Update 9.0 with various locale changes from 8.x

docs/reference/mapping/params/format.asciidoc

@@ -31,8 +31,13 @@ down to the nearest day.
 [[custom-date-formats]]
 ==== Custom date formats
 
-Completely customizable date formats are supported. The syntax for these is explained
-https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html[DateTimeFormatter docs].
+Completely customizable date formats are supported. The syntax for these is explained in
+https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/time/format/DateTimeFormatter.html[DateTimeFormatter docs].
+
+Note that whilst the built-in formats for week dates use the ISO definition of weekyears,
+custom formatters using the `Y`, `W`, or `w` field specifiers use the JDK locale definition
+of weekyears. This can result in different values between the built-in formats and custom formats
+for week dates.
 
 [[built-in-date-formats]]
 ==== Built In Formats
@@ -256,31 +261,37 @@ The following tables lists all the defaults ISO formats supported:
 `week_date` or `strict_week_date`::
 
     A formatter for a full date as four digit weekyear, two digit week of
-    weekyear, and one digit day of week: `xxxx-'W'ww-e`.
+    weekyear, and one digit day of week: `YYYY-'W'ww-e`.
+    This uses the ISO week-date definition.
 
 `week_date_time` or `strict_week_date_time`::
 
     A formatter that combines a full weekyear date and time, separated by a
-    'T': `xxxx-'W'ww-e'T'HH:mm:ss.SSSZ`.
+    'T': `YYYY-'W'ww-e'T'HH:mm:ss.SSSZ`.
+    This uses the ISO week-date definition.
 
 `week_date_time_no_millis` or `strict_week_date_time_no_millis`::
 
     A formatter that combines a full weekyear date and time without millis,
-    separated by a 'T': `xxxx-'W'ww-e'T'HH:mm:ssZ`.
+    separated by a 'T': `YYYY-'W'ww-e'T'HH:mm:ssZ`.
+    This uses the ISO week-date definition.
 
 `weekyear` or `strict_weekyear`::
 
-    A formatter for a four digit weekyear: `xxxx`.
+    A formatter for a four digit weekyear: `YYYY`.
+    This uses the ISO week-date definition.
 
 `weekyear_week` or `strict_weekyear_week`::
 
     A formatter for a four digit weekyear and two digit week of weekyear:
-    `xxxx-'W'ww`.
+    `YYYY-'W'ww`.
+    This uses the ISO week-date definition.
 
 `weekyear_week_day` or `strict_weekyear_week_day`::
 
     A formatter for a four digit weekyear, two digit week of weekyear, and one
-    digit day of week: `xxxx-'W'ww-e`.
+    digit day of week: `YYYY-'W'ww-e`.
+    This uses the ISO week-date definition.
 
 `year` or `strict_year`::
 

docs/reference/mapping/types/date.asciidoc

@@ -126,7 +126,7 @@ The following parameters are accepted by `date` fields:
 
     The locale to use when parsing dates since months do not have the same names
     and/or abbreviations in all languages. The default is the
-    https://docs.oracle.com/javase/8/docs/api/java/util/Locale.html#ROOT[`ROOT` locale],
+    https://docs.oracle.com/javase/8/docs/api/java/util/Locale.html#ROOT[`ROOT` locale].
 
 <<ignore-malformed,`ignore_malformed`>>::
 

modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/DateFormat.java

@@ -97,6 +97,7 @@ Function<String, ZonedDateTime> getFunction(String format, ZoneId zoneId, Locale
                 // fill the rest of the date up with the parsed date
                 if (accessor.isSupported(ChronoField.YEAR) == false
                     && accessor.isSupported(ChronoField.YEAR_OF_ERA) == false
+                    && accessor.isSupported(WeekFields.ISO.weekBasedYear()) == false
                     && accessor.isSupported(WeekFields.of(locale).weekBasedYear()) == false
                     && accessor.isSupported(ChronoField.INSTANT_SECONDS) == false) {
                     int year = LocalDate.now(ZoneOffset.UTC).getYear();

server/src/main/java/org/elasticsearch/common/time/DateFormatters.java

@@ -82,7 +82,7 @@ private static DateFormatter newDateFormatter(String format, DateTimeFormatter p
         );
     }
 
-    public static final WeekFields WEEK_FIELDS_ROOT = WeekFields.of(Locale.ROOT);
+    public static final WeekFields WEEK_FIELDS_ROOT = WeekFields.ISO;
 
     private static final DateTimeFormatter TIME_ZONE_FORMATTER_NO_COLON = new DateTimeFormatterBuilder().appendOffset("+HHmm", "Z")
         .toFormatter(Locale.ROOT)
@@ -2389,6 +2389,7 @@ public static ZonedDateTime from(TemporalAccessor accessor, Locale locale, ZoneI
         boolean isLocalTimeSet = localTime != null;
 
