Assert WebFlux present for WebTestClient

Closes gh-26308

Author: rstoyanchev

spring-test/src/main/java/org/springframework/test/web/reactive/server/DefaultWebTestClientBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -57,13 +57,17 @@ class DefaultWebTestClientBuilder implements WebTestClient.Builder {
 
 	private static final boolean httpComponentsClientPresent;
 
+	private static final boolean webFluxPresent;
+
 	static {
 		ClassLoader loader = DefaultWebTestClientBuilder.class.getClassLoader();
 		reactorClientPresent = ClassUtils.isPresent("reactor.netty.http.client.HttpClient", loader);
 		jettyClientPresent = ClassUtils.isPresent("org.eclipse.jetty.client.HttpClient", loader);
 		httpComponentsClientPresent =
 				ClassUtils.isPresent("org.apache.hc.client5.http.impl.async.CloseableHttpAsyncClient", loader) &&
 						ClassUtils.isPresent("org.apache.hc.core5.reactive.ReactiveDataConsumer", loader);
+		webFluxPresent = ClassUtils.isPresent(
+				"org.springframework.web.reactive.function.client.ExchangeFunction", loader);
 	}
 
 
@@ -119,6 +123,10 @@ class DefaultWebTestClientBuilder implements WebTestClient.Builder {
 		Assert.isTrue(httpHandlerBuilder == null || connector == null,
 				"Expected WebHttpHandlerBuilder or ClientHttpConnector but not both.");
 
+		// Helpful message especially for MockMvcWebTestClient users
+		Assert.state(webFluxPresent,
+				"To use WebTestClient, please add spring-webflux to the test classpath.");
+
 		this.connector = connector;
 		this.httpHandlerBuilder = (httpHandlerBuilder != null ? httpHandlerBuilder.clone() : null);
 	}