StringUtils.parseLocale consistently handles invalid locale values

Closes gh-22603

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

@@ -773,7 +773,9 @@ public abstract class StringUtils {
 		String[] tokens = tokenizeLocaleSource(localeValue);
 		if (tokens.length == 1) {
 			Locale resolved = Locale.forLanguageTag(localeValue);
-			return (resolved.getLanguage().length() > 0 ? resolved : null);
+			if (resolved.getLanguage().length() > 0) {
+				return resolved;
+			}
 		}
 		return parseLocaleTokens(localeValue, tokens);
 	}
@@ -820,7 +822,7 @@ public abstract class StringUtils {
 			}
 		}
 
-		if ("".equals(variant) && country.startsWith("#")) {
+		if (variant.isEmpty() && country.startsWith("#")) {
 			variant = country;
 			country = "";
 		}
@@ -1193,7 +1195,7 @@ public abstract class StringUtils {
 		}
 
 		List<String> result = new ArrayList<>();
-		if ("".equals(delimiter)) {
+		if (delimiter.isEmpty()) {
 			for (int i = 0; i < str.length(); i++) {
 				result.add(deleteAny(str.substring(i, i + 1), charsToDelete));
 			}

spring-core/src/test/java/org/springframework/util/StringUtilsTests.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.
@@ -765,4 +765,20 @@ public class StringUtilsTests {
 		}
 	}
 
+	@Test
+	public void testInvalidLocaleWithLocaleString() {
+		assertEquals(new Locale("invalid"), StringUtils.parseLocaleString("invalid"));
+		assertEquals(new Locale("invalidvalue"), StringUtils.parseLocaleString("invalidvalue"));
+		assertEquals(new Locale("invalidvalue", "foo"), StringUtils.parseLocaleString("invalidvalue_foo"));
+		assertNull(StringUtils.parseLocaleString(""));
+	}
+
+	@Test
+	public void testInvalidLocaleWithLanguageTag() {
+		assertEquals(new Locale("invalid"), StringUtils.parseLocale("invalid"));
+		assertEquals(new Locale("invalidvalue"), StringUtils.parseLocale("invalidvalue"));
+		assertEquals(new Locale("invalidvalue", "foo"), StringUtils.parseLocale("invalidvalue_foo"));
+		assertNull(StringUtils.parseLocale(""));
+	}
+
 }