Switch back to unicode for the DOT substitute character.

MongoDB doesn't support "." in field names, so a Private Use Area character was used. This was originally stored in unicode format, but delomboking the code caused it to get transformed into another encoding. This causes issues on certain systems when building the software, so we are converting it back to its unicode representation. The character has been the same throughout, ensuring binary compatilibity.

See: https://www.compart.com/en/unicode/U+F607

Related: spring-projects/spring-session-data-mongodb@d601e27#diff-57190a47726099e31fdf86b12b80206e2ae24feb28aacaf494b99557583df150L47
Closes #2053.

Author: gregturn

spring-session-data-mongodb/src/main/java/org/springframework/session/data/mongo/MongoSession.java

@@ -16,19 +16,14 @@
 
 package org.springframework.session.data.mongo;
 
+import org.springframework.lang.Nullable;
+import org.springframework.session.Session;
+
 import java.time.Duration;
 import java.time.Instant;
-import java.util.Date;
-import java.util.HashMap;
-import java.util.Map;
-import java.util.Objects;
-import java.util.Set;
-import java.util.UUID;
+import java.util.*;
 import java.util.stream.Collectors;
 
-import org.springframework.lang.Nullable;
-import org.springframework.session.Session;
-
 /**
  * Session object providing additional information about the datetime of expiration.
  *
@@ -39,10 +34,15 @@
 public class MongoSession implements Session {
 
 	/**
-	 * Mongo doesn't support {@literal dot} in field names. We replace it with a very
-	 * rarely used character
+	 * Mongo doesn't support {@literal dot} in field names. We replace it with a unicode character from the Private Use Area.
+	 * <p>
+	 * NOTE: This was originally stored in unicode format. Delomboking the code caused it to get converted to another encoding,
+	 * which isn't supported on all systems, so we migrated back to unicode. The same character is being represented ensuring
+	 * binary compatibility.
+	 *
+	 * @see https://www.compart.com/en/unicode/U+F607
 	 */
-	private static final char DOT_COVER_CHAR = '';
+	private static final char DOT_COVER_CHAR = '\uF607';
 
 	private String id;
 
@@ -106,8 +106,7 @@ public void setAttribute(String attributeName, Object attributeValue) {
 
 		if (attributeValue == null) {
 			removeAttribute(coverDot(attributeName));
-		}
-		else {
+		} else {
 			this.attrs.put(coverDot(attributeName), attributeValue);
 		}
 	}