Validate combined RestTemplate and RestClient usage in mock REST config

Fixes gh-38820

spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/web/client/MockRestServiceServerAutoConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -136,9 +136,12 @@ private RequestExpectationManager getDelegate() {
 				.getExpectationManagers();
 			Map<RestClient.Builder, RequestExpectationManager> restClientExpectationManagers = this.restClientCustomizer
 				.getExpectationManagers();
-			Assert.state(!(restTemplateExpectationManagers.isEmpty() && restClientExpectationManagers.isEmpty()),
-					"Unable to use auto-configured MockRestServiceServer since "
-							+ "a mock server customizer has not been bound to a RestTemplate or RestClient");
+			boolean neitherBound = restTemplateExpectationManagers.isEmpty() && restClientExpectationManagers.isEmpty();
+			boolean bothBound = !restTemplateExpectationManagers.isEmpty() && !restClientExpectationManagers.isEmpty();
+			Assert.state(!neitherBound, "Unable to use auto-configured MockRestServiceServer since "
+					+ "a mock server customizer has not been bound to a RestTemplate or RestClient");
+			Assert.state(!bothBound, "Unable to use auto-configured MockRestServiceServer since "
+					+ "mock server customizers have been bound to both a RestTemplate and a RestClient");
 			if (!restTemplateExpectationManagers.isEmpty()) {
 				Assert.state(restTemplateExpectationManagers.size() == 1,
 						"Unable to use auto-configured MockRestServiceServer since "

spring-boot-project/spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/web/client/RestClientTestRestTemplateAndRestClientTogetherIntegrationTests.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2012-2024 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.autoconfigure.web.client;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.web.client.MockServerRestClientCustomizer;
+import org.springframework.boot.test.web.client.MockServerRestTemplateCustomizer;
+import org.springframework.http.MediaType;
+import org.springframework.test.web.client.MockRestServiceServer;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
+import static org.springframework.test.web.client.match.MockRestRequestMatchers.requestTo;
+import static org.springframework.test.web.client.response.MockRestResponseCreators.withSuccess;
+
+/**
+ * Tests for {@link RestClientTest @RestClientTest} with a {@code RestTemplate} and a
+ * {@code RestClient} clients.
+ *
+ * @author Scott Frederick
+ */
+@RestClientTest({ ExampleRestTemplateService.class, ExampleRestClientService.class })
+class RestClientTestRestTemplateAndRestClientTogetherIntegrationTests {
+
+	@Autowired
+	private ExampleRestTemplateService restTemplateClient;
+
+	@Autowired
+	private ExampleRestClientService restClientClient;
+
+	@Autowired
+	private MockServerRestTemplateCustomizer templateCustomizer;
+
+	@Autowired
+	private MockServerRestClientCustomizer clientCustomizer;
+
+	@Autowired
+	private MockRestServiceServer server;
+
+	@Test
+	void serverShouldNotWork() {
+		assertThatIllegalStateException().isThrownBy(
+				() -> this.server.expect(requestTo(uri("/test"))).andRespond(withSuccess("hello", MediaType.TEXT_HTML)))
+			.withMessageContaining("Unable to use auto-configured");
+	}
+
+	@Test
+	void restTemplateClientRestCallViaCustomizer() {
+		this.templateCustomizer.getServer()
+			.expect(requestTo("/test"))
+			.andRespond(withSuccess("hello", MediaType.TEXT_HTML));
+		assertThat(this.restTemplateClient.test()).isEqualTo("hello");
+	}
+
+	@Test
+	void restClientClientRestCallViaCustomizer() {
+		this.clientCustomizer.getServer()
+			.expect(requestTo(uri("/test")))
+			.andRespond(withSuccess("there", MediaType.TEXT_HTML));
+		assertThat(this.restClientClient.test()).isEqualTo("there");
+	}
+
+	private static String uri(String path) {
+		return "https://example.com" + path;
+	}
+
+}