Unwrap request factory when creating RestClient from RestTemplate

This commit makes sure that, when building a RestClient based on the
configuration of a RestTemplate, the request factory is unwrapped if it
is a InterceptingClientHttpRequestFactory.

Closes gh-32038

spring-web/src/test/java/org/springframework/web/client/RestClientBuilderTests.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2002-2024 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.web.client;
+
+import java.lang.reflect.Field;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.client.ClientHttpRequestInitializer;
+import org.springframework.http.client.ClientHttpRequestInterceptor;
+import org.springframework.http.client.JettyClientHttpRequestFactory;
+import org.springframework.http.client.support.BasicAuthenticationInterceptor;
+import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.StringHttpMessageConverter;
+import org.springframework.web.util.DefaultUriBuilderFactory;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+
+/**
+ * @author Arjen Poutsma
+ */
+public class RestClientBuilderTests {
+
+	@SuppressWarnings("unchecked")
+	@Test
+	void createFromRestTemplate() throws NoSuchFieldException, IllegalAccessException {
+		JettyClientHttpRequestFactory requestFactory = new JettyClientHttpRequestFactory();
+		DefaultUriBuilderFactory uriBuilderFactory = new DefaultUriBuilderFactory();
+		ResponseErrorHandler errorHandler = new DefaultResponseErrorHandler();
+		List<HttpMessageConverter<?>> restTemplateMessageConverters = List.of(new StringHttpMessageConverter());
+		ClientHttpRequestInterceptor interceptor = new BasicAuthenticationInterceptor("foo", "bar");
+		ClientHttpRequestInitializer initializer = request -> {};
+
+		RestTemplate restTemplate = new RestTemplate(requestFactory);
+		restTemplate.setUriTemplateHandler(uriBuilderFactory);
+		restTemplate.setErrorHandler(errorHandler);
+		restTemplate.setMessageConverters(restTemplateMessageConverters);
+		restTemplate.setInterceptors(List.of(interceptor));
+		restTemplate.setClientHttpRequestInitializers(List.of(initializer));
+
+		RestClient.Builder builder = RestClient.builder(restTemplate);
+		assertThat(builder).isInstanceOf(DefaultRestClientBuilder.class);
+		DefaultRestClientBuilder defaultBuilder = (DefaultRestClientBuilder) builder;
+
+		assertThat(fieldValue("requestFactory", defaultBuilder)).isSameAs(requestFactory);
+		assertThat(fieldValue("uriBuilderFactory", defaultBuilder)).isSameAs(uriBuilderFactory);
+
+		List<StatusHandler> statusHandlers = (List<StatusHandler>) fieldValue("statusHandlers", defaultBuilder);
+		assertThat(statusHandlers).hasSize(1);
+
+		List<HttpMessageConverter<?>> restClientMessageConverters =
+				(List<HttpMessageConverter<?>>) fieldValue("messageConverters", defaultBuilder);
+		assertThat(restClientMessageConverters).containsExactlyElementsOf(restClientMessageConverters);
+
+		List<ClientHttpRequestInterceptor> interceptors =
+				(List<ClientHttpRequestInterceptor>) fieldValue("interceptors", defaultBuilder);
+		assertThat(interceptors).containsExactly(interceptor);
+
+		List<ClientHttpRequestInitializer> initializers =
+				(List<ClientHttpRequestInitializer>) fieldValue("initializers", defaultBuilder);
+		assertThat(initializers).containsExactly(initializer);
+	}
+
+	private static Object fieldValue(String name, DefaultRestClientBuilder instance)
+			throws NoSuchFieldException, IllegalAccessException {
+
+		Field field = DefaultRestClientBuilder.class.getDeclaredField(name);
+		field.setAccessible(true);
+
+		return field.get(instance);
+	}
+}