GH-9489: Remove Content-Length HTTP before sending GET request

Fixes: #9489
Issue link: #9489

If request message has a `Content-Length` HTTP, it is still mapped to the target HTTP request
even if that one is indicated as "no-body" (`GET`, `HEAD`, `TRACE`).
In this case Netty fails to decode such a missed body with error:
```
java.lang.IllegalArgumentException: text is empty (possibly HTTP/0.9)), version: HTTP/1.0
```

* Since `Content-Length` is not supposed to be supported for those methods,
remove it altogether from the HTTP request headers
* Add nullability API into the `org.springframework.integration.http.outbound`
* Check received HTTP request on the server side that it does not have such a header for `GET`

**Auto-cherry-pick to `6.3.x` & `6.2.x`**

spring-integration-http/src/main/java/org/springframework/integration/http/outbound/AbstractHttpRequestExecutingMessageHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2023 the original author or authors.
+ * Copyright 2017-2024 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.
@@ -32,7 +32,6 @@
 import javax.xml.transform.Source;
 
 import org.springframework.beans.factory.BeanFactory;
-import org.springframework.context.ApplicationContext;
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.Expression;
@@ -100,6 +99,7 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 
 	private boolean expectReply = true;
 
+	@Nullable
 	private Expression expectedResponseTypeExpression;
 
 	private boolean extractPayload = true;
@@ -114,6 +114,7 @@ public abstract class AbstractHttpRequestExecutingMessageHandler extends Abstrac
 
 	private HeaderMapper<HttpHeaders> headerMapper = DefaultHttpHeaderMapper.outboundMapper();
 
+	@Nullable
 	private Expression uriVariablesExpression;
 
 	public AbstractHttpRequestExecutingMessageHandler(Expression uriExpression) {
@@ -196,7 +197,7 @@ public void setExpectReply(boolean expectReply) {
 	 * Specify the expected response type for the REST request.
 	 * Otherwise, it is null and an empty {@link ResponseEntity} is returned from HTTP client.
 	 * To take advantage of the HttpMessageConverters
-	 * registered on this adapter, provide a different type).
+	 * registered on this adapter, provide a different type.
 	 * @param expectedResponseType The expected type.
 	 * Also see {@link #setExpectedResponseTypeExpression(Expression)}
 	 */
@@ -322,7 +323,7 @@ protected Object handleRequestMessage(Message<?> requestMessage) {
 
 	@Nullable
 	protected abstract Object exchange(Object uri, HttpMethod httpMethod, HttpEntity<?> httpRequest,
-			Object expectedResponseType, Message<?> requestMessage, Map<String, ?> uriVariables);
+			@Nullable Object expectedResponseType, Message<?> requestMessage, @Nullable Map<String, ?> uriVariables);
 
 	protected Object getReply(ResponseEntity<?> httpResponse) {
 		HttpHeaders httpHeaders = httpResponse.getHeaders();
@@ -377,6 +378,7 @@ private HttpEntity<?> createHttpEntityFromPayload(Message<?> message, HttpMethod
 		}
 		HttpHeaders httpHeaders = mapHeaders(message);
 		if (!shouldIncludeRequestBody(httpMethod)) {
+			httpHeaders.remove(HttpHeaders.CONTENT_LENGTH);
 			return new HttpEntity<>(httpHeaders);
 		}
 		// otherwise, we are creating a request with a body and need to deal with the content-type header as well
@@ -514,6 +516,7 @@ private HttpMethod determineHttpMethod(Message<?> requestMessage) {
 		}
 	}
 
+	@Nullable
 	private Object determineExpectedResponseType(Message<?> requestMessage) {
 		return evaluateTypeFromExpression(requestMessage, this.expectedResponseTypeExpression, "expectedResponseType");
 	}
@@ -536,9 +539,7 @@ protected Object evaluateTypeFromExpression(Message<?> requestMessage, @Nullable
 							"evaluation resulted in a " + typeClass + ".");
 			if (type instanceof String && StringUtils.hasText((String) type)) {
 				try {
-					ApplicationContext applicationContext = getApplicationContext();
-					type = ClassUtils.forName((String) type,
-							applicationContext == null ? null : applicationContext.getClassLoader());
+					type = ClassUtils.forName((String) type, getApplicationContext().getClassLoader());
 				}
 				catch (ClassNotFoundException e) {
 					throw new IllegalStateException("Cannot load class for name: " + type, e);

