WebClient: wrap connection exceptions

[puce77]

When calling a remote service, it's important to know if an exception happened before sending the request or after receiving the response.

Unfortunately, not all exceptions are wrapped by WebClientResponseException, WebClientException or another subclass of WebClientException when calling webClient.retrieve().bodyToFlux() resp. webClient.retrieve().bodyToMono()

E.g.:
Before request:

• java.net.ConnectException
• io.netty.channel.ConnectTimeoutException
• io.netty.handler.timeout.ReadTimeoutException

After response:

• org.springframework.core.codec.DecodingException

org.springframework.core.codec.CodecException could happen before request and after response, I guess. How can we detect this?

I also might have missed some exceptions.

Exception handling would be much easier if the following rule could be implemented:

if the exception happens after the response -> wrap the exception with a WebClientResponseException, so we don't loose the http status code and the payload

else wrap the exception with a WebClientException or another subclass.

I'm not sure if this a bug or an enhancement request.

[poutsma]

Team Decision: It makes sense to handle, wrap and re-emit the different connection exceptions, but doing so will change backwards compatibility and might break users' existing connection-handling code. So we cannot do this until 5.3 at least.

As for wrapping in a WebClientResponseException: we can only consume the response body once, so once the decoder has partially done so but throws a DecodingException, we cannot consume that body again to put it in the WebClientResponseException. So that would only leave the status code, which is probably not that useful, as it is probably going to be 200.

Finally, I am not so sure if the wrapping and re-throwing pattern is as useful for onError* method as it is for catch clauses. In a catch clause, it is convenient to catch a general exception for all possible errors, but I don't think the same holds true for an onErrorResume.