backport 4c695fa8a459adcdb8cdfe9e90783007c65fb90e

Author: GoeLin

src/java.base/share/classes/java/lang/AbstractStringBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2003, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -28,7 +28,6 @@
 import jdk.internal.math.DoubleToDecimal;
 import jdk.internal.math.FloatToDecimal;
 
-import java.io.IOException;
 import java.nio.CharBuffer;
 import java.util.Arrays;
 import java.util.Spliterator;
@@ -356,8 +355,12 @@ public void setLength(int newLength) {
      */
     @Override
     public char charAt(int index) {
+        byte coder = this.coder;
+        byte[] value = this.value;
+        // Ensure count is less than or equal to capacity (racy reads and writes can produce inconsistent values)
+        int count = Math.min(this.count, value.length >> coder);
         checkIndex(index, count);
-        if (isLatin1()) {
+        if (coder == LATIN1) {
             return (char)(value[index] & 0xff);
         }
         return StringUTF16.getChar(value, index);
@@ -416,6 +419,7 @@ public int codePointAt(int index) {
      *            of this sequence.
      */
     public int codePointBefore(int index) {
+        byte[] value = this.value;
         int i = index - 1;
         checkIndex(i, count);
         if (isLatin1()) {
@@ -1720,7 +1724,7 @@ private final void putCharsAt(int index, CharSequence s, int off, int end) {
                 } else {
                     inflate();
                     // store c to make sure it has a UTF16 char
-                    StringUTF16.putChar(this.value, j++, c);
+                    StringUTF16.putCharSB(this.value, j++, c);
                     i++;
                     StringUTF16.putCharsSB(this.value, j, s, i, end);
                     return;
@@ -1810,7 +1814,7 @@ private final void appendChars(CharSequence s, int off, int end) {
                     count = j;
                     inflate();
                     // Store c to make sure sb has a UTF16 char
-                    StringUTF16.putChar(this.value, j++, c);
+                    StringUTF16.putCharSB(this.value, j++, c);
                     count = j;
                     i++;
                     StringUTF16.putCharsSB(this.value, j, s, i, end);