Add support for setting the "Vary" response header

Issue: SPR-14070

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -38,6 +38,7 @@ import java.util.TimeZone;
 import org.springframework.util.Assert;
 import org.springframework.util.LinkedCaseInsensitiveMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -947,6 +948,24 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 		return getFirst(UPGRADE);
 	}
 
+	/**
+	 * Set the request header names (e.g. "Accept-Language") for which the
+	 * response is subject to content negotiation and variances based on the
+	 * value of those request headers.
+	 * @param requestHeaders the request header names
+	 * @since 4.3
+	 */
+	public void setVary(List<String> requestHeaders) {
+		set(VARY, toCommaDelimitedString(requestHeaders));
+	}
+
+	/**
+	 * Return the request header names subject to content negotiation.
+	 */
+	public List<String> getVary() {
+		return getFirstValueAsList(VARY);
+	}
+
 	/**
 	 * Parse the first header value for the given header name as a date,
 	 * return -1 if there is no value, or raise {@link IllegalArgumentException}

spring-web/src/main/java/org/springframework/http/ResponseEntity.java

@@ -332,6 +332,17 @@ public class ResponseEntity<T> extends HttpEntity<T> {
 		 */
 		B cacheControl(CacheControl cacheControl);
 
+		/**
+		 * Configure one or more request header names (e.g. "Accept-Language") to
+		 * add to the "Vary" response header to inform clients that the response is
+		 * subject to content negotiation and variances based on the value of the
+		 * given request headers. The configured request header names are added only
+		 * if not already present in the response "Vary" header.
+		 * @param requestHeaders request header names
+		 * @since 4.3
+		 */
+		B varyBy(String... requestHeaders);
+
 		/**
 		 * Build the response entity with no body.
 		 * @return the response entity
@@ -454,6 +465,12 @@ public class ResponseEntity<T> extends HttpEntity<T> {
 			return this;
 		}
 
+		@Override
+		public BodyBuilder varyBy(String... requestHeaders) {
+			this.headers.setVary(Arrays.asList(requestHeaders));
+			return this;
+		}
+
 		@Override
 		public ResponseEntity<Void> build() {
 			return new ResponseEntity<Void>(null, this.headers, this.status);