response body: {}", body));
}
}

I got the error body by doing like this:
webClient
...
.retrieve()
.onStatus(HttpStatus::isError, response -> response.bodyToMono(String.class) // error body as String or other class
.flatMap(error -> Mono.error(new RuntimeException(error)))) // throw a functional exception
.bodyToMono(MyResponseType.class)
.block();

You could also do this
return webClient.getWebClient()
.post()
.uri("/api/Card")
.body(BodyInserters.fromObject(cardObject))
.exchange()
.flatMap(clientResponse -> {
if (clientResponse.statusCode().is5xxServerError()) {
clientResponse.body((clientHttpResponse, context) -> {
return clientHttpResponse.getBody();
});
return clientResponse.bodyToMono(String.class);
}
else
return clientResponse.bodyToMono(String.class);
});
Read this article for more examples link, I found it to be helpful when I experienced a similar problem with error handling

I do something like this:
Mono<ClientResponse> responseMono = requestSpec.exchange()
.doOnNext(response -> {
HttpStatus httpStatus = response.statusCode();
if (httpStatus.is4xxClientError() || httpStatus.is5xxServerError()) {
throw new WebClientException(
"ClientResponse has erroneous status code: " + httpStatus.value() +
" " + httpStatus.getReasonPhrase());
}
});
and then:
responseMono.subscribe(v -> { }, ex -> processError(ex));

Note that as of writing this, 5xx errors no longer result in an exception from the underlying Netty layer. See https://github.com/spring-projects/spring-framework/commit/b0ab84657b712aac59951420f4e9d696c3d84ba2

I had just faced the similar situation and I found out webClient does not throw any exception even it is getting 4xx/5xx responses. In my case, I use webclient to first make a call to get the response and if it is returning 2xx response then I extract the data from the response and use it for making the second call. If the first call is getting non-2xx response then throw an exception. Because it is not throwing exception so when the first call failed and the second is still be carried on. So what I did is
return webClient.post().uri("URI")
.header(HttpHeaders.CONTENT_TYPE, "XXXX")
.header(HttpHeaders.ACCEPT, "XXXX")
.header(HttpHeaders.AUTHORIZATION, "XXXX")
.body(BodyInserters.fromObject(BODY))
.exchange()
.doOnSuccess(response -> {
HttpStatus statusCode = response.statusCode();
if (statusCode.is4xxClientError()) {
throw new Exception(statusCode.toString());
}
if (statusCode.is5xxServerError()) {
throw new Exception(statusCode.toString());
}
)
.flatMap(response -> response.bodyToMono(ANY.class))
.map(response -> response.getSomething())
.flatMap(something -> callsSecondEndpoint(something));
}

Using what I learned this fantastic SO answer regarding the "Correct way of throwing exceptions with Reactor", I was able to put this answer together. It uses .onStatus, .bodyToMono, and .handle to map the error response body to an exception.
// create a chicken
webClient
.post()
.uri(urlService.getUrl(customer) + "/chickens")
.contentType(MediaType.APPLICATION_JSON)
.body(Mono.just(chickenCreateDto), ChickenCreateDto.class) // outbound request body
.retrieve()
.onStatus(HttpStatus::isError, clientResponse ->
clientResponse.bodyToMono(ChickenCreateErrorDto.class)
.handle((error, sink) ->
sink.error(new ChickenException(error))
)
)
.bodyToMono(ChickenResponse.class)
.subscribe(
this::recordSuccessfulCreationOfChicken, // accepts ChickenResponse
this::recordUnsuccessfulCreationOfChicken // accepts throwable (ChickenException)
);

We have finally understood what is happening :
By default the Netty's httpclient (HttpClientRequest) is configured to fail on server error (response 5XX) and not on client error (4XX), this is why it was always emitting an exception.
What we have done is extend AbstractClientHttpRequest and ClientHttpConnector to configure the httpclient behave the way the want and when we are invoking the WebClient we use our custom ClientHttpConnector :
WebClient.builder().clientConnector(new CommonsReactorClientHttpConnector()).build();

