Introduce Date-to-Instant and Instant-to-Date converters

In order to avoid unnecessary use of reflection and to simplify native
image deployments, this commit introduces explicit support for
automatic conversions from java.util.Date to java.time.Instant and vice
versa.

To achieve that, this commit introduces an InstantToDateConverter and a
DateToInstantConverter and registers them automatically in
DefaultConversionService.

See gh-35156
Closes gh-35175

spring-core/src/test/java/org/springframework/core/convert/converter/DefaultConversionServiceTests.java

@@ -22,12 +22,16 @@ import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
+import java.sql.Time;
+import java.sql.Timestamp;
+import java.time.Instant;
 import java.time.ZoneId;
 import java.util.AbstractList;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Currency;
+import java.util.Date;
 import java.util.EnumSet;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
@@ -973,6 +977,88 @@ class DefaultConversionServiceTests {
 		assertThat((Object) conversionService.convert(Optional.empty(), Optional.class)).isSameAs(Optional.empty());
 	}
 
+	@Test  // gh-35175
+	void convertDateToInstant() {
+		TypeDescriptor dateDescriptor = TypeDescriptor.valueOf(Date.class);
+		TypeDescriptor instantDescriptor = TypeDescriptor.valueOf(Instant.class);
+		Date date = new Date();
+
+		// Conversion performed by DateToInstantConverter.
+		assertThat(conversionService.convert(date, dateDescriptor, instantDescriptor))
+				.isEqualTo(date.toInstant());
+	}
+
+	@Test  // gh-35175
+	void convertSqlDateToInstant() {
+		TypeDescriptor sqlDateDescriptor = TypeDescriptor.valueOf(java.sql.Date.class);
+		TypeDescriptor instantDescriptor = TypeDescriptor.valueOf(Instant.class);
+		java.sql.Date sqlDate = new java.sql.Date(System.currentTimeMillis());
+
+		// DateToInstantConverter blindly invokes toInstant() on any java.util.Date
+		// subtype, which results in an UnsupportedOperationException since
+		// java.sql.Date does not have a time component. However, even if
+		// DateToInstantConverter were not registered, ObjectToObjectConverter
+		// would still attempt to invoke toInstant() on a java.sql.Date by convention,
+		// which results in the same UnsupportedOperationException.
+		assertThatExceptionOfType(ConversionFailedException.class)
+				.isThrownBy(() -> conversionService.convert(sqlDate, sqlDateDescriptor, instantDescriptor))
+				.withCauseExactlyInstanceOf(UnsupportedOperationException.class);
+	}
+
+	@Test  // gh-35175
+	void convertSqlTimeToInstant() {
+		TypeDescriptor timeDescriptor = TypeDescriptor.valueOf(Time.class);
+		TypeDescriptor instantDescriptor = TypeDescriptor.valueOf(Instant.class);
+		Time time = new Time(System.currentTimeMillis());
+
+		// DateToInstantConverter blindly invokes toInstant() on any java.util.Date
+		// subtype, which results in an UnsupportedOperationException since
+		// java.sql.Date does not have a time component. However, even if
+		// DateToInstantConverter were not registered, ObjectToObjectConverter
+		// would still attempt to invoke toInstant() on a java.sql.Date by convention,
+		// which results in the same UnsupportedOperationException.
+		assertThatExceptionOfType(ConversionFailedException.class)
+				.isThrownBy(() -> conversionService.convert(time, timeDescriptor, instantDescriptor))
+				.withCauseExactlyInstanceOf(UnsupportedOperationException.class);
+	}
+
+	@Test  // gh-35175
+	void convertSqlTimestampToInstant() {
+		TypeDescriptor timestampDescriptor = TypeDescriptor.valueOf(Timestamp.class);
+		TypeDescriptor instantDescriptor = TypeDescriptor.valueOf(Instant.class);
+		Timestamp timestamp = new Timestamp(System.currentTimeMillis());
+
+		// Conversion performed by DateToInstantConverter.
+		assertThat(conversionService.convert(timestamp, timestampDescriptor, instantDescriptor))
+				.isEqualTo(timestamp.toInstant());
+	}
+
+	@Test  // gh-35175
+	void convertInstantToDate() {
+		TypeDescriptor instantDescriptor = TypeDescriptor.valueOf(Instant.class);
+		TypeDescriptor dateDescriptor = TypeDescriptor.valueOf(Date.class);
+		Date date = new Date();
+		Instant instant = date.toInstant();
+
+		// Conversion performed by InstantToDateConverter.
+		assertThat(conversionService.convert(instant, instantDescriptor, dateDescriptor))
+				.isExactlyInstanceOf(Date.class)
+				.isEqualTo(date);
+	}
+
+	@Test
+	void convertInstantToSqlTimestamp() {
+		TypeDescriptor instantDescriptor = TypeDescriptor.valueOf(Instant.class);
+		TypeDescriptor timestampDescriptor = TypeDescriptor.valueOf(Timestamp.class);
+		Timestamp timestamp = new Timestamp(System.currentTimeMillis());
+		Instant instant = timestamp.toInstant();
+
+		// Conversion performed by ObjectToObjectConverter.
+		assertThat(conversionService.convert(instant, instantDescriptor, timestampDescriptor))
+				.isExactlyInstanceOf(Timestamp.class)
+				.isEqualTo(timestamp);
+	}
+
 
 	// test fields and helpers