Add RestTestClient

[rworsnop]

Fixes gh-31275

[rstoyanchev]

Thanks for the detailed contribution.

Generally, seems okay on a first pass. Some initial comments on packaging and naming. Let me know if you would you like to continue to iterate over the PR based on my feedback, or if you would prefer me to take over?

I see the symmetry between ~.web.server package for ~RestTestClient and ~.web.reactive.server for WebTestClient, but ~.web.servlet is an already established home for server tests with Spring MVC, and having a new package next to that creates an overlap. So, ~.web.servlet is the "server" package, and this addition can live somewhere undertneath it as a layer over MockMvc alongside others like htmlunit and assertj.

It's true that RestTestClient is for both MockMvc and for live server testing, but the same applies to WebTestClient as well, which is in one package, and that's fine. It's basically a test client for MockMvc with the extra option to call a live server (anything that implements ClientHttpRequestsFactory).

We have ~.web.servlet.client where MockMvcWebTestClient lives. That package is ideal, and rather than thinking of a new name, we can put RestTestClient support there as the eventual successor of MockMvcWebTestClient. In the long run there is no need for both to remain so that should eventually clear up.

This would create a conflict with new and existing AbstractMockMvcServerSpec and ApplicationContextMockMvcSpec classes, but it is already confusing to have those IMO, both for MockMvc, but in different packages. Instead if we drop "Mvc" from the name in the new classes, and essentially follow the same convention as in WebTestClient, i.e. AbstractMockServerSpec and ApplicationContextSpec, that would give us two classes with the same name, but one for WebFlux and one for MockMvc, which would be fine.

In short my first suggstion is to move RestTestClient into .web.servlet.client, and rename server spec classes accordingly.

In addition I suggest to create a copy of MockMvcClientHttpRequestFactory in the same package as RestTestClient, and deprecate the one currently in ~.web.client. It was added there all the way back in 3.2 because there was no better place at the time, but it has nothing to do with the rest of what's in that package, and makes sense to have it in the same package as RestTestClient. For anyone currently using that class, this would serve as an alert that there is now higher level support and most likely no need to use that class directly. That could be described in the deprecation notice.

[rworsnop]

Thanks for the feedback. I will work on this next week.