GH-10830: Add RestClient support for HTTP outbound endpoints

Fixes: Fixes: #10830

* add `rest-client` XSD attribute for HTTP outbound gateway/channel adapter
* extend XML parsers to wire `RestClient` and reject mixed `rest-template` + `rest-client`
* add `HttpRequestExecutingMessageHandler` constructors and execution path for `RestClient`
* add DSL overloads in `Http` and `HttpMessageHandlerSpec` for preconfigured `RestClient`
* guard local client options (`request-factory`, `error-handler`, `message-converters`, `encoding-mode`) when external client is used
* add parser/DSL/handler tests, including invalid dual-client configuration cases
* update HTTP reference docs (`outbound`, `timeout`, `java-config`) for `RestClient` usage

GH-10830: Deprecate RestTemplate outbound configuration and align with RestClient-first APIs

* deprecate RestTemplate-based constructors and mutators in `HttpRequestExecutingMessageHandler` (`since = "7.1", forRemoval = true`)
* keep current runtime behavior stable in the handler (RestTemplate local path + RestClient external path) to avoid backward-compatibility regressions
* deprecate RestTemplate-based DSL factory methods in `Http` and align `RestClient` methods to `7.1` documentation
* rework `HttpMessageHandlerSpec` to use `RestClient.create(restTemplate)` for RestTemplate-based construction and simplify client state tracking
* deprecate RestTemplate-focused spec mutators (`requestFactory`, `errorHandler`, `messageConverters`) in favor of configuring `RestClient`
* update HTTP reference docs (`java-config`, `outbound`, `timeout`) with 7.1 RestClient guidance and RestTemplate deprecation notes
* adjust related tests/imports to match deprecation and client-selection changes

GH-10830: Align HTTP outbound local client configuration with RestClient.Builder

* keep no-client `HttpRequestExecutingMessageHandler` constructors non-deprecated and use `@SuppressWarnings("removal")` where they delegate to deprecated RestTemplate constructors
* switch local (no external client) setup to `RestClient.Builder` and build the local `RestClient` in `doInit()`
* keep local mutator APIs (`setRequestFactory`, `setErrorHandler`, `setMessageConverters`) and apply them to the local `RestClient.Builder`
* rebuild the local `RestClient` after local mutator updates so post-init changes are visible at runtime
* retain guards that reject local-only options when an external RestTemplate/RestClient is provided
* handle `Void` response type in RestClient exchange path via `toBodilessEntity()` restoring prior behavior for bodiless replies.
* make `restClient` `volatile` because it is rebuilt and read across lifecycle/request threads
* remove deprecations from matching local mutators in `HttpMessageHandlerSpec`
* update outbound/parser/DSL tests to align with local RestClient-backed behavior
* adjust proxy and cookie test fixtures for RestClient path (including non-null request attributes in cookie request stub)

GH-10830: Refine HTTP RestClient delegation and local client lifecycle

* refactor nullable `RestClient` HTTP DSL factory methods to delegate via RestClient-first paths and remove duplicate fallback branches
* add shared `outboundGatewaySpec(...)` helpers in `Http` to centralize URI/String/Expression client selection
* add non-deprecated local constructors in `HttpMessageHandlerSpec` for URI/String/Expression paths
* keep deprecated RestTemplate DSL overloads and add `@SuppressWarnings("removal")` where they intentionally call deprecated constructors
* build local `RestClient` once in `HttpRequestExecutingMessageHandler#doInit()` and stop rebuilding on each local mutator call
* keep local mutators (`setRequestFactory`, `setErrorHandler`, `setMessageConverters`) applying to local `RestClient.Builder`
* align HTTP proxy test flow with lifecycle semantics by re-initializing after mutator-based setup
* revise HTTP proxy/cookie test fixtures and formatting updates for the current RestClient-backed local behavior
* add `[[x7.1-http-changes]]` section to `whats-new.adoc` documenting HTTP 7.1 updates

GH-10830: Refine HTTP RestClient delegation

* route deprecated RestTemplate HTTP DSL/spec paths through RestClient.create(RestTemplate)
* remove deprecated internal-call suppressions in Http factory methods and simplify nullable RestClient delegation
* align HttpRequestExecutingMessageHandler default constructor flow with RestClient-first initialization
* switch local message converter setup to non-deprecated RestClient.Builder.configureMessageConverters API
* update HTTP parser/DSL tests and reduce deprecated-constructor warnings in outbound handler tests
* What's New entry for HTTP RestClient/RestTemplate migration notes

Signed-off-by: Arun Sethumadhavan <[email protected]>

Author: mailaruns

spring-integration-http/src/main/java/org/springframework/integration/http/config/HttpAdapterParsingUtils.java

@@ -36,6 +36,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Shiliang Li
+ * @author Arun Sethumadhavan
  *
  * @since 2.0.2
  */
@@ -62,6 +63,23 @@ static void verifyNoRestTemplateAttributes(Element element, ParserContext parser
 		}
 	}
 
+	static void verifyNoRestClientAttributes(Element element, ParserContext parserContext) {
+		for (String attributeName : SYNC_REST_TEMPLATE_REFERENCE_ATTRIBUTES) {
+			if (element.hasAttribute(attributeName)) {
+				parserContext.getReaderContext().error("When providing a 'rest-client' reference, the '"
+								+ attributeName + "' attribute is not allowed, " +
+								"it must be set on the provided client instead",
+						parserContext.extractSource(element));
+			}
+		}
+
+		if (element.hasAttribute("encoding-mode")) {
+			parserContext.getReaderContext().error("When providing a 'rest-client' reference, " +
+							"the 'encoding-mode' must be set on the 'RestClient.Builder.uriBuilderFactory' property.",
+					parserContext.extractSource(element));
+		}
+	}
+
 	static void configureUriVariableExpressions(BeanDefinitionBuilder builder, ParserContext parserContext,
 			Element element) {
 
