Avoid LinkedList performance issues through use of ArrayDeque

Closes gh-25652

Author: jhoeller

spring-beans/src/main/java/org/springframework/beans/factory/parsing/ParseState.java

@@ -16,12 +16,12 @@
 
 package org.springframework.beans.factory.parsing;
 
-import java.util.LinkedList;
+import java.util.ArrayDeque;
 
 import org.springframework.lang.Nullable;
 
 /**
- * Simple {@link LinkedList}-based structure for tracking the logical position during
+ * Simple {@link ArrayDeque}-based structure for tracking the logical position during
  * a parsing process. {@link Entry entries} are added to the LinkedList at
  * each point during the parse phase in a reader-specific manner.
  *
@@ -30,6 +30,7 @@
  * error messages.
  *
  * @author Rob Harrop
+ * @author Juergen Hoeller
  * @since 2.0
  */
 public final class ParseState {
@@ -40,45 +41,44 @@ public final class ParseState {
 	private static final char TAB = '\t';
 
 	/**
-	 * Internal {@link LinkedList} storage.
+	 * Internal {@link ArrayDeque} storage.
 	 */
-	private final LinkedList<Entry> state;
+	private final ArrayDeque<Entry> state;
 
 
 	/**
-	 * Create a new {@code ParseState} with an empty {@link LinkedList}.
+	 * Create a new {@code ParseState} with an empty {@link ArrayDeque}.
 	 */
 	public ParseState() {
-		this.state = new LinkedList<>();
+		this.state = new ArrayDeque<>();
 	}
 
 	/**
-	 * Create a new {@code ParseState} whose {@link LinkedList} is a {@link Object#clone clone}
+	 * Create a new {@code ParseState} whose {@link ArrayDeque} is a {@link Object#clone clone}
 	 * of that of the passed in {@code ParseState}.
 	 */
-	@SuppressWarnings("unchecked")
 	private ParseState(ParseState other) {
-		this.state = (LinkedList<Entry>) other.state.clone();
+		this.state = other.state.clone();
 	}
 
 
 	/**
-	 * Add a new {@link Entry} to the {@link LinkedList}.
+	 * Add a new {@link Entry} to the {@link ArrayDeque}.
 	 */
 	public void push(Entry entry) {
 		this.state.push(entry);
 	}
 
 	/**
-	 * Remove an {@link Entry} from the {@link LinkedList}.
+	 * Remove an {@link Entry} from the {@link ArrayDeque}.
 	 */
 	public void pop() {
 		this.state.pop();
 	}
 
 	/**
-	 * Return the {@link Entry} currently at the top of the {@link LinkedList} or
-	 * {@code null} if the {@link LinkedList} is empty.
+	 * Return the {@link Entry} currently at the top of the {@link ArrayDeque} or
+	 * {@code null} if the {@link ArrayDeque} is empty.
 	 */
 	@Nullable
 	public Entry peek() {
@@ -100,15 +100,17 @@ public ParseState snapshot() {
 	@Override
 	public String toString() {
 		StringBuilder sb = new StringBuilder();
-		for (int x = 0; x < this.state.size(); x++) {
-			if (x > 0) {
+		int i = 0;
+		for (ParseState.Entry entry : this.state) {
+			if (i > 0) {
 				sb.append('\n');
-				for (int y = 0; y < x; y++) {
+				for (int j = 0; j < i; j++) {
 					sb.append(TAB);
 				}
 				sb.append("-> ");
 			}
-			sb.append(this.state.get(x));
+			sb.append(entry);
+			i++;
 		}
 		return sb.toString();
 	}
@@ -118,7 +120,6 @@ public String toString() {
 	 * Marker interface for entries into the {@link ParseState}.
 	 */
 	public interface Entry {
-
 	}
 
