Add missing "Content-Length: 0" header with HttpComponents

Prior to this commit, HTTP requests sent with the
`HttpComponentsClientHttpRequestFactory` would not set a
"Content-Length" header for empty request bodies. Setting a request
entity is the expected behavior for unsafe HTTP methods, and this would
align the behavior with other HTTP clients.
Developers would often rely on `BufferingClientHttpRequestFactory` to
set this information on the request.

This commit ensures that a `NullEntity` is used for unsafe HTTP methods,
when no body has been set for the request. This result in a
"Content-Length:0" request header.

Fixes gh-32678

spring-web/src/main/java/org/springframework/http/client/HttpComponentsClientHttpRequest.java

@@ -30,6 +30,8 @@
 import org.apache.hc.core5.http.ClassicHttpResponse;
 import org.apache.hc.core5.http.Header;
 import org.apache.hc.core5.http.HttpEntity;
+import org.apache.hc.core5.http.Method;
+import org.apache.hc.core5.http.io.entity.NullEntity;
 import org.apache.hc.core5.http.protocol.HttpContext;
 
 import org.springframework.http.HttpHeaders;
@@ -89,8 +91,10 @@ protected ClientHttpResponse executeInternal(HttpHeaders headers, @Nullable Body
 		addHeaders(this.httpRequest, headers);
 
 		if (body != null) {
-			HttpEntity requestEntity = new BodyEntity(headers, body);
-			this.httpRequest.setEntity(requestEntity);
+			this.httpRequest.setEntity(new BodyEntity(headers, body));
+		}
+		else if (!Method.isSafe(this.httpRequest.getMethod())) {
+			this.httpRequest.setEntity(NullEntity.INSTANCE);
 		}
 		ClassicHttpResponse httpResponse = this.httpClient.executeOpen(null, this.httpRequest, this.httpContext);
 		return new HttpComponentsClientHttpResponse(httpResponse);

spring-web/src/test/java/org/springframework/http/client/AbstractMockWebServerTests.java

@@ -106,6 +106,10 @@ else if(request.getPath().startsWith("/methods/")) {
 					assertThat(request.getMethod()).isEqualTo(expectedMethod);
 					return new MockResponse();
 				}
+				else if(request.getPath().startsWith("/header/")) {
+					String headerName = request.getPath().replace("/header/","");
+					return new MockResponse().setBody(headerName + ":" + request.getHeader(headerName)).setResponseCode(200);
+				}
 				return new MockResponse().setResponseCode(404);
 			}
 			catch (Throwable exc) {

spring-web/src/test/java/org/springframework/http/client/HttpComponentsClientHttpRequestFactoryTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * 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.
@@ -16,7 +16,9 @@
 
 package org.springframework.http.client;
 
+import java.io.InputStreamReader;
 import java.net.URI;
+import java.util.stream.Stream;
 
 import org.apache.hc.client5.http.classic.HttpClient;
 import org.apache.hc.client5.http.config.Configurable;
@@ -26,8 +28,12 @@
 import org.apache.hc.client5.http.protocol.HttpClientContext;
 import org.apache.hc.core5.util.Timeout;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
 
 import org.springframework.http.HttpMethod;
+import org.springframework.http.HttpStatus;
+import org.springframework.util.FileCopyUtils;
 
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static org.assertj.core.api.Assertions.assertThat;
@@ -37,6 +43,7 @@
 
 /**
  * @author Stephane Nicoll
+ * @author Brian Clozel
  */
 class HttpComponentsClientHttpRequestFactoryTests extends AbstractHttpRequestFactoryTests {
 
@@ -145,6 +152,36 @@ public HttpClient getHttpClient() {
 		assertThat(requestConfig2.getConnectionRequestTimeout()).isEqualTo(Timeout.of(7000, MILLISECONDS));
 	}
 
+	@ParameterizedTest
+	@MethodSource("unsafeHttpMethods")
+	void shouldSetContentLengthWhenEmptyBody(HttpMethod method) throws Exception {
+		ClientHttpRequest request = factory.createRequest(URI.create(baseUrl + "/header/Content-Length"), method);
+		try (ClientHttpResponse response = request.execute()) {
+			assertThat(response.getStatusCode()).as("Invalid status code").isEqualTo(HttpStatus.OK);
+			String result = FileCopyUtils.copyToString(new InputStreamReader(response.getBody()));
+			assertThat(result).as("Invalid body").isEqualTo("Content-Length:0");
+		}
+	}
+
+	static Stream<HttpMethod> unsafeHttpMethods() {
+		return Stream.of(HttpMethod.POST, HttpMethod.PUT, HttpMethod.DELETE, HttpMethod.PATCH);
+	}
+
+	@ParameterizedTest
+	@MethodSource("safeHttpMethods")
+	void shouldNotSetContentLengthWhenEmptyBodyAndSafeMethod(HttpMethod method) throws Exception {
+		ClientHttpRequest request = factory.createRequest(URI.create(baseUrl + "/header/Content-Length"), method);
+		try (ClientHttpResponse response = request.execute()) {
+			assertThat(response.getStatusCode()).as("Invalid status code").isEqualTo(HttpStatus.OK);
+			String result = FileCopyUtils.copyToString(new InputStreamReader(response.getBody()));
+			assertThat(result).as("Invalid body").isEqualTo("Content-Length:null");
+		}
+	}
+
+	static Stream<HttpMethod> safeHttpMethods() {
+		return Stream.of(HttpMethod.GET, HttpMethod.OPTIONS, HttpMethod.TRACE);
+	}
+
 	private RequestConfig retrieveRequestConfig(HttpComponentsClientHttpRequestFactory factory) throws Exception {
 		URI uri = URI.create(baseUrl + "/status/ok");
 		HttpComponentsClientHttpRequest request = (HttpComponentsClientHttpRequest)