Support parsing long millisecond timestamps in InstantFormatter

This commit adds support of parsing a simple long from a String and
turning it to an `Instant` by considering it represents a timestamp in
milliseconds (see `Instant.ofEpochMilli`). Failing to parse a long from
the String, the previous algorithm is used: first check for an RFC-1123
representation then an ISO_INSTANT representation.

See gh-30312
Closes gh-30546

spring-context/src/test/java/org/springframework/format/datetime/standard/DateTimeFormattingTests.java

@@ -621,6 +621,19 @@ class DateTimeFormattingTests {
 								.hasMessageStartingWith("Text '210302'")
 								.hasNoCause();
 		}
+
+		@Test
+		void testBindInstantAsLongEpochMillis() {
+			MutablePropertyValues propertyValues = new MutablePropertyValues();
+			propertyValues.add("instant", 1234L);
+			binder.bind(propertyValues);
+			assertThat(binder.getBindingResult().getErrorCount()).isZero();
+			assertThat(binder.getBindingResult().getRawFieldValue("instant"))
+					.isInstanceOf(Instant.class)
+					.isEqualTo(Instant.ofEpochMilli(1234L));
+			assertThat(binder.getBindingResult().getFieldValue("instant"))
+					.hasToString("1970-01-01T00:00:01.234Z");
+		}
 	}
 
 

spring-context/src/test/java/org/springframework/format/datetime/standard/InstantFormatterTests.java

@@ -19,6 +19,7 @@ package org.springframework.format.datetime.standard;
 import java.text.ParseException;
 import java.time.Instant;
 import java.time.format.DateTimeFormatter;
+import java.time.temporal.ChronoUnit;
 import java.util.Random;
 import java.util.stream.Stream;
 
@@ -79,6 +80,16 @@ class InstantFormatterTests {
 		assertThat(actual).isEqualTo(expected);
 	}
 
+	@ParameterizedTest
+	@ArgumentsSource(RandomEpochMillisProvider.class)
+	void should_parse_into_an_Instant_from_epoch_mili(Instant input) throws ParseException {
+		Instant expected = input;
+
+		Instant actual = instantFormatter.parse(Long.toString(input.toEpochMilli()), null);
+
+		assertThat(actual).isEqualTo(expected);
+	}
+
 	private static class RandomInstantProvider implements ArgumentsProvider {
 
 		private static final long DATA_SET_SIZE = 10;
@@ -121,5 +132,19 @@ class InstantFormatterTests {
 					.map(DateTimeFormatter.RFC_1123_DATE_TIME.withZone(systemDefault())::format);
 		}
 	}
+	private static final class RandomEpochMillisProvider implements ArgumentsProvider {
+
+		private static final long DATA_SET_SIZE = 10;
+
+		private static final Random random = new Random();
+
+		@Override
+		public Stream<Arguments> provideArguments(ExtensionContext context) {
+			return random.longs(DATA_SET_SIZE, Long.MIN_VALUE, Long.MAX_VALUE)
+					.mapToObj(Instant::ofEpochMilli)
+					.map(instant -> instant.truncatedTo(ChronoUnit.MILLIS))
+					.map(Arguments::of);
+		}
+	}
 
 }