         // the first two cases are the most common, so this allows us to exit early when parsing dates
+        WeekFields localeWeekFields;
         if (isLocalDateSet && isLocalTimeSet) {
             return of(localDate, localTime, zoneId);
         } else if (accessor.isSupported(ChronoField.INSTANT_SECONDS) && accessor.isSupported(NANO_OF_SECOND)) {
@@ -2407,17 +2408,18 @@ public static ZonedDateTime from(TemporalAccessor accessor, Locale locale, ZoneI
         } else if (accessor.isSupported(MONTH_OF_YEAR)) {
             // missing year, falling back to the epoch and then filling
             return getLocalDate(accessor, locale).atStartOfDay(zoneId);
-        } else if (accessor.isSupported(WeekFields.of(locale).weekBasedYear())) {
-            return localDateFromWeekBasedDate(accessor, locale).atStartOfDay(zoneId);
+        } else if (accessor.isSupported(WeekFields.ISO.weekBasedYear())) {
+            return localDateFromWeekBasedDate(accessor, locale, WeekFields.ISO).atStartOfDay(zoneId);
+        } else if (accessor.isSupported((localeWeekFields = WeekFields.of(locale)).weekBasedYear())) {
+            return localDateFromWeekBasedDate(accessor, locale, localeWeekFields).atStartOfDay(zoneId);
         }
 
         // we should not reach this piece of code, everything being parsed we should be able to
         // convert to a zoned date time! If not, we have to extend the above methods
         throw new IllegalArgumentException("temporal accessor [" + accessor + "] cannot be converted to zoned date time");
     }
 
-    private static LocalDate localDateFromWeekBasedDate(TemporalAccessor accessor, Locale locale) {
-        WeekFields weekFields = WeekFields.of(locale);
+    private static LocalDate localDateFromWeekBasedDate(TemporalAccessor accessor, Locale locale, WeekFields weekFields) {
         if (accessor.isSupported(weekFields.weekOfWeekBasedYear())) {
             return LocalDate.ofEpochDay(0)
                 .with(weekFields.weekBasedYear(), accessor.get(weekFields.weekBasedYear()))
@@ -2459,8 +2461,11 @@ public String toString() {
     };
 
     private static LocalDate getLocalDate(TemporalAccessor accessor, Locale locale) {
-        if (accessor.isSupported(WeekFields.of(locale).weekBasedYear())) {
-            return localDateFromWeekBasedDate(accessor, locale);
+        WeekFields localeWeekFields;
+        if (accessor.isSupported(WeekFields.ISO.weekBasedYear())) {
+            return localDateFromWeekBasedDate(accessor, locale, WeekFields.ISO);
+        } else if (accessor.isSupported((localeWeekFields = WeekFields.of(locale)).weekBasedYear())) {
+            return localDateFromWeekBasedDate(accessor, locale, localeWeekFields);
         } else if (accessor.isSupported(MONTH_OF_YEAR)) {
             int year = getYear(accessor);
             if (accessor.isSupported(DAY_OF_MONTH)) {

server/src/test/java/org/elasticsearch/common/time/DateFormattersTests.java

@@ -112,6 +112,24 @@ public void testWeekBasedDates() {
 
         assertThat(DateFormatters.from(dateFormatter.parse("2016")), equalTo(ZonedDateTime.of(2015, 12, 27, 0, 0, 0, 0, ZoneOffset.UTC)));
         assertThat(DateFormatters.from(dateFormatter.parse("2015")), equalTo(ZonedDateTime.of(2014, 12, 28, 0, 0, 0, 0, ZoneOffset.UTC)));
+
+        // the built-in formats use different week definitions (ISO instead of locale)
+        dateFormatter = DateFormatters.forPattern("weekyear_week");
+
+        assertThat(
+            DateFormatters.from(dateFormatter.parse("2016-W01")),
+            equalTo(ZonedDateTime.of(2016, 01, 04, 0, 0, 0, 0, ZoneOffset.UTC))
+        );
+
+        assertThat(
+            DateFormatters.from(dateFormatter.parse("2015-W01")),
+            equalTo(ZonedDateTime.of(2014, 12, 29, 0, 0, 0, 0, ZoneOffset.UTC))
+        );
+
+        dateFormatter = DateFormatters.forPattern("weekyear");
+
+        assertThat(DateFormatters.from(dateFormatter.parse("2016")), equalTo(ZonedDateTime.of(2016, 01, 04, 0, 0, 0, 0, ZoneOffset.UTC)));
+        assertThat(DateFormatters.from(dateFormatter.parse("2015")), equalTo(ZonedDateTime.of(2014, 12, 29, 0, 0, 0, 0, ZoneOffset.UTC)));
     }
 
     public void testEpochMillisParser() {