Make WebTestClient creation back off when no HTTP client available

Fixes gh-27520

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/web/reactive/server/WebTestClientContextCustomizerWithoutSupportedHttpClientTests.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2012-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.test.web.reactive.server;
+
+import java.util.Collections;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.testsupport.classpath.ClassPathExclusions;
+import org.springframework.test.context.ContextCustomizer;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link WebTestClientContextCustomizerFactory} when no supported HTTP client
+ * is on the classpath.
+ *
+ * @author Andy Wilkinson
+ */
+@ClassPathExclusions({ "reactor-netty*.jar", "jetty-client*.jar" })
+public class WebTestClientContextCustomizerWithoutSupportedHttpClientTests {
+
+	@Test
+	void createContextCustomizerWhenNoSupportedHttpClientIsAvailableShouldReturnNull() {
+		WebTestClientContextCustomizerFactory contextCustomizerFactory = new WebTestClientContextCustomizerFactory();
+		ContextCustomizer contextCustomizer = contextCustomizerFactory.createContextCustomizer(TestClass.class,
+				Collections.emptyList());
+		assertThat(contextCustomizer).isNull();
+	}
+
+	@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
+	private static class TestClass {
+
+	}
+
+}