Improve MIME type subtype suffix handling

Prior to this commit, the subtype suffix of a MIME type (see RFC 6839)
was not properly taken into account when checking compatibility between
MIME types.

For example, `"application/*"` was not considered as compatible with
`"application/vnd.io.spring+json"`.

This commit adds a new `MimeType#getSubtypeSuffix()` method to easily
extract the subtype suffix information. This method is then reused in
the `isCompatibleWith` implementation to better handle these cases.

Fixes gh-25350

Author: bclozel

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

@@ -284,6 +284,19 @@ public String getSubtype() {
 		return this.subtype;
 	}
 
+	/**
+	 * Return the subtype suffix as defined in RFC 6839.
+	 * @since 5.3
+	 */
+	@Nullable
+	public String getSubtypeSuffix() {
+		int suffixIndex = this.subtype.lastIndexOf('+');
+		if (suffixIndex != -1 && this.subtype.length() > suffixIndex) {
+			return this.subtype.substring(suffixIndex + 1);
+		}
+		return null;
+	}
+
 	/**
 	 * Return the character set, as indicated by a {@code charset} parameter, if any.
 	 * @return the character set, or {@code null} if not available
@@ -377,22 +390,20 @@ else if (getType().equals(other.getType())) {
 			if (getSubtype().equals(other.getSubtype())) {
 				return true;
 			}
-			// Wildcard with suffix? e.g. application/*+xml
 			if (isWildcardSubtype() || other.isWildcardSubtype()) {
-				int thisPlusIdx = getSubtype().lastIndexOf('+');
-				int otherPlusIdx = other.getSubtype().lastIndexOf('+');
-				if (thisPlusIdx == -1 && otherPlusIdx == -1) {
+				String thisSuffix = getSubtypeSuffix();
+				String otherSuffix = other.getSubtypeSuffix();
+				if (getSubtype().equals(WILDCARD_TYPE)
+						|| other.getSubtype().equals(WILDCARD_TYPE)) {
 					return true;
 				}
-				else if (thisPlusIdx != -1 && otherPlusIdx != -1) {
-					String thisSubtypeNoSuffix = getSubtype().substring(0, thisPlusIdx);
-					String otherSubtypeNoSuffix = other.getSubtype().substring(0, otherPlusIdx);
-					String thisSubtypeSuffix = getSubtype().substring(thisPlusIdx + 1);
-					String otherSubtypeSuffix = other.getSubtype().substring(otherPlusIdx + 1);
-					if (thisSubtypeSuffix.equals(otherSubtypeSuffix) &&
-							(WILDCARD_TYPE.equals(thisSubtypeNoSuffix) || WILDCARD_TYPE.equals(otherSubtypeNoSuffix))) {
-						return true;
-					}
+				else if (isWildcardSubtype() && thisSuffix != null) {
+					return thisSuffix.equals(other.getSubtype())
+							|| thisSuffix.equals(otherSuffix);
+				}
+				else if (other.isWildcardSubtype() && otherSuffix != null) {
+					return this.getSubtype().equals(otherSuffix)
+							|| otherSuffix.equals(thisSuffix);
 				}
 			}
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -320,6 +320,25 @@ void parseMimeTypesWithQuotedParameters() {
 		testWithQuotedParameters("foo/bar;param=\"\\,\\\"");
 	}
 
+	@Test
+	void parseSubtypeSuffix() {
+		MimeType type = new MimeType("application", "vdn.something+json");
+		assertThat(type.getSubtypeSuffix()).isEqualTo("json");
+		type = new MimeType("application", "vdn.something");
+		assertThat(type.getSubtypeSuffix()).isNull();
+		type = new MimeType("application", "vdn.something+");
+		assertThat(type.getSubtypeSuffix()).isEqualTo("");
+		type = new MimeType("application", "vdn.some+thing+json");
+		assertThat(type.getSubtypeSuffix()).isEqualTo("json");
+	}
+
+	@Test // gh-25350
+	void wildcardSubtypeCompatibleWithSuffix() {
+		MimeType applicationStar = new MimeType("application", "*");
+		MimeType applicationVndJson = new MimeType("application", "vnd.something+json");
+		assertThat(applicationStar.isCompatibleWith(applicationVndJson)).isTrue();
+	}
+
 	private void testWithQuotedParameters(String... mimeTypes) {
 		String s = String.join(",", mimeTypes);
 		List<MimeType> actual = MimeTypeUtils.parseMimeTypes(s);