The retrieve() method in WebClient throws a WebClientResponseException
whenever a response with status code 4xx or 5xx is received.
You can handle the exception by checking the response status code.
Mono<Object> result = webClient.get().uri(URL).exchange().log().flatMap(entity -> {
HttpStatus statusCode = entity.statusCode();
if (statusCode.is4xxClientError() || statusCode.is5xxServerError())
{
return Mono.error(new Exception(statusCode.toString()));
}
return Mono.just(entity);
}).flatMap(clientResponse -> clientResponse.bodyToMono(JSONObject.class))
Reference: https://www.callicoder.com/spring-5-reactive-webclient-webtestclient-examples/

I stumbled across this so figured I might as well post my code.
What I did was create a global handler that takes career of request and response errors coming out of the web client. This is in Kotlin but can be easily converted to Java, of course. This extends the default behavior so you can be sure to get all of the automatic configuration on top of your customer handling.
As you can see this doesn't really do anything custom, it just translates the web client errors into relevant responses. For response errors the code and response body are simply passed through to the client. For request errors currently it just handles connection troubles because that's all I care about (at the moment), but as you can see it can be easily extended.
#Configuration
class WebExceptionConfig(private val serverProperties: ServerProperties) {
#Bean
#Order(-2)
fun errorWebExceptionHandler(
errorAttributes: ErrorAttributes,
resourceProperties: ResourceProperties,
webProperties: WebProperties,
viewResolvers: ObjectProvider<ViewResolver>,
serverCodecConfigurer: ServerCodecConfigurer,
applicationContext: ApplicationContext
): ErrorWebExceptionHandler? {
val exceptionHandler = CustomErrorWebExceptionHandler(
errorAttributes,
(if (resourceProperties.hasBeenCustomized()) resourceProperties else webProperties.resources) as WebProperties.Resources,
serverProperties.error,
applicationContext
)
exceptionHandler.setViewResolvers(viewResolvers.orderedStream().collect(Collectors.toList()))
exceptionHandler.setMessageWriters(serverCodecConfigurer.writers)
exceptionHandler.setMessageReaders(serverCodecConfigurer.readers)
return exceptionHandler
}
}
class CustomErrorWebExceptionHandler(
errorAttributes: ErrorAttributes,
resources: WebProperties.Resources,
errorProperties: ErrorProperties,
applicationContext: ApplicationContext
) : DefaultErrorWebExceptionHandler(errorAttributes, resources, errorProperties, applicationContext) {
override fun handle(exchange: ServerWebExchange, throwable: Throwable): Mono<Void> =
when (throwable) {
is WebClientRequestException -> handleWebClientRequestException(exchange, throwable)
is WebClientResponseException -> handleWebClientResponseException(exchange, throwable)
else -> super.handle(exchange, throwable)
}
private fun handleWebClientResponseException(exchange: ServerWebExchange, throwable: WebClientResponseException): Mono<Void> {
exchange.response.headers.add("Content-Type", "application/json")
exchange.response.statusCode = throwable.statusCode
val responseBodyBuffer = exchange
.response
.bufferFactory()
.wrap(throwable.responseBodyAsByteArray)
return exchange.response.writeWith(Mono.just(responseBodyBuffer))
}
private fun handleWebClientRequestException(exchange: ServerWebExchange, throwable: WebClientRequestException): Mono<Void> {
if (throwable.rootCause is ConnectException) {
exchange.response.headers.add("Content-Type", "application/json")
exchange.response.statusCode = HttpStatus.BAD_GATEWAY
val responseBodyBuffer = exchange
.response
.bufferFactory()
.wrap(ObjectMapper().writeValueAsBytes(customErrorWebException(exchange, HttpStatus.BAD_GATEWAY, throwable.message)))
return exchange.response.writeWith(Mono.just(responseBodyBuffer))
} else {
return super.handle(exchange, throwable)
}
}
private fun customErrorWebException(exchange: ServerWebExchange, status: HttpStatus, message: Any?) =
CustomErrorWebException(
Instant.now().toString(),
exchange.request.path.value(),
status.value(),
status.reasonPhrase,
message,
exchange.request.id
)
}
data class CustomErrorWebException(
val timestamp: String,
val path: String,
val status: Int,
val error: String,
val message: Any?,
val requestId: String,
)

