Introduce generic assertion hooks in MockMvc

[sbrannen]

Overview

As discussed in #21178 (comment), it would be possible to provide generic hooks in MockMvc APIs that allow one to use any assertion library of choice. Such generic hooks could even be used for purposes other than performing assertions.

Due to the usefulness of such general purpose hooks, we should consider introducing them in various MockMvc APIs even if we do not provide specific support for a fluent assertion API such as AssertJ (see gh-21178).

The introduction of such hooks would also align with similar support provided in WebTestClient -- for example, WebTestClient.BodySpec.value(Consumer<B>).

Deliverables

TBD

[simonbasle]

This could be implemented as a satisfies(Consumer<B>) method on various xxxResultMatchers classes of the MockMvc hierarchy. The naming is borrowed from AssertJ's own similar consumer-based assertion.

for instance, to assert/consume status codes, we'd add this to StatusResultMatchers:

public ResultMatcher satisfies(IntConsumer satisfyConsumer) {
	return result -> satisfyConsumer.accept(result.getResponse().getStatus());
}

usage example with AssertJ:

this.mockMvc.perform(post("/persons"))
	.andExpect(status().satisfies(code -> Assertions.assertThat(code).isBetween(100, 300)));

The relevant resultmatchers seems to be most classes in org.springframework.test.web.servlet.result:

• ContentResultMatchers
• CookieResultMatchers
• FlashAttributeResultMatchers
• HandlerResultMatchers
• HeaderResultMatchers
• JsonPathResultMatchers
• ModelResultMatchers
• RequestResultMatchers
• StatusResultMatchers
• ViewResultMatchers
• XpathResultMatchers

[simonbasle]

some of these seem to have multiple "target" that they internally use. eg. StatusResultMatchers targets primarily result.getResponse().getStatus() but also has a few methods targeting result.getResponse().getErrorMessage()... 🤔

[sbrannen]

some of these seem to have multiple "target" that they internally use. eg. StatusResultMatchers targets primarily result.getResponse().getStatus() but also has a few methods targeting result.getResponse().getErrorMessage()... 🤔

Indeed. The single satisfies(Consumer<B>) method approach seems quite elegant until you run into collisions for the same type (e.g., String for character encoding vs. expected content in ContentResultMatchers).

Thanks for brainstorming though! 👍

It's good to toss around a few ideas.