AcceptHeaderLocaleResolver keeps language match among supported locales

Issue: SPR-16599

spring-web/src/main/java/org/springframework/web/server/i18n/AcceptHeaderLocaleContextResolver.java

@@ -100,23 +100,26 @@ public class AcceptHeaderLocaleContextResolver implements LocaleContextResolver
 		if (CollectionUtils.isEmpty(requestLocales)) {
 			return this.defaultLocale;  // may be null
 		}
-		List<Locale> supported = getSupportedLocales();
-		if (supported.isEmpty()) {
+		List<Locale> supportedLocales = getSupportedLocales();
+		if (supportedLocales.isEmpty()) {
 			return requestLocales.get(0);  // never null
 		}
 
 		Locale languageMatch = null;
 		for (Locale locale : requestLocales) {
-			if (supported.contains(locale)) {
-				// Full match: typically language + country
-				return locale;
+			if (supportedLocales.contains(locale)) {
+				if (languageMatch == null || languageMatch.getLanguage().equals(locale.getLanguage())) {
+					// Full match: language + country, possibly narrowed from earlier language-only match
+					return locale;
+				}
 			}
 			else if (languageMatch == null) {
 				// Let's try to find a language-only match as a fallback
-				for (Locale candidate : supported) {
+				for (Locale candidate : supportedLocales) {
 					if (!StringUtils.hasLength(candidate.getCountry()) &&
 							candidate.getLanguage().equals(locale.getLanguage())) {
 						languageMatch = candidate;
+						break;
 					}
 				}
 			}

spring-web/src/test/java/org/springframework/web/server/i18n/AcceptHeaderLocaleContextResolverTests.java

@@ -72,6 +72,18 @@ public class AcceptHeaderLocaleContextResolverTests {
 		assertEquals(ENGLISH, this.resolver.resolveLocaleContext(exchange(GERMANY, US, UK)).getLocale());
 	}
 
+	@Test
+	public void resolvePreferredAgainstCountryIfPossible() {
+		this.resolver.setSupportedLocales(Arrays.asList(ENGLISH, UK));
+		assertEquals(UK, this.resolver.resolveLocaleContext(exchange(GERMANY, US, UK)).getLocale());
+	}
+
+	@Test
+	public void resolvePreferredAgainstLanguageWithMultipleSupportedLocales() {
+		this.resolver.setSupportedLocales(Arrays.asList(GERMAN, US));
+		assertEquals(GERMAN, this.resolver.resolveLocaleContext(exchange(GERMANY, US, UK)).getLocale());
+	}
+
 	@Test
 	public void resolveMissingAcceptLanguageHeader() {
 		MockServerHttpRequest request = MockServerHttpRequest.get("/").build();

spring-webmvc/src/main/java/org/springframework/web/servlet/i18n/AcceptHeaderLocaleResolver.java

@@ -98,38 +98,36 @@ public class AcceptHeaderLocaleResolver implements LocaleResolver {
 			return defaultLocale;
 		}
 		Locale requestLocale = request.getLocale();
-		if (isSupportedLocale(requestLocale)) {
+		List<Locale> supportedLocales = getSupportedLocales();
+		if (supportedLocales.isEmpty() || supportedLocales.contains(requestLocale)) {
 			return requestLocale;
 		}
-		Locale supportedLocale = findSupportedLocale(request);
+		Locale supportedLocale = findSupportedLocale(request, supportedLocales);
 		if (supportedLocale != null) {
 			return supportedLocale;
 		}
 		return (defaultLocale != null ? defaultLocale : requestLocale);
 	}
 
-	private boolean isSupportedLocale(Locale locale) {
-		List<Locale> supportedLocales = getSupportedLocales();
-		return (supportedLocales.isEmpty() || supportedLocales.contains(locale));
-	}
-
 	@Nullable
-	private Locale findSupportedLocale(HttpServletRequest request) {
+	private Locale findSupportedLocale(HttpServletRequest request, List<Locale> supportedLocales) {
 		Enumeration<Locale> requestLocales = request.getLocales();
-		List<Locale> supported = getSupportedLocales();
 		Locale languageMatch = null;
 		while (requestLocales.hasMoreElements()) {
 			Locale locale = requestLocales.nextElement();
-			if (supported.contains(locale)) {
-				// Full match: typically language + country
-				return locale;
+			if (supportedLocales.contains(locale)) {
+				if (languageMatch == null || languageMatch.getLanguage().equals(locale.getLanguage())) {
+					// Full match: language + country, possibly narrowed from earlier language-only match
+					return locale;
+				}
 			}
 			else if (languageMatch == null) {
 				// Let's try to find a language-only match as a fallback
-				for (Locale candidate : supported) {
+				for (Locale candidate : supportedLocales) {
 					if (!StringUtils.hasLength(candidate.getCountry()) &&
 							candidate.getLanguage().equals(locale.getLanguage())) {
 						languageMatch = candidate;
+						break;
 					}
 				}
 			}

spring-webmvc/src/test/java/org/springframework/web/servlet/i18n/AcceptHeaderLocaleResolverTests.java

@@ -63,6 +63,18 @@ public class AcceptHeaderLocaleResolverTests {
 		assertEquals(ENGLISH, this.resolver.resolveLocale(request(GERMANY, US, UK)));
 	}
 
+	@Test
+	public void resolvePreferredAgainstCountryIfPossible() {
+		this.resolver.setSupportedLocales(Arrays.asList(ENGLISH, UK));
+		assertEquals(UK, this.resolver.resolveLocale(request(GERMANY, US, UK)));
+	}
+
+	@Test
+	public void resolvePreferredAgainstLanguageWithMultipleSupportedLocales() {
+		this.resolver.setSupportedLocales(Arrays.asList(GERMAN, US));
+		assertEquals(GERMAN, this.resolver.resolveLocale(request(GERMANY, US, UK)));
+	}
+
 	@Test
 	public void resolvePreferredNotSupportedWithDefault() {
 		this.resolver.setSupportedLocales(Arrays.asList(US, JAPAN));