Always buffer response body in RestTestClient

Prior to this commit, `RestTestClient` tests could only perform
expectations on the response without consuming the body. In this case,
the client could leak HTTP connections with the underlying HTTP library
because the response was not entirely read.

This commit ensures that the response is always fully drained before
performing expectations. The client is configured to buffer the response
content, so further body expectations are always possible.

Fixes gh-35784

spring-test/spring-test.gradle

@@ -64,6 +64,7 @@ dependencies {
 	testImplementation(testFixtures(project(":spring-web")))
 	testImplementation("com.fasterxml.jackson.core:jackson-databind")
 	testImplementation("com.rometools:rome")
+	testImplementation("com.squareup.okhttp3:mockwebserver3")
 	testImplementation("com.thoughtworks.xstream:xstream")
 	testImplementation("de.bechte.junit:junit-hierarchicalcontextrunner")
 	testImplementation("io.projectreactor.netty:reactor-netty-http")
@@ -73,11 +74,10 @@ dependencies {
 	testImplementation("jakarta.mail:jakarta.mail-api")
 	testImplementation("jakarta.validation:jakarta.validation-api")
 	testImplementation("javax.cache:cache-api")
-	testImplementation("org.apache.httpcomponents:httpclient") {
-		exclude group: "commons-logging", module: "commons-logging"
-	}
+	testImplementation("org.apache.httpcomponents.client5:httpclient5")
 	testImplementation("org.awaitility:awaitility")
 	testImplementation("org.easymock:easymock")
+	testImplementation("org.eclipse.jetty:jetty-reactive-httpclient")
 	testImplementation("org.hibernate.orm:hibernate-core")
 	testImplementation("org.hibernate.validator:hibernate-validator")
 	testImplementation("org.hsqldb:hsqldb")

spring-test/src/main/java/org/springframework/test/web/servlet/client/ExchangeResult.java

@@ -92,6 +92,8 @@ public class ExchangeResult {
 		this.uriTemplate = uriTemplate;
 		this.requestBody = requestBody;
 		this.converterDelegate = converter;
+		// buffer response body in all cases, or connections might leak if expectations do not read the response
+		bufferResponseBody();
 	}
 
 	ExchangeResult(ExchangeResult result) {
@@ -241,6 +243,15 @@ public class ExchangeResult {
 				formatBody(getResponseHeaders().getContentType(), getResponseBodyContent()) +"\n";
 	}
 
+	private void bufferResponseBody() {
+		try {
+			StreamUtils.drain(this.clientResponse.getBody());
+		}
+		catch (IOException ex) {
+			throw new IllegalStateException("Failed to get response content: " + ex);
+		}
+	}
+
 	private String formatStatus(HttpStatusCode statusCode) {
 		String result = statusCode.toString();
 		if (statusCode instanceof HttpStatus status) {

spring-test/src/test/java/org/springframework/test/web/servlet/client/RestTestClientIntegrationTests.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright 2002-present 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.test.web.servlet.client;
+
+import java.io.IOException;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+import mockwebserver3.MockResponse;
+import mockwebserver3.MockWebServer;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import org.springframework.http.client.ClientHttpRequestFactory;
+import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
+import org.springframework.http.client.JdkClientHttpRequestFactory;
+import org.springframework.http.client.JettyClientHttpRequestFactory;
+import org.springframework.http.client.ReactorClientHttpRequestFactory;
+import org.springframework.http.client.SimpleClientHttpRequestFactory;
+
+import static org.junit.jupiter.params.provider.Arguments.argumentSet;
+
+/**
+ * Integration tests for {@link RestTestClient} against a live server.
+ */
+class RestTestClientIntegrationTests {
+
+	private RestTestClient client;
+
+	@Retention(RetentionPolicy.RUNTIME)
+	@Target(ElementType.METHOD)
+	@ParameterizedTest
+	@MethodSource("clientHttpRequestFactories")
+	@interface ParameterizedRestClientTest {
+	}
+
+	static Stream<Arguments> clientHttpRequestFactories() {
+		return Stream.of(
+				argumentSet("JDK HttpURLConnection", new SimpleClientHttpRequestFactory()),
+				argumentSet("Jetty", new JettyClientHttpRequestFactory()),
+				argumentSet("JDK HttpClient", new JdkClientHttpRequestFactory()),
+				argumentSet("Reactor Netty", new ReactorClientHttpRequestFactory()),
+				argumentSet("HttpComponents", new HttpComponentsClientHttpRequestFactory())
+		);
+	}
+
+	private MockWebServer server;
+
+	private RestTestClient testClient;
+
+
+	private void startServer(ClientHttpRequestFactory requestFactory) throws IOException {
+		this.server = new MockWebServer();
+		this.server.start();
+		this.testClient = RestTestClient.bindToServer(requestFactory)
+				.baseUrl(this.server.url("/").toString())
+				.build();
+	}
+
+	@ParameterizedRestClientTest // gh-35784
+	void sequentialRequestsNotConsumingBody(ClientHttpRequestFactory requestFactory) throws IOException {
+		startServer(requestFactory);
+		for (int i = 0; i < 10; i++) {
+			prepareResponse(builder ->
+					builder.setHeader("Content-Type", "text/plain").body("Hello Spring!"));
+			this.testClient.get().uri("/").exchange().expectStatus().isOk();
+		}
+	}
+
+	private void prepareResponse(Function<MockResponse.Builder, MockResponse.Builder> f) {
+		MockResponse.Builder builder = new MockResponse.Builder();
+		this.server.enqueue(f.apply(builder).build());
+	}
+
+	@AfterEach
+	void shutdown() throws IOException {
+		if (server != null) {
+			this.server.close();
+		}
+	}
+
+}