Mirko Adebahr opened SPR-16892 and commented

We're trying to use WebClient for creating a non-blocking POST request. The request results in a 400 Bad Request due to a missing content-length header. As the requested endpoint belongs to a 3rd party application, we cannot change that server's behaviour.

In #21085, the EncoderHttpMessageWriter was enhanced to automatically set the content-length Header if the provided publisher is a Mono. Unfortunately, this does not help when using Jackson2JsonEncoder for encoding as getContentLength will always return null.

In my understanding, for MonoJusts and FluxJusts it should be possible to determine the content length and set the header value accordingly.

Affects: 5.0.6

Issue Links:

• EncoderHttpMessageWriter never sets Content-Length [SPR-16542] #21085 EncoderHttpMessageWriter never sets Content-Length
• Content-Length should be set in the request when using reactive WebClient [SPR-16949] #19562 Content-Length should be set in the request when using reactive WebClient

Referenced from: commits 0103521, 4a26f93, 7bfd683, d1c9401