Refine ContentNegotiationStrategy contract

Consistently return "*/*" if no media types were requested rather than
an empty list. Existing code has to check for both in any case to see
if nothing was requested.

Issue: SPR-16624

spring-web/src/main/java/org/springframework/web/accept/AbstractMappingContentNegotiationStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -122,7 +122,7 @@ public abstract class AbstractMappingContentNegotiationStrategy extends MappingM
 				return Collections.singletonList(mediaType);
 			}
 		}
-		return Collections.emptyList();
+		return MEDIA_TYPE_ALL_LIST;
 	}
 
 

spring-web/src/main/java/org/springframework/web/accept/ContentNegotiationManager.java

@@ -45,9 +45,6 @@ import org.springframework.web.context.request.NativeWebRequest;
  */
 public class ContentNegotiationManager implements ContentNegotiationStrategy, MediaTypeFileExtensionResolver {
 
-	private static final List<MediaType> MEDIA_TYPE_ALL = Collections.singletonList(MediaType.ALL);
-
-
 	private final List<ContentNegotiationStrategy> strategies = new ArrayList<>();
 
 	private final Set<MediaTypeFileExtensionResolver> resolvers = new LinkedHashSet<>();
@@ -125,12 +122,12 @@ public class ContentNegotiationManager implements ContentNegotiationStrategy, Me
 	public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
 		for (ContentNegotiationStrategy strategy : this.strategies) {
 			List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
-			if (mediaTypes.isEmpty() || mediaTypes.equals(MEDIA_TYPE_ALL)) {
+			if (mediaTypes.equals(MEDIA_TYPE_ALL_LIST)) {
 				continue;
 			}
 			return mediaTypes;
 		}
-		return Collections.emptyList();
+		return MEDIA_TYPE_ALL_LIST;
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/web/accept/ContentNegotiationStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.accept;
 
+import java.util.Collections;
 import java.util.List;
 
 import org.springframework.http.MediaType;
@@ -31,11 +32,20 @@ import org.springframework.web.context.request.NativeWebRequest;
 @FunctionalInterface
 public interface ContentNegotiationStrategy {
 
+	/**
+	 * A singleton list with {@link MediaType#ALL} that is returned from
+	 * {@link #resolveMediaTypes} when no specific media types are requested.
+	 * @since 5.0.5
+	 */
+	List<MediaType> MEDIA_TYPE_ALL_LIST = Collections.singletonList(MediaType.ALL);
+
+
 	/**
 	 * Resolve the given request to a list of media types. The returned list is
 	 * ordered by specificity first and by quality parameter second.
 	 * @param webRequest the current request
-	 * @return the requested media types or an empty list (never {@code null})
+	 * @return the requested media types, or {@link #MEDIA_TYPE_ALL_LIST} if none
+	 * were requested.
 	 * @throws HttpMediaTypeNotAcceptableException if the requested media
 	 * types cannot be parsed
 	 */

spring-web/src/main/java/org/springframework/web/accept/HeaderContentNegotiationStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -17,12 +17,12 @@
 package org.springframework.web.accept;
 
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.List;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.InvalidMediaTypeException;
 import org.springframework.http.MediaType;
+import org.springframework.util.CollectionUtils;
 import org.springframework.web.HttpMediaTypeNotAcceptableException;
 import org.springframework.web.context.request.NativeWebRequest;
 
@@ -45,14 +45,14 @@ public class HeaderContentNegotiationStrategy implements ContentNegotiationStrat
 
 		String[] headerValueArray = request.getHeaderValues(HttpHeaders.ACCEPT);
 		if (headerValueArray == null) {
-			return Collections.emptyList();
+			return MEDIA_TYPE_ALL_LIST;
 		}
 
 		List<String> headerValues = Arrays.asList(headerValueArray);
 		try {
 			List<MediaType> mediaTypes = MediaType.parseMediaTypes(headerValues);
 			MediaType.sortBySpecificityAndQuality(mediaTypes);
-			return mediaTypes;
+			return !CollectionUtils.isEmpty(mediaTypes) ? mediaTypes : MEDIA_TYPE_ALL_LIST;
 		}
 		catch (InvalidMediaTypeException ex) {
 			throw new HttpMediaTypeNotAcceptableException(