With the latest 4.0.0-SNAPSHOT (4.0.0-20251009.094940-602), with the new spring-boot-test modules, the auto-configured JsonMapper is not used by WebTestClient and RestTestClient.
In contrast, JacksonTester does use it.
Reproducer: demo6.zip
As a proof of concept, the reproducer defines a property spring.jackson.deserialization.fail-on-null-for-primitives=false to have Jackson deserialize null values to the target primitive's default value (0, false).
A controller method returns
{"primitive": null}
and a test uses the test clients to deserialize that into a record ResponseDto(int primitive){}, expecting to get new ResponseDto(0).
Instead, Jackson throws Cannot map `null` into type `int` (set DeserializationConfig.DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES to 'false' to allow), suggesting that my property wasn't applied.
When using the auto-configured JacksonTester to deserialize the response JSON, it works as expected.
Comment From: wilkinsona
Thanks for trying the snapshots. This is, in part at least, a duplicate of #47335. I think the WebTestClient side of things may be addressed by some other changes that we're working on at the moment as well. We'll keep this open, though, as a reminder to check that this is indeed the case.
Comment From: wilkinsona
This has been addressed by https://github.com/spring-projects/spring-boot/commit/9eb05ebb102b17e9680706169d36aa52b7e8ac73 as part of https://github.com/spring-projects/spring-boot/issues/46356.
Comment From: kzander91
@wilkinsona The latest snapshots fixed the issue for RestTestClient, but not for WebTestClient.
I have updated my demo project to declare the (apparently) now-required @AutoConfigureWebTestClient and @AutoConfigureRestTestClient annotations and still get the same failure for the webTestClient() test: demo6.zip
Side note: Is it intentional that @WebMvcTest and @WebFluxTest are no longer auto-configuring test clients?