diff --git a/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFOptimiser.java b/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFOptimiser.java
new file mode 100755
index 0000000000..9d2ad208b6
--- /dev/null
+++ b/poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFOptimiser.java
@@ -0,0 +1,249 @@
+/* ====================================================================
+ Licensed to the Apache Software Foundation (ASF) under one or more
+ contributor license agreements. See the NOTICE file distributed with
+ this work for additional information regarding copyright ownership.
+ The ASF licenses this file to You under the Apache License, Version 2.0
+ (the "License"); you may not use this file except in compliance with
+ the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+==================================================================== */
+
+package org.apache.poi.xssf.usermodel;
+
+import org.apache.poi.ss.usermodel.Cell;
+import org.apache.poi.ss.usermodel.Color;
+import org.apache.poi.ss.usermodel.Row;
+import org.apache.poi.xssf.model.StylesTable;
+import org.openxmlformats.schemas.spreadsheetml.x2006.main.CTCellXfs;
+import org.openxmlformats.schemas.spreadsheetml.x2006.main.CTStylesheet;
+import org.openxmlformats.schemas.spreadsheetml.x2006.main.CTXf;
+
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.Locale;
+import java.util.Map;
+
+/**
+ * XSSF equivalent of HSSFOptimiser.
+ *
+ * Provides methods to optimise cell styles and fonts in XSSF workbooks
+ * by remapping cells that use duplicate styles/fonts to use canonical
+ * (first) occurrences.
+ *
+ * This helps avoid "style explosion" issues where Excel workbooks accumulate
+ * too many style definitions, causing performance problems or file corruption.
+ *
+ * Usage:
+ *
+ * XSSFWorkbook workbook = new XSSFWorkbook(file);
+ * XSSFOptimiser.optimiseFonts(workbook);
+ * XSSFOptimiser.optimiseCellStyles(workbook);
+ * workbook.write(outputStream);
+ *
+ *
+ * Note: Unlike HSSFOptimiser which can remove unused styles from the binary
+ * format, this implementation remaps cells to canonical styles without removing
+ * entries from the XML. This is because XSSF's StylesTable maintains a separate
+ * Java list from the CTStylesheet XML, and removing entries can cause
+ * XmlValueDisconnectedException on save. The remapping approach prevents
+ * further style explosion and is safe.
+ *
+ * @see org.apache.poi.hssf.usermodel.HSSFOptimiser
+ * @since 6.0.0
+ */
+public final class XSSFOptimiser {
+
+ private XSSFOptimiser() {
+ // no instances of this class
+ }
+
+ /**
+ * Goes through the Workbook, optimising the cell styles by remapping
+ * cells that use duplicate styles to use the canonical (first) occurrence.
+ *
+ * Two styles are considered duplicates if they have identical properties:
+ * font properties (not just font index), data format, alignment, borders,
+ * fill colors, etc.
+ *
+ * For best results, call {@link #optimiseFonts(XSSFWorkbook)} first.
+ *
+ * @param workbook The workbook to optimise
+ * @return The number of cells that were remapped to canonical styles
+ */
+ public static int optimiseCellStyles(XSSFWorkbook workbook) {
+ int numStyles = workbook.getNumCellStyles();
+
+ if (numStyles <= 1) {
+ return 0; // Nothing to optimize
+ }
+
+ // Step 1: Build signature map for duplicate detection
+ Map signatureToCanonical = new LinkedHashMap<>();
+ Map oldToCanonical = new HashMap<>();
+
+ for (int i = 0; i < numStyles; i++) {
+ XSSFCellStyle style = workbook.getCellStyleAt(i);
+ String signature = getStyleSignature(style);
+
+ if (signatureToCanonical.containsKey(signature)) {
+ // Duplicate found - map to earlier canonical style
+ oldToCanonical.put(i, signatureToCanonical.get(signature));
+ } else {
+ // First occurrence - this is the canonical style
+ signatureToCanonical.put(signature, i);
+ oldToCanonical.put(i, i);
+ }
+ }
+
+ // Step 2: Remap all cells to use canonical styles
+ int cellsRemapped = 0;
+ for (int sheetIdx = 0; sheetIdx < workbook.getNumberOfSheets(); sheetIdx++) {
+ XSSFSheet sheet = workbook.getSheetAt(sheetIdx);
+ for (Row row : sheet) {
+ if (row == null) continue;
+ for (Cell cell : row) {
+ if (cell == null) continue;
+ int currentIdx = cell.getCellStyle().getIndex();
+ int canonicalIdx = oldToCanonical.getOrDefault(currentIdx, currentIdx);
+
+ if (canonicalIdx != currentIdx) {
+ cell.setCellStyle(workbook.getCellStyleAt(canonicalIdx));
+ cellsRemapped++;
+ }
+ }
+ }
+ }
+
+ return cellsRemapped;
+ }
+
+ /**
+ * Goes through the Workbook, optimising the fonts by updating cell styles
+ * that reference duplicate fonts to use the canonical (first) occurrence.
