Support for Instant type (#21)

Since `Instant` requires a timezone which is not stored/accessible from the database,
all `Instant` objects are instantiated with UTC timezone by default

src/main/java/tech/ailef/dbadmin/external/dbmapping/DbFieldType.java

@@ -25,9 +25,11 @@ import java.math.BigInteger;
 import java.sql.Date;
 import java.text.ParseException;
 import java.text.SimpleDateFormat;
+import java.time.Instant;
 import java.time.LocalDate;
 import java.time.LocalDateTime;
 import java.time.OffsetDateTime;
+import java.time.ZoneOffset;
 import java.util.List;
 import java.util.Locale;
 
@@ -270,6 +272,28 @@ public enum DbFieldType {
 			return List.of(CompareOperator.AFTER, CompareOperator.STRING_EQ, CompareOperator.BEFORE);
 		}
 	},
+	INSTANT {
+		@Override
+		public String getFragmentName() {
+			return "datetime";
+		}
+
+		@Override
+		public Object parseValue(Object value) {
+			if (value == null || value.toString().isBlank()) return null;
+			return LocalDateTime.parse(value.toString()).toInstant(ZoneOffset.UTC);
+		}
+
+		@Override
+		public Class<?> getJavaClass() {
+			return Instant.class;
+		}
+		
+		@Override
+		public List<CompareOperator> getCompareOperators() {
+			return List.of(CompareOperator.AFTER, CompareOperator.STRING_EQ, CompareOperator.BEFORE);
+		}
+	},
 	STRING {
 		@Override
 		public String getFragmentName() {
@@ -626,6 +650,8 @@ public enum DbFieldType {
 			return DATE;
 		} else if (klass == LocalDateTime.class) {
 			return LOCAL_DATE_TIME;
+		} else if (klass == Instant.class) {
+			return INSTANT;
 		} else if (klass == Float.class || klass == float.class) {
 			return FLOAT;
 		} else if (klass == Double.class || klass == double.class) {

src/main/java/tech/ailef/dbadmin/external/dbmapping/DbFieldValue.java

@@ -19,8 +19,10 @@
 
 package tech.ailef.dbadmin.external.dbmapping;
 
+import java.time.Instant;
 import java.time.LocalDateTime;
 import java.time.OffsetDateTime;
+import java.time.ZoneId;
 import java.util.Objects;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
@@ -41,7 +43,7 @@ public class DbFieldValue {
 
 	public Object getValue() {
 		/*
-		 * Special handling of OffsetDateTime to be compatbile
+		 * Special handling of OffsetDateTime and Instant to be compatabile
 		 * with the HTML datetime-local input field: we "cast"
 		 * it to a LocalDateTime so the toString() method will
 		 * not produce the ending "Z" which prevents the datepicker
@@ -51,6 +53,12 @@ public class DbFieldValue {
 			return LocalDateTime.from((OffsetDateTime)value);
 		}
 		
+		if (value instanceof Instant) {
+			Instant i = (Instant)value;
+			LocalDateTime result = LocalDateTime.ofInstant(i, ZoneId.of("UTC"));
+			return result;
+		}
+		
 		return value;
 	}