From ef3fe11ae673558e278d8c30902253365950da78 Mon Sep 17 00:00:00 2001
From: Sanne Grinovero <sanne@hibernate.org>
Date: Thu, 14 Oct 2021 12:56:27 +0100
Subject: [PATCH] HHH-14882 Minor implementation optimisations in internal
 StandardStack

The current StandardStack implementation is based on a LinkedList, which is well known to be a suboptimal choice; this should be switched to a lazily allocated ArrayDeque.
---
 .../util/collections/StandardStack.java       | 32 +++++++++++++++----
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/hibernate-core/src/main/java/org/hibernate/internal/util/collections/StandardStack.java b/hibernate-core/src/main/java/org/hibernate/internal/util/collections/StandardStack.java
index db11ba4e5e..91cf3f0257 100644
--- a/hibernate-core/src/main/java/org/hibernate/internal/util/collections/StandardStack.java
+++ b/hibernate-core/src/main/java/org/hibernate/internal/util/collections/StandardStack.java
@@ -6,9 +6,8 @@
  */
 package org.hibernate.internal.util.collections;
 
-import java.util.LinkedList;
-import java.util.function.Consumer;
-import java.util.function.Function;
+import java.util.ArrayDeque;
+import java.util.Deque;
 
 /**
  * A general-purpose stack impl.
@@ -19,39 +18,58 @@ import java.util.function.Function;
  */
 public final class StandardStack<T> implements Stack<T> {
 
-	private final LinkedList<T> internalStack = new LinkedList<>();
+	private ArrayDeque<T> internalStack;
 
 	public StandardStack() {
 	}
 
 	@Override
 	public void push(T newCurrent) {
-		internalStack.addFirst( newCurrent );
+		stackInstanceExpected().addFirst( newCurrent );
+	}
+
+	private Deque<T> stackInstanceExpected() {
+		if ( internalStack == null ) {
+			//"7" picked to use 8, but skipping the odd initialCapacity method
+			internalStack = new ArrayDeque<>(7);
+		}
+		return internalStack;
 	}
 
 	@Override
 	public T pop() {
-		return internalStack.removeFirst();
+		return stackInstanceExpected().removeFirst();
 	}
 
 	@Override
 	public T getCurrent() {
+		if ( internalStack == null ) {
+			return null;
+		}
 		return internalStack.peek();
 	}
 
 	@Override
 	public int depth() {
+		if ( internalStack == null ) {
+			return 0;
+		}
 		return internalStack.size();
 	}
 
 	@Override
 	public boolean isEmpty() {
+		if ( internalStack == null ) {
+			return true;
+		}
 		return internalStack.isEmpty();
 	}
 
 	@Override
 	public void clear() {
-		internalStack.clear();
+		if ( internalStack != null ) {
+			internalStack.clear();
+		}
 	}
 
 }