Improve docs on {@code Accept-Language} negotiation

Closes spring-projectsgh-28673

Author: rstoyanchev

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -29,9 +29,12 @@
 import org.springframework.web.server.ServerWebExchange;
 
 /**
- * {@link LocaleContextResolver} implementation that simply uses the primary locale
- * specified in the "Accept-Language" header of the HTTP request (that is,
- * the locale sent by the client browser, normally that of the client's OS).
+ * {@link LocaleContextResolver} implementation that looks for a match between
+ * locales in the {@code Accept-Language} header and a list of configured
+ * supported locales.
+ *
+ * <p>See {@link #setSupportedLocales(List)} for further details on how
+ * supported and requested locales are matched.
  *
  * <p>Note: Does not support {@link #setLocaleContext}, since the accept header
  * can only be changed through changing the client's locale settings.
@@ -50,8 +53,20 @@ public class AcceptHeaderLocaleContextResolver implements LocaleContextResolver
 
 
 	/**
-	 * Configure supported locales to check against the requested locales
-	 * determined via {@link HttpHeaders#getAcceptLanguageAsLocales()}.
+	 * Configure the list of supported locales to compare and match against
+	 * {@link HttpHeaders#getAcceptLanguageAsLocales() requested locales}.
+	 * <p>In order for a supported locale to be considered a match, it must match
+	 * on both country and language. If you want to support a language-only match
+	 * as a fallback, you must configure the language explicitly as a supported
+	 * locale.
+	 * <p>For example, if the supported locales are {@code ["de-DE","en-US"]},
+	 * then a request for {@code "en-GB"} will not match, and neither will a
+	 * request for {@code "en"}. If you want to support additional locales for a
+	 * given language such as {@code "en"}, then you must add it to the list of
+	 * supported locales.
+	 * <p>If there is no match, then the {@link #setDefaultLocale(Locale)
+	 * defaultLocale} is used, if configured, or otherwise falling back on
+	 * the first requested locale.
 	 * @param locales the supported locales
 	 */
 	public void setSupportedLocales(List<Locale> locales) {
@@ -117,10 +132,10 @@ private Locale resolveSupportedLocale(@Nullable List<Locale> requestLocales) {
 			}
 			else if (languageMatch == null) {
 				// Let's try to find a language-only match as a fallback
-				for (Locale candidate : supportedLocales) {
-					if (!StringUtils.hasLength(candidate.getCountry()) &&
-							candidate.getLanguage().equals(locale.getLanguage())) {
-						languageMatch = candidate;
+				for (Locale supportedLocale : supportedLocales) {
+					if (!StringUtils.hasLength(supportedLocale.getCountry()) &&
+							supportedLocale.getLanguage().equals(locale.getLanguage())) {
+						languageMatch = supportedLocale;
 						break;
 					}
 				}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -29,12 +29,16 @@
 import org.springframework.web.servlet.LocaleResolver;
 
 /**
- * {@link LocaleResolver} implementation that simply uses the primary locale
- * specified in the {@code Accept-Language} header of the HTTP request (that is,
- * the locale sent by the client browser, normally that of the client's OS).
+ * {@link LocaleResolver} implementation that looks for a match between locales
+ * in the {@code Accept-Language} header and a list of configured supported
+ * locales.
  *
- * <p>Note: Does not support {@link #setLocale} since the {@code Accept-Language}
- * header can only be changed by changing the client's locale settings.
+ * <p>See {@link #setSupportedLocales(List)} for further details on how
+ * supported and requested locales are matched.
+ *
+ * <p>Note: This implementation does not support {@link #setLocale} since the
+ * {@code Accept-Language} header can only be changed by changing the client's
+ * locale settings.
  *
  * @author Juergen Hoeller
  * @author Rossen Stoyanchev
@@ -47,9 +51,20 @@ public class AcceptHeaderLocaleResolver extends AbstractLocaleResolver {
 
 
 	/**
-	 * Configure supported locales to check against the requested locales
-	 * determined via {@link HttpServletRequest#getLocales()}. If this is not
-	 * configured then {@link HttpServletRequest#getLocale()} is used instead.
+	 * Configure the list of supported locales to compare and match against
+	 * {@link HttpServletRequest#getLocales() requested locales}.
+	 * <p>In order for a supported locale to be considered a match, it must match
+	 * on both country and language. If you want to support a language-only match
+	 * as a fallback, you must configure the language explicitly as a supported
+	 * locale.
+	 * <p>For example, if the supported locales are {@code ["de-DE","en-US"]},
+	 * then a request for {@code "en-GB"} will not match, and neither will a
+	 * request for {@code "en"}. If you want to support additional locales for a
+	 * given language such as {@code "en"}, then you must add it to the list of
+	 * supported locales.
+	 * <p>If there is no match, then the {@link #setDefaultLocale(Locale)
+	 * defaultLocale} is used, if configured, or otherwise falling back on
+	 * {@link HttpServletRequest#getLocale()}.
 	 * @param locales the supported locales
 	 * @since 4.3
 	 */
@@ -99,10 +114,10 @@ private Locale findSupportedLocale(HttpServletRequest request, List<Locale> supp
 			}
 			else if (languageMatch == null) {
 				// Let's try to find a language-only match as a fallback
-				for (Locale candidate : supportedLocales) {
-					if (!StringUtils.hasLength(candidate.getCountry()) &&
-							candidate.getLanguage().equals(locale.getLanguage())) {
-						languageMatch = candidate;
+				for (Locale supportedLocale : supportedLocales) {
+					if (!StringUtils.hasLength(supportedLocale.getCountry()) &&
+							supportedLocale.getLanguage().equals(locale.getLanguage())) {
+						languageMatch = supportedLocale;
 						break;
 					}
 				}