Add getAcceptLanguageAsLocales

The use of Locale.LanguageRange for the Accept-Language header makes
sense as it gives the most flexibility for a client to set a weighted
list and for a server to do filtering via Locale#filter.

This commit adds an additional convenience method that turns
the LangugeRange list to a list of Locale's also filtering out a
wildcard (i.e. "*"). A List<Locale> is the most basic way to access
prefered languages and needed when filtering is not required.

Issue: SPR-15024

Author: rstoyanchev

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

@@ -443,13 +443,16 @@ public List<MediaType> getAccept() {
 	 * Set the acceptable language ranges,
 	 * as specified by the {@literal Accept-Language} header.
 	 * @see Locale.LanguageRange
+	 * @since 5.0
 	 */
 	public void setAcceptLanguage(List<Locale.LanguageRange> languages) {
 		Assert.notNull(languages, "'languages' must not be null");
-		DecimalFormat df = new DecimalFormat("0.0", DECIMAL_FORMAT_SYMBOLS);
-		List<String> values = languages
-				.stream()
-				.map(r -> (r.getWeight() == Locale.LanguageRange.MAX_WEIGHT ? r.getRange() : r.getRange() + ";q=" + df.format(r.getWeight())))
+		DecimalFormat decimal = new DecimalFormat("0.0", DECIMAL_FORMAT_SYMBOLS);
+		List<String> values = languages.stream()
+				.map(range ->
+						range.getWeight() == Locale.LanguageRange.MAX_WEIGHT ?
+								range.getRange() :
+								range.getRange() + ";q=" + decimal.format(range.getWeight()))
 				.collect(Collectors.toList());
 		set(ACCEPT_LANGUAGE, toCommaDelimitedString(values));
 	}
@@ -458,6 +461,7 @@ public void setAcceptLanguage(List<Locale.LanguageRange> languages) {
 	 * Return the acceptable language ranges,
 	 * as specified by the {@literal Accept-Language} header
 	 * @see Locale.LanguageRange
+	 * @since 5.0
 	 */
 	public List<Locale.LanguageRange> getAcceptLanguage() {
 		String value = getFirst(ACCEPT_LANGUAGE);
@@ -467,6 +471,22 @@ public List<Locale.LanguageRange> getAcceptLanguage() {
 		return Collections.emptyList();
 	}
 
+	/**
+	 * A variant of {@link #getAcceptLanguage()} that converts each
+	 * {@link java.util.Locale.LanguageRange} to a {@link Locale}.
+	 * @since 5.0
+	 */
+	public List<Locale> getAcceptLanguageAsLocales() {
+		List<Locale.LanguageRange> ranges = getAcceptLanguage();
+		if (ranges.isEmpty()) {
+			return Collections.emptyList();
+		}
+		return ranges.stream()
+				.map(range -> Locale.forLanguageTag(range.getRange()))
+				.filter(locale -> StringUtils.hasText(locale.getDisplayName()))
+				.collect(Collectors.toList());
+	}
+
 	/**
 	 * Set the (new) value of the {@code Access-Control-Allow-Credentials} response header.
 	 */
@@ -767,6 +787,7 @@ public ContentDisposition getContentDisposition() {
 	 * as specified by the {@literal Content-Language} header.
 	 * <p>Use {@code set(CONTENT_LANGUAGE, ...)} if you need
 	 * to set multiple content languages.</p>
+	 * @since 5.0
 	 */
 	public void setContentLanguage(Locale locale) {
 		Assert.notNull(locale, "'locale' must not be null");
@@ -779,6 +800,7 @@ public void setContentLanguage(Locale locale) {
 	 * <p>Returns {@code null} when the content language is unknown.
 	 * <p>Use {@code getValuesAsList(CONTENT_LANGUAGE)} if you need
 	 * to get multiple content languages.</p>
+	 * @since 5.0
 	 */
 	public Locale getContentLanguage() {
 		return getValuesAsList(CONTENT_LANGUAGE)

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

@@ -424,30 +424,39 @@ public void accessControlRequestMethod() {
 
 	@Test
 	public void acceptLanguage() {
-		assertTrue(headers.getAcceptLanguage().isEmpty());
-		String headerValue = "fr-ch, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5";
+		String headerValue = "fr-ch, fr;q=0.9, en-*;q=0.8, de;q=0.7, *;q=0.5";
 		headers.setAcceptLanguage(Locale.LanguageRange.parse(headerValue));
 		assertEquals(headerValue, headers.getFirst(HttpHeaders.ACCEPT_LANGUAGE));
-		List<Locale.LanguageRange> languages = headers.getAcceptLanguage();
-		Locale.LanguageRange[] languageArray = new Locale.LanguageRange[]{
+
+		List<Locale.LanguageRange> expectedRanges = Arrays.asList(
 				new Locale.LanguageRange("fr-ch"),
 				new Locale.LanguageRange("fr", 0.9),
-				new Locale.LanguageRange("en", 0.8),
+				new Locale.LanguageRange("en-*", 0.8),
 				new Locale.LanguageRange("de", 0.7),
 				new Locale.LanguageRange("*", 0.5)
-		};
-		assertArrayEquals(languageArray, languages.toArray());
+		);
+		assertEquals(expectedRanges, headers.getAcceptLanguage());
+
+		List<Locale> expectedLocales = Arrays.asList(
+				Locale.forLanguageTag("fr-ch"),
+				Locale.forLanguageTag("fr"),
+				Locale.forLanguageTag("en"),
+				Locale.forLanguageTag("de")
+		);
+		assertEquals(expectedLocales, headers.getAcceptLanguageAsLocales());
 	}
 
 	@Test
 	public void contentLanguage() {
-		assertNull(headers.getContentLanguage());
 		headers.setContentLanguage(Locale.FRANCE);
 		assertEquals(Locale.FRANCE, headers.getContentLanguage());
 		assertEquals("fr-FR", headers.getFirst(HttpHeaders.CONTENT_LANGUAGE));
-		headers.clear();
-		headers.set(HttpHeaders.CONTENT_LANGUAGE, Locale.GERMAN.toLanguageTag() + ", " + Locale.CANADA);
-		assertEquals(Locale.GERMAN, headers.getContentLanguage());
+	}
+
+	@Test
+	public void contentLanguageSerialized() {
+		headers.set(HttpHeaders.CONTENT_LANGUAGE,  "de, en_CA");
+		assertEquals("Expected one (first) locale", Locale.GERMAN, headers.getContentLanguage());
 	}
 
 }