spring-integration-http/src/main/java/org/springframework/integration/http/outbound/HttpRequestExecutingMessageHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 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.
@@ -97,7 +97,7 @@ public HttpRequestExecutingMessageHandler(Expression uriExpression) {
 	 * @param uri The URI.
 	 * @param restTemplate The rest template.
 	 */
-	public HttpRequestExecutingMessageHandler(String uri, RestTemplate restTemplate) {
+	public HttpRequestExecutingMessageHandler(String uri, @Nullable RestTemplate restTemplate) {
 		this(new LiteralExpression(uri), restTemplate);
 		/*
 		 *  We'd prefer to do this assertion first, but the compiler doesn't allow it. However,
@@ -173,7 +173,7 @@ public void setEncodingMode(DefaultUriBuilderFactory.EncodingMode encodingMode)
 	@Override
 	@Nullable
 	protected Object exchange(Object uri, HttpMethod httpMethod, HttpEntity<?> httpRequest,
-			Object expectedResponseType, Message<?> requestMessage, Map<String, ?> uriVariables) {
+			@Nullable Object expectedResponseType, Message<?> requestMessage, @Nullable Map<String, ?> uriVariables) {
 
 		ResponseEntity<?> httpResponse;
 		try {

spring-integration-http/src/main/java/org/springframework/integration/http/outbound/package-info.java

@@ -1,4 +1,6 @@
 /**
  * Provides classes supporting outbound endpoints.
  */
+@org.springframework.lang.NonNullApi
+@org.springframework.lang.NonNullFields
 package org.springframework.integration.http.outbound;

spring-integration-webflux/src/test/java/org/springframework/integration/webflux/outbound/WebFluxRequestExecutingMessageHandlerTests.java

@@ -32,6 +32,7 @@
 import org.springframework.core.io.buffer.DataBufferLimitException;
 import org.springframework.expression.Expression;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.client.reactive.ClientHttpConnector;
@@ -95,6 +96,47 @@ void testReactiveReturn() {
 				.verify(Duration.ofSeconds(10));
 	}
 
+	@Test
+	void noContentLengthHeaderForGetMethod() {
+		ClientHttpConnector httpConnector =
+				new HttpHandlerConnector((request, response) -> {
+					assertThat(request.getHeaders())
+							.doesNotContainKey(org.springframework.http.HttpHeaders.CONTENT_LENGTH);
+					response.setStatusCode(HttpStatus.OK);
+					return Mono.defer(response::setComplete);
+				});
+
+		WebClient webClient = WebClient.builder()
+				.clientConnector(httpConnector)
+				.build();
+
+		String destinationUri = "https://www.springsource.org/spring-integration";
+		WebFluxRequestExecutingMessageHandler reactiveHandler =
+				new WebFluxRequestExecutingMessageHandler(destinationUri, webClient);
+		reactiveHandler.setHttpMethod(HttpMethod.GET);
+
+		FluxMessageChannel ackChannel = new FluxMessageChannel();
+		reactiveHandler.setOutputChannel(ackChannel);
+		String testPayload = "hello, world";
+		Message<?> testMessage =
+				MessageBuilder.withPayload(testPayload)
+						.setHeader(org.springframework.http.HttpHeaders.CONTENT_LENGTH, testPayload.length())
+						.build();
+		reactiveHandler.handleMessage(testMessage);
+		reactiveHandler.handleMessage(testMessage);
+
+		StepVerifier.create(ackChannel, 2)
+				.assertNext(m ->
+						assertThat(m.getHeaders())
+								.containsEntry(HttpHeaders.STATUS_CODE, HttpStatus.OK)
+								// The reply message headers are copied from the request message
+								.containsEntry(org.springframework.http.HttpHeaders.CONTENT_LENGTH, testPayload.length()))
+				.assertNext(m -> assertThat(m.getHeaders()).containsEntry(HttpHeaders.STATUS_CODE, HttpStatus.OK))
+				.expectNoEvent(Duration.ofMillis(100))
+				.thenCancel()
+				.verify(Duration.ofSeconds(10));
+	}
+
 	@Test
 	void testReactiveErrorOneWay() {
 		ClientHttpConnector httpConnector =