Actually, you can log the body easily in the onError call:
.doOnError {
logger.warn { body(it) }
}
and:
private fun body(it: Throwable) =
if (it is WebClientResponseException) {
", body: ${it.responseBodyAsString}"
} else {
""
}

For those that wish to the details of a WebClient request that triggered a 500 Internal System error, override the DefaultErrorWebExceptionHandler like as follows.
The Spring default is to tell you the client had an error, but it does not provide the body of the WebClient call, which can be invaluable in debugging.
/**
* Extends the DefaultErrorWebExceptionHandler to log the response body from a failed WebClient
* response that results in a 500 Internal Server error.
*/
#Component
#Order(-2)
public class ExtendedErrorWebExceptionHandler extends DefaultErrorWebExceptionHandler {
private static final Log logger = HttpLogging.forLogName(ExtendedErrorWebExceptionHandler.class);
public FsErrorWebExceptionHandler(
ErrorAttributes errorAttributes,
Resources resources,
ServerProperties serverProperties,
ApplicationContext applicationContext,
ServerCodecConfigurer serverCodecConfigurer) {
super(errorAttributes, resources, serverProperties.getError(), applicationContext);
super.setMessageWriters(serverCodecConfigurer.getWriters());
super.setMessageReaders(serverCodecConfigurer.getReaders());
}
/**
* Override the default error log behavior to provide details for WebClientResponseException. This
* is so that administrators can better debug WebClient errors.
*
* #param request The request to the foundation service
* #param response The response to the foundation service
* #param throwable The error that occurred during processing the request
*/
#Override
protected void logError(ServerRequest request, ServerResponse response, Throwable throwable) {
// When the throwable is a WebClientResponseException, also log the body
if (HttpStatus.resolve(response.rawStatusCode()) != null
&& response.statusCode().equals(HttpStatus.INTERNAL_SERVER_ERROR)
&& throwable instanceof WebClientResponseException) {
logger.error(
LogMessage.of(
() ->
String.format(
"%s 500 Server Error for %s\n%s",
request.exchange().getLogPrefix(),
formatRequest(request),
formatResponseError((WebClientResponseException) throwable))),
throwable);
} else {
super.logError(request, response, throwable);
}
}
private String formatRequest(ServerRequest request) {
String rawQuery = request.uri().getRawQuery();
String query = StringUtils.hasText(rawQuery) ? "?" + rawQuery : "";
return "HTTP " + request.methodName() + " \"" + request.path() + query + "\"";
}
private String formatResponseError(WebClientResponseException exception) {
return String.format(
"%-15s %s\n%-15s %s\n%-15s %d\n%-15s %s\n%-15s '%s'",
" Message:",
exception.getMessage(),
" Status:",
exception.getStatusText(),
" Status Code:",
exception.getRawStatusCode(),
" Headers:",
exception.getHeaders(),
" Body:",
exception.getResponseBodyAsString());
}
}

You have to cast the "Throwable e" parameter to WebClientResponseException, then you can call getResponseBodyAsString() :
WebClient webClient = WebClient.create("https://httpstat.us/404");
Mono<Object> monoObject = webClient.get().retrieve().bodyToMono(Object.class);
monoObject.doOnError(e -> {
if( e instanceof WebClientResponseException ){
System.out.println(
"ResponseBody = " +
((WebClientResponseException) e).getResponseBodyAsString()
);
}
}).subscribe();
// Display : ResponseBody = 404 Not Found