@@ -97,6 +115,7 @@ static void configureUriVariableExpressions(BeanDefinitionBuilder builder, Parse
 
 	static void configureUrlConstructorArg(Element element, ParserContext parserContext,
 			BeanDefinitionBuilder builder) {
+
 		String urlAttribute = element.getAttribute("url");
 		String urlExpressionAttribute = element.getAttribute("url-expression");
 		boolean hasUrlAttribute = StringUtils.hasText(urlAttribute);
@@ -114,6 +133,7 @@ static void configureUrlConstructorArg(Element element, ParserContext parserCont
 			expressionDef = new RootBeanDefinition(ExpressionFactoryBean.class);
 			expressionDef.getConstructorArgumentValues().addGenericArgumentValue(urlExpressionAttribute);
 		}
+
 		builder.addConstructorArgValue(expressionDef);
 
 	}
@@ -147,6 +167,7 @@ else if (hasHttpMethodExpression) {
 
 	static void setExpectedResponseOrExpression(Element element, ParserContext parserContext,
 			BeanDefinitionBuilder builder) {
+
 		String expectedResponseType = element.getAttribute("expected-response-type");
 		String expectedResponseTypeExpression = element.getAttribute("expected-response-type-expression");
 

spring-integration-http/src/main/java/org/springframework/integration/http/config/HttpOutboundChannelAdapterParser.java

@@ -36,6 +36,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Shiliang Li
+ * @author Arun Sethumadhavan
  *
  * @since 2.0
  */
@@ -78,13 +79,26 @@ protected BeanDefinitionBuilder getBuilder(Element element, ParserContext parser
 				BeanDefinitionBuilder.genericBeanDefinition(HttpRequestExecutingMessageHandler.class);
 
 		String restTemplateRef = element.getAttribute("rest-template");
+		String restClientRef = element.getAttribute("rest-client");
+
+		if (StringUtils.hasText(restTemplateRef) && StringUtils.hasText(restClientRef)) {
+			parserContext.getReaderContext()
+					.error("Only one of 'rest-template' and 'rest-client' references is allowed.",
+							parserContext.extractSource(element));
+		}
 
 		if (StringUtils.hasText(restTemplateRef)) {
 			HttpAdapterParsingUtils.verifyNoRestTemplateAttributes(element, parserContext);
 			builder.getBeanDefinition()
 					.getConstructorArgumentValues()
 					.addIndexedArgumentValue(1, new RuntimeBeanReference(restTemplateRef));
 		}
+		else if (StringUtils.hasText(restClientRef)) {
+			HttpAdapterParsingUtils.verifyNoRestClientAttributes(element, parserContext);
+			builder.getBeanDefinition()
+					.getConstructorArgumentValues()
+					.addIndexedArgumentValue(1, new RuntimeBeanReference(restClientRef));
+		}
 		else {
 			for (String referenceAttributeName : HttpAdapterParsingUtils.SYNC_REST_TEMPLATE_REFERENCE_ATTRIBUTES) {
 				IntegrationNamespaceUtils.setReferenceIfAttributeDefined(builder, element, referenceAttributeName);

spring-integration-http/src/main/java/org/springframework/integration/http/config/HttpOutboundGatewayParser.java

@@ -34,6 +34,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Shiliang Li
+ * @author Arun Sethumadhavan
  */
 public class HttpOutboundGatewayParser extends AbstractConsumerEndpointParser {
 
@@ -91,13 +92,26 @@ protected BeanDefinitionBuilder getBuilder(Element element, ParserContext parser
 				BeanDefinitionBuilder.genericBeanDefinition(HttpRequestExecutingMessageHandler.class);
 
 		String restTemplateRef = element.getAttribute("rest-template");
+		String restClientRef = element.getAttribute("rest-client");
+
+		if (StringUtils.hasText(restTemplateRef) && StringUtils.hasText(restClientRef)) {
+			parserContext.getReaderContext()
+					.error("Only one of 'rest-template' and 'rest-client' references is allowed.",
+							parserContext.extractSource(element));
+		}
 
 		if (StringUtils.hasText(restTemplateRef)) {
 			HttpAdapterParsingUtils.verifyNoRestTemplateAttributes(element, parserContext);
 			builder.getBeanDefinition()
 					.getConstructorArgumentValues()
 					.addIndexedArgumentValue(1, new RuntimeBeanReference(restTemplateRef));
 		}
+		else if (StringUtils.hasText(restClientRef)) {
+			HttpAdapterParsingUtils.verifyNoRestClientAttributes(element, parserContext);
+			builder.getBeanDefinition()
+					.getConstructorArgumentValues()
+					.addIndexedArgumentValue(1, new RuntimeBeanReference(restClientRef));
+		}
 		else {
 			for (String referenceAttributeName : HttpAdapterParsingUtils.SYNC_REST_TEMPLATE_REFERENCE_ATTRIBUTES) {
 				IntegrationNamespaceUtils.setReferenceIfAttributeDefined(builder, element, referenceAttributeName);