HHH-9414: Convert MarkerObject to enum for serialization safety

hibernate-core/src/main/java/org/hibernate/collection/spi/AbstractPersistentCollection.java

@@ -383,7 +383,7 @@ public boolean elementExists(Object element) {
 		}
 	}
 
-	protected static final Object UNKNOWN = new MarkerObject( "UNKNOWN" );
+	protected static final MarkerObject UNKNOWN = MarkerObject.UNKNOWN;
 
 	protected Object readElementByIndex(final Object index) {
 		if ( !initialized ) {

hibernate-core/src/main/java/org/hibernate/engine/spi/PersistenceContext.java

@@ -45,7 +45,7 @@ public interface PersistenceContext {
 	/**
 	 * Marker object used to indicate (via reference checking) that no row was returned.
 	 */
-	Object NO_ROW = new MarkerObject( "NO_ROW" );
+	MarkerObject NO_ROW = MarkerObject.NO_ROW;
 
 	boolean isStateless();
 

hibernate-core/src/main/java/org/hibernate/internal/util/MarkerObject.java

@@ -4,18 +4,52 @@
  */
 package org.hibernate.internal.util;
 
-import java.io.Serializable;
-
 /**
- * @deprecated This is a legacy of very ancient versions of Hibernate.
+ * Marker objects used throughout Hibernate to represent special sentinel values.
+ * These are used for reference-equality checks (==) to indicate special states
+ * like "no row found", "unfetched collection", etc.
+ *
+ * <p>Implemented as an enum to ensure proper singleton behavior across serialization
+ * and deserialization, fixing issues where marker object identity was lost after
+ * deserialization (e.g., in clustered environments or session serialization scenarios).
  *
- * @author Gavin King
+ * @see <a href="https://hibernate.atlassian.net/browse/HHH-9414">HHH-9414</a>
  */
-@Deprecated
-public class MarkerObject implements Serializable {
+public enum MarkerObject {
+	/**
+	 * Marker indicating that no row was returned from the database.
+	 */
+	NO_ROW("NO_ROW"),
+
+	/**
+	 * Marker used in proxy implementations to signal that the actual
+	 * implementation method should be invoked.
+	 */
+	INVOKE_IMPLEMENTATION("INVOKE_IMPLEMENTATION"),
+
+	/**
+	 * Marker indicating an unknown state in persistent collections.
+	 */
+	UNKNOWN("UNKNOWN"),
+
+	/**
+	 * Marker indicating a collection that has not yet been fetched from the database.
+	 */
+	UNFETCHED_COLLECTION("UNFETCHED COLLECTION"),
+
+	/**
+	 * Marker representing a null discriminator value.
+	 */
+	NULL_DISCRIMINATOR("<null discriminator>"),
+
+	/**
+	 * Marker representing a not-null discriminator value.
+	 */
+	NOT_NULL_DISCRIMINATOR("<not null discriminator>");
+
 	private final String name;
 
-	public MarkerObject(String name) {
+	MarkerObject(String name) {
 		this.name = name;
 	}
 

hibernate-core/src/main/java/org/hibernate/persister/entity/DiscriminatorHelper.java

@@ -29,8 +29,8 @@
 @Internal
 public class DiscriminatorHelper {
 
-	public static final Object NULL_DISCRIMINATOR = new MarkerObject( "<null discriminator>" );
-	public static final Object NOT_NULL_DISCRIMINATOR = new MarkerObject( "<not null discriminator>" );
+	public static final MarkerObject NULL_DISCRIMINATOR = MarkerObject.NULL_DISCRIMINATOR;
+	public static final MarkerObject NOT_NULL_DISCRIMINATOR = MarkerObject.NOT_NULL_DISCRIMINATOR;
 
 	/**
 	 * The underlying BasicType as the "JDBC mapping" between the relational {@link org.hibernate.type.descriptor.java.JavaType}

hibernate-core/src/main/java/org/hibernate/proxy/pojo/BasicLazyInitializer.java

@@ -21,7 +21,7 @@
  */
 public abstract class BasicLazyInitializer extends AbstractLazyInitializer {
 
-	protected static final Object INVOKE_IMPLEMENTATION = new MarkerObject( "INVOKE_IMPLEMENTATION" );
+	protected static final MarkerObject INVOKE_IMPLEMENTATION = MarkerObject.INVOKE_IMPLEMENTATION;
 
 	protected final Class<?> persistentClass;
 	protected final Method getIdentifierMethod;

hibernate-core/src/main/java/org/hibernate/type/CollectionType.java

@@ -51,7 +51,7 @@
 public abstract class CollectionType extends AbstractType implements AssociationType {
 
 	@Internal
-	public static final Object UNFETCHED_COLLECTION = new MarkerObject( "UNFETCHED COLLECTION" );
+	public static final MarkerObject UNFETCHED_COLLECTION = MarkerObject.UNFETCHED_COLLECTION;
 
 	private final String role;
 	private final String foreignKeyPropertyName;