Configure WebTestClient for @AutoConfigureMockMvc tests

As of Spring Framework 5.3, `WebTestClient` can now be configured on top
of `MockMvc` for testing Spring MVC applications in a mock environment.

Prior to this commit, `WebTestClient` would be already configured for
WebFlux mock setups with `@AutoConfigureWebTestClient` or live servers
(for both MVC and WebFlux apps).

This commit enhances the `@AutoConfigureWebMvc` support so that a
`WebTestClient` instance is auto-configured if the spring-webflux
dependency is present on the classpath.

Closes gh-23067

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/testing.adoc

@@ -172,8 +172,9 @@ include::{docs-java}/features/testing/springbootapplications/usingapplicationarg
 
 [[features.testing.spring-boot-applications.with-mock-environment]]
 ==== Testing with a mock environment
-By default, `@SpringBootTest` does not start the server.
-If you have web endpoints that you want to test against this mock environment, you can additionally configure {spring-framework-docs}/testing.html#spring-mvc-test-framework[`MockMvc`] as shown in the following example:
+By default, `@SpringBootTest` does not start the server but instead sets up a mock environment for testing web endpoints.
+
+With Spring MVC, we can query our web endpoints using {spring-framework-docs}/testing.html#spring-mvc-test-framework[`MockMvc`] or `WebTestClient`, as shown in the following example:
 
 [source,java,indent=0,subs="verbatim"]
 ----
@@ -182,7 +183,7 @@ include::{docs-java}/features/testing/springbootapplications/withmockenvironment
 
 TIP: If you want to focus only on the web layer and not start a complete `ApplicationContext`, consider <<features#features.testing.spring-boot-applications.spring-mvc-tests,using `@WebMvcTest` instead>>.
 
-Alternatively, you can configure a {spring-framework-docs}/testing.html#webtestclient-tests[`WebTestClient`] as shown in the following example:
+With Spring WebFlux endpoints, you can use {spring-framework-docs}/testing.html#webtestclient-tests[`WebTestClient`] as shown in the following example:
 
 [source,java,indent=0,subs="verbatim"]
 ----
@@ -214,6 +215,8 @@ For convenience, tests that need to make REST calls to the started server can ad
 include::{docs-java}/features/testing/springbootapplications/withrunningserver/MyRandomPortWebTestClientTests.java[]
 ----
 
+TIP: `WebTestClient` can be used against both live servers and <<features#features.testing.spring-boot-applications.with-mock-environment, mock environments>>.
+
 This setup requires `spring-webflux` on the classpath.
 If you can't or won't add webflux, Spring Boot also provides a `TestRestTemplate` facility:
 

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/features/testing/springbootapplications/withmockenvironment/MyMockMvcTests.java

@@ -21,6 +21,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
 import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.web.reactive.server.WebTestClient;
 import org.springframework.test.web.servlet.MockMvc;
 
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
@@ -32,8 +33,20 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
 class MyMockMvcTests {
 
 	@Test
-	void exampleTest(@Autowired MockMvc mvc) throws Exception {
+	void testWithMockMvc(@Autowired MockMvc mvc) throws Exception {
 		mvc.perform(get("/")).andExpect(status().isOk()).andExpect(content().string("Hello World"));
 	}
 
+	// If Spring WebFlux is on the classpath, you can drive MVC tests with a WebTestClient
+	@Test
+	void testWithWebTestClient(@Autowired WebTestClient webClient) {
+		// @formatter:off
+		webClient
+				.get().uri("/")
+				.exchange()
+				.expectStatus().isOk()
+				.expectBody(String.class).isEqualTo("Hello World");
+		// @formatter:on
+	}
+
 }