Add support for headers in @HttpExchange

On the client side, supports `name=value` pairs. Placeholders in values
are resolved by the `embeddedValueResolver`.
On the server side, additionally supports `name` and `!name` syntax.

Closes gh-33309

spring-web/src/main/java/org/springframework/web/service/annotation/DeleteExchange.java

@@ -60,4 +60,10 @@ public @interface DeleteExchange {
 	@AliasFor(annotation = HttpExchange.class)
 	String[] accept() default {};
 
+	/**
+	 * Alias for {@link HttpExchange#headers()}.
+	 */
+	@AliasFor(annotation = HttpExchange.class)
+	String[] headers() default {};
+
 }

spring-web/src/main/java/org/springframework/web/service/annotation/HttpExchange.java

@@ -173,4 +173,16 @@ public @interface HttpExchange {
 	 */
 	String[] accept() default {};
 
+	/**
+	 * The additional headers to use, as an array of {@code name=value} pairs.
+	 * <p>Multiple comma-separated values are accepted, and placeholders are
+	 * supported in these values. However, Accept and Content-Type headers are
+	 * ignored: see {@link #accept()} and {@link #contentType()}.
+	 * <p>Supported at the type level as well as at the method level, in which
+	 * case the method-level values override type-level values.
+	 * <p>By default, this is empty.
+	 * @since 6.2
+	 */
+	String[] headers() default {};
+
 }

spring-web/src/main/java/org/springframework/web/service/annotation/PatchExchange.java

@@ -60,4 +60,10 @@ public @interface PatchExchange {
 	@AliasFor(annotation = HttpExchange.class)
 	String[] accept() default {};
 
+	/**
+	 * Alias for {@link HttpExchange#headers()}.
+	 */
+	@AliasFor(annotation = HttpExchange.class)
+	String[] headers() default {};
+
 }

spring-web/src/main/java/org/springframework/web/service/annotation/PostExchange.java

@@ -60,4 +60,10 @@ public @interface PostExchange {
 	@AliasFor(annotation = HttpExchange.class)
 	String[] accept() default {};
 
+	/**
+	 * Alias for {@link HttpExchange#headers()}.
+	 */
+	@AliasFor(annotation = HttpExchange.class)
+	String[] headers() default {};
+
 }

spring-web/src/main/java/org/springframework/web/service/annotation/PutExchange.java

@@ -60,4 +60,10 @@ public @interface PutExchange {
 	@AliasFor(annotation = HttpExchange.class)
 	String[] accept() default {};
 
+	/**
+	 * Alias for {@link HttpExchange#headers()}.
+	 */
+	@AliasFor(annotation = HttpExchange.class)
+	String[] headers() default {};
+
 }

spring-web/src/main/java/org/springframework/web/service/invoker/HttpServiceMethod.java

@@ -19,8 +19,10 @@ package org.springframework.web.service.invoker;
 import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Method;
 import java.time.Duration;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
+import java.util.Set;
 import java.util.function.Function;
 import java.util.function.Supplier;
 
@@ -46,6 +48,8 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.LinkedMultiValueMap;
+import org.springframework.util.MultiValueMap;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.util.StringValueResolver;
@@ -156,6 +160,7 @@ final class HttpServiceMethod {
 	private record HttpRequestValuesInitializer(
 			@Nullable HttpMethod httpMethod, @Nullable String url,
 			@Nullable MediaType contentType, @Nullable List<MediaType> acceptMediaTypes,
+			@Nullable MultiValueMap<String, String> otherHeaders,
 			Supplier<HttpRequestValues.Builder> requestValuesSupplier) {
 
 		public HttpRequestValues.Builder initializeRequestValuesBuilder() {
@@ -172,6 +177,16 @@ final class HttpServiceMethod {
 			if (this.acceptMediaTypes != null) {
 				requestValues.setAccept(this.acceptMediaTypes);
 			}
+			if (this.otherHeaders != null) {
+				this.otherHeaders.forEach((name, values) -> {
+					if (values.size() == 1) {
+						requestValues.addHeader(name, values.get(0));
+					}
+					else {
+						requestValues.addHeader(name, values.toArray(new String[0]));
+					}
+				});
+			}
 			return requestValues;
 		}
 
@@ -202,9 +217,10 @@ final class HttpServiceMethod {
 			String url = initUrl(typeAnnotation, methodAnnotation, embeddedValueResolver);
 			MediaType contentType = initContentType(typeAnnotation, methodAnnotation);
 			List<MediaType> acceptableMediaTypes = initAccept(typeAnnotation, methodAnnotation);
-
-			return new HttpRequestValuesInitializer(
-					httpMethod, url, contentType, acceptableMediaTypes, requestValuesSupplier);
+			MultiValueMap<String, String> headers = initHeaders(typeAnnotation, methodAnnotation,
+					embeddedValueResolver);
+			return new HttpRequestValuesInitializer(httpMethod, url, contentType,
+					acceptableMediaTypes, headers, requestValuesSupplier);
 		}
 
 		@Nullable
@@ -280,6 +296,50 @@ final class HttpServiceMethod {
 			return null;
 		}
 
+		private static MultiValueMap<String, String> parseHeaders(String[] headersArray,
+				@Nullable StringValueResolver embeddedValueResolver) {
+			MultiValueMap<String, String> headers = new LinkedMultiValueMap<>();
+			for (String h: headersArray) {
+				String[] headerPair = StringUtils.split(h, "=");
+				if (headerPair != null) {
+					String headerName = headerPair[0].trim();
+					List<String> headerValues = new ArrayList<>();
+					Set<String> parsedValues = StringUtils.commaDelimitedListToSet(headerPair[1]);
+					for (String headerValue : parsedValues) {
+						if (embeddedValueResolver != null) {
+							headerValue = embeddedValueResolver.resolveStringValue(headerValue);
+						}
+						if (headerValue != null) {
+							headerValue = headerValue.trim();
+							headerValues.add(headerValue);
+						}
+					}
+					if (!headerValues.isEmpty()) {
+						headers.addAll(headerName, headerValues);
+					}
+				}
+			}
+			return headers;
+		}
+
+		@Nullable
+		private static MultiValueMap<String, String> initHeaders(@Nullable HttpExchange typeAnnotation, HttpExchange methodAnnotation,
+				@Nullable StringValueResolver embeddedValueResolver) {
+			MultiValueMap<String, String> methodLevelHeaders = parseHeaders(methodAnnotation.headers(),
+					embeddedValueResolver);
+			if (!ObjectUtils.isEmpty(methodLevelHeaders)) {
+				return methodLevelHeaders;
+			}
+
+			MultiValueMap<String, String> typeLevelHeaders = (typeAnnotation != null ?
+					parseHeaders(typeAnnotation.headers(), embeddedValueResolver) : null);
+			if (!ObjectUtils.isEmpty(typeLevelHeaders)) {
+				return typeLevelHeaders;
+			}
+
+			return null;
+		}
+
 		private static List<AnnotationDescriptor> getAnnotationDescriptors(AnnotatedElement element) {
 			return MergedAnnotations.from(element, SearchStrategy.TYPE_HIERARCHY, RepeatableContainers.none())
 					.stream(HttpExchange.class)