Ignore empty entries when parsing MediaTypes and MimeTypes

Prior to Spring Framework 5.1.3, MimeTypeUtils.parseMimeTypes() and
MediaType.parseMediaTypes() ignored empty entries, but 5.1.3 introduced
a regression in that an empty entry -- for example, due to a trailing
comma in the list of media types in an HTTP Accept header -- would result
in a "406 Not Acceptable" response status.

This commit fixes this by filtering out empty entries before parsing
them into MimeType and MediaType instances. Empty entries are therefore
effectively ignored.

Fixes gh-23241

spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java

@@ -38,6 +38,7 @@
  * @author Arjen Poutsma
  * @author Rossen Stoyanchev
  * @author Dimitrios Liapis
+ * @author Sam Brannen
  * @since 4.0
  */
 public abstract class MimeTypeUtils {
@@ -259,10 +260,11 @@ public static List<MimeType> parseMimeTypes(String mimeTypes) {
 			return Collections.emptyList();
 		}
 		return tokenize(mimeTypes).stream()
-				.map(MimeTypeUtils::parseMimeType).collect(Collectors.toList());
+				.filter(StringUtils::hasText)
+				.map(MimeTypeUtils::parseMimeType)
+				.collect(Collectors.toList());
 	}
 
-
 	/**
 	 * Tokenize the given comma-separated string of {@code MimeType} objects
 	 * into a {@code List<String>}. Unlike simple tokenization by ",", this

spring-core/src/test/java/org/springframework/util/MimeTypeTests.java

@@ -277,6 +277,13 @@ public void parseMimeTypes() {
 		assertEquals("Invalid amount of mime types", 0, mimeTypes.size());
 	}
 
+	@Test // gh-23241
+	public void parseMimeTypesWithTrailingComma() {
+		List<MimeType> mimeTypes = MimeTypeUtils.parseMimeTypes("text/plain, text/html,");
+		assertNotNull("No mime types returned", mimeTypes);
+		assertEquals("Incorrect number of mime types", 2, mimeTypes.size());
+	}
+
 	@Test // SPR-17459
 	public void parseMimeTypesWithQuotedParameters() {
 		testWithQuotedParameters("foo/bar;param=\",\"");

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -44,6 +44,7 @@
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
  * @author Kazuki Shimizu
+ * @author Sam Brannen
  * @since 3.0
  * @see <a href="https://tools.ietf.org/html/rfc7231#section-3.1.1.1">
  *     HTTP 1.1: Semantics and Content, section 3.1.1.1</a>
@@ -553,7 +554,9 @@ public static List<MediaType> parseMediaTypes(@Nullable String mediaTypes) {
 			return Collections.emptyList();
 		}
 		return MimeTypeUtils.tokenize(mediaTypes).stream()
-				.map(MediaType::parseMediaType).collect(Collectors.toList());
+				.filter(StringUtils::hasText)
+				.map(MediaType::parseMediaType)
+				.collect(Collectors.toList());
 	}
 
 	/**

spring-web/src/test/java/org/springframework/http/MediaTypeTests.java

@@ -143,6 +143,13 @@ public void parseMediaTypes() throws Exception {
 		assertEquals("Invalid amount of media types", 0, mediaTypes.size());
 	}
 
+	@Test // gh-23241
+	public void parseMediaTypesWithTrailingComma() {
+		List<MediaType> mediaTypes = MediaType.parseMediaTypes("text/plain, text/html, ");
+		assertNotNull("No media types returned", mediaTypes);
+		assertEquals("Incorrect number of media types", 2, mediaTypes.size());
+	}
+
 	@Test
 	public void compareTo() {
 		MediaType audioBasic = new MediaType("audio", "basic");