HHH-16047 Allow reusing of FlushEntityEvent instances

hibernate-core/src/main/java/org/hibernate/event/internal/AbstractFlushingEventListener.java

@@ -203,15 +203,24 @@ public abstract class AbstractFlushingEventListener implements JpaBootstrapSensi
 		final Map.Entry<Object,EntityEntry>[] entityEntries = persistenceContext.reentrantSafeEntityEntries();
 		final int count = entityEntries.length;
 
+		FlushEntityEvent entityEvent = null; //allow reuse of the event as it's heavily allocated in certain use cases
+		int eventGenerationId = 0; //Used to double-check the instance reuse won't cause problems
+
 		for ( Map.Entry<Object,EntityEntry> me : entityEntries ) {
 			// Update the status of the object and if necessary, schedule an update
 
 			EntityEntry entry = me.getValue();
 			Status status = entry.getStatus();
 
+
 			if ( status != Status.LOADING && status != Status.GONE ) {
-				final FlushEntityEvent entityEvent = new FlushEntityEvent( source, me.getKey(), entry );
+				entityEvent = createOrReuseEventInstance( entityEvent, source, me.getKey(), entry );
+
+				entityEvent.setInstanceGenerationId( ++eventGenerationId );
+
 				flushListeners.fireEventOnEachListener( entityEvent, FlushEntityEventListener::onFlushEntity );
+				entityEvent.setAllowedToReuse( true );
+				assert entityEvent.getInstanceGenerationId() == eventGenerationId;
 			}
 		}
 
@@ -220,6 +229,26 @@ public abstract class AbstractFlushingEventListener implements JpaBootstrapSensi
 		return count;
 	}
 
+	/**
+	 * Reuses a FlushEntityEvent for a new purpose, if possible;
+	 * if not possible a new actual instance is returned.
+	 */
+	private FlushEntityEvent createOrReuseEventInstance(
+			FlushEntityEvent possiblyValidExistingInstance,
+			EventSource source,
+			Object key,
+			EntityEntry entry) {
+		FlushEntityEvent entityEvent = possiblyValidExistingInstance;
+		if ( entityEvent == null || (! entityEvent.isAllowedToReuse() ) ) {
+			//need to create a new instance
+			entityEvent = new FlushEntityEvent( source, key, entry );
+		}
+		else {
+			entityEvent.resetAndReuseEventInstance( key, entry );
+		}
+		return entityEvent;
+	}
+
 	/**
 	 * process any unreferenced collections and then inspect all known collections,
 	 * scheduling creates/removes/updates

hibernate-core/src/main/java/org/hibernate/event/spi/FlushEntityEvent.java

@@ -12,14 +12,17 @@ import org.hibernate.engine.spi.EntityEntry;
  * @author Gavin King
  */
 public class FlushEntityEvent extends AbstractEvent {
-	private final Object entity;
+
+	private Object entity;
 	private Object[] propertyValues;
 	private Object[] databaseSnapshot;
 	private int[] dirtyProperties;
 	private boolean hasDirtyCollection;
 	private boolean dirtyCheckPossible;
 	private boolean dirtyCheckHandledByInterceptor;
-	private final EntityEntry entityEntry;
+	private EntityEntry entityEntry;
+	private boolean allowedToReuse;//allows this event instance to be reused for multiple events: special case to GC
+	private int instanceGenerationId;//in support of event instance reuse: to double check no recursive/nested use is happening
 
 	public FlushEntityEvent(EventSource source, Object entity, EntityEntry entry) {
 		super(source);
@@ -75,4 +78,45 @@ public class FlushEntityEvent extends AbstractEvent {
 	public Object getEntity() {
 		return entity;
 	}
+
+	/**
+	 * This is a terrible anti-pattern, but particular circumstances call for being
+	 * able to reuse the same event instance: this is otherwise allocated in hot loops
+	 * and since each event is escaping the scope it's actually causing allocation issues.
+	 * The flush event does not appear to be used recursively so this is currently safe to
+	 * do, nevertheless we add an allowedToReuse flag to ensure only instances whose
+	 * purpose has completed are being reused.
+	 * N.B. two out of three parameters from the constructor are reset: the same EventSource is implied
+	 * on reuse.
+	 * @param entity same as constructor parameter
+	 * @param entry same as constructor parameter
+	 */
+	public void resetAndReuseEventInstance(Object entity, EntityEntry entry) {
+		this.entity = entity;
+		this.entityEntry = entry;
+		this.allowedToReuse = false;
+		//and reset other fields to the default:
+		this.propertyValues = null;
+		this.databaseSnapshot = null;
+		this.dirtyProperties = null;
+		this.hasDirtyCollection = false;
+		this.dirtyCheckPossible = false;
+		this.dirtyCheckHandledByInterceptor = false;
+	}
+
+	public boolean isAllowedToReuse() {
+		return this.allowedToReuse;
+	}
+
+	public void setAllowedToReuse(final boolean allowedToReuse) {
+		this.allowedToReuse = allowedToReuse;
+	}
+
+	public int getInstanceGenerationId() {
+		return this.instanceGenerationId;
+	}
+
+	public void setInstanceGenerationId(final int instanceGenerationId) {
+		this.instanceGenerationId = instanceGenerationId;
+	}
 }