 }

spring-core/src/main/java/org/springframework/util/FastByteArrayOutputStream.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,8 +20,9 @@
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.security.MessageDigest;
+import java.util.ArrayDeque;
+import java.util.Deque;
 import java.util.Iterator;
-import java.util.LinkedList;
 
 import org.springframework.lang.Nullable;
 
@@ -31,7 +32,7 @@
  * its sibling {@link ResizableByteArrayOutputStream}.
  *
  * <p>Unlike {@link java.io.ByteArrayOutputStream}, this implementation is backed
- * by a {@link java.util.LinkedList} of {@code byte[]} instead of 1 constantly
+ * by an {@link java.util.ArrayDeque} of {@code byte[]} instead of 1 constantly
  * resizing {@code byte[]}. It does not copy buffers when it gets expanded.
  *
  * <p>The initial buffer is only created when the stream is first written.
@@ -50,7 +51,7 @@ public class FastByteArrayOutputStream extends OutputStream {
 
 
 	// The buffers used to store the content bytes
-	private final LinkedList<byte[]> buffers = new LinkedList<>();
+	private final Deque<byte[]> buffers = new ArrayDeque<>();
 
 	// The size, in bytes, to use when allocating the first byte[]
 	private final int initialBlockSize;
@@ -289,7 +290,7 @@ else if (size() == targetCapacity && this.buffers.getFirst().length == targetCap
 	}
 
 	/**
-	 * Create a new buffer and store it in the LinkedList
+	 * Create a new buffer and store it in the ArrayDeque.
 	 * <p>Adds a new buffer that can store at least {@code minCapacity} bytes.
 	 */
 	private void addBuffer(int minCapacity) {

spring-core/src/main/java/org/springframework/util/StringUtils.java

@@ -18,14 +18,15 @@
 
 import java.io.ByteArrayOutputStream;
 import java.nio.charset.Charset;
+import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Deque;
 import java.util.Enumeration;
 import java.util.Iterator;
 import java.util.LinkedHashSet;
-import java.util.LinkedList;
 import java.util.List;
 import java.util.Locale;
 import java.util.Properties;
@@ -655,6 +656,9 @@ public static String applyRelativePath(String path, String relativePath) {
 	 * inner simple dots.
 	 * <p>The result is convenient for path comparison. For other uses,
 	 * notice that Windows separators ("\") are replaced by simple slashes.
+	 * <p><strong>NOTE</strong> that {@code cleanPath} should not be depended
+	 * upon in a security context. Other mechanisms should be used to prevent
+	 * path-traversal issues.
 	 * @param path the original path
 	 * @return the normalized path
 	 */
@@ -690,7 +694,7 @@ public static String cleanPath(String path) {
 		}
 
 		String[] pathArray = delimitedListToStringArray(pathToUse, FOLDER_SEPARATOR);
-		LinkedList<String> pathElements = new LinkedList<>();
+		Deque<String> pathElements = new ArrayDeque<>();
 		int tops = 0;
 
 		for (int i = pathArray.length - 1; i >= 0; i--) {
@@ -709,7 +713,7 @@ else if (TOP_PATH.equals(element)) {
 				}
 				else {
 					// Normal path element found.
-					pathElements.add(0, element);
+					pathElements.addFirst(element);
 				}
 			}
 		}
@@ -720,11 +724,11 @@ else if (TOP_PATH.equals(element)) {
 		}
 		// Remaining top paths need to be retained.
 		for (int i = 0; i < tops; i++) {
-			pathElements.add(0, TOP_PATH);
+			pathElements.addFirst(TOP_PATH);
 		}
 		// If nothing else left, at least explicitly point to current path.
 		if (pathElements.size() == 1 && pathElements.getLast().isEmpty() && !prefix.endsWith(FOLDER_SEPARATOR)) {
-			pathElements.add(0, CURRENT_PATH);
+			pathElements.addFirst(CURRENT_PATH);
 		}
 
 		return prefix + collectionToDelimitedString(pathElements, FOLDER_SEPARATOR);