+ *
+ * Two fonts are considered duplicates if they have identical properties:
+ * name, size, bold, italic, strikeout, underline, color, typeOffset, charset.
+ *
+ * @param workbook The workbook to optimise
+ * @return The number of duplicate fonts found
+ */
+ public static int optimiseFonts(XSSFWorkbook workbook) {
+ int numFonts = workbook.getNumberOfFonts();
+
+ if (numFonts <= 1) {
+ return 0; // Nothing to optimize
+ }
+
+ // Step 1: Build signature map for duplicate detection
+ Map signatureToCanonical = new LinkedHashMap<>();
+ Map oldToCanonical = new HashMap<>();
+ int duplicatesFound = 0;
+
+ for (int i = 0; i < numFonts; i++) {
+ XSSFFont font = workbook.getFontAt(i);
+ String signature = getFontSignature(font);
+
+ if (signatureToCanonical.containsKey(signature)) {
+ oldToCanonical.put(i, signatureToCanonical.get(signature));
+ duplicatesFound++;
+ } else {
+ signatureToCanonical.put(signature, i);
+ oldToCanonical.put(i, i);
+ }
+ }
+
+ if (duplicatesFound == 0) {
+ return 0; // No duplicates to remap
+ }
+
+ // Step 2: Update font references in cell styles' CTXf entries
+ StylesTable stylesTable = workbook.getStylesSource();
+ CTStylesheet ctStylesheet = stylesTable.getCTStylesheet();
+ CTCellXfs ctXfs = ctStylesheet.getCellXfs();
+
+ if (ctXfs != null) {
+ for (int i = 0; i < ctXfs.sizeOfXfArray(); i++) {
+ CTXf xf = ctXfs.getXfArray(i);
+ if (xf.isSetFontId()) {
+ int oldFontId = (int) xf.getFontId();
+ int canonicalFontId = oldToCanonical.getOrDefault(oldFontId, oldFontId);
+ if (canonicalFontId != oldFontId) {
+ xf.setFontId(canonicalFontId);
+ }
+ }
+ }
+ }
+
+ return duplicatesFound;
+ }
+
+ /**
+ * Generate a signature string for a cell style to detect duplicates.
+ * Uses font PROPERTIES (not index) so styles with identical but separate fonts
+ * are detected as duplicates.
+ */
+ private static String getStyleSignature(XSSFCellStyle style) {
+ StringBuilder sb = new StringBuilder();
+ // Use font properties, not index, so identical fonts are detected
+ XSSFFont font = style.getFont();
+ sb.append("font:").append(getFontSignature(font)).append(";");
+ sb.append("fmt:").append(style.getDataFormat()).append(";");
+ sb.append("align:").append(style.getAlignment()).append(";");
+ sb.append("valign:").append(style.getVerticalAlignment()).append(";");
+ sb.append("wrap:").append(style.getWrapText()).append(";");
+ sb.append("rotation:").append(style.getRotation()).append(";");
+ sb.append("indent:").append(style.getIndention()).append(";");
+ sb.append("borderL:").append(style.getBorderLeft()).append(";");
+ sb.append("borderR:").append(style.getBorderRight()).append(";");
+ sb.append("borderT:").append(style.getBorderTop()).append(";");
+ sb.append("borderB:").append(style.getBorderBottom()).append(";");
+ sb.append("fillFg:").append(colorToString(style.getFillForegroundColorColor())).append(";");
+ sb.append("fillBg:").append(colorToString(style.getFillBackgroundColorColor())).append(";");
+ sb.append("fillPat:").append(style.getFillPattern()).append(";");
+ sb.append("locked:").append(style.getLocked()).append(";");
+ sb.append("hidden:").append(style.getHidden()).append(";");
+ return sb.toString();
+ }
+
+ /**
+ * Generate a signature string for a font to detect duplicates.
+ */
+ private static String getFontSignature(XSSFFont font) {
+ StringBuilder sb = new StringBuilder();
+ sb.append("name:").append(font.getFontName()).append(";");
+ sb.append("size:").append(font.getFontHeightInPoints()).append(";");
+ sb.append("bold:").append(font.getBold()).append(";");
+ sb.append("italic:").append(font.getItalic()).append(";");
+ sb.append("strike:").append(font.getStrikeout()).append(";");
+ sb.append("underline:").append(font.getUnderline()).append(";");
+ sb.append("color:").append(colorToString(font.getXSSFColor())).append(";");
+ sb.append("typeOffset:").append(font.getTypeOffset()).append(";");
+ sb.append("charset:").append(font.getCharSet()).append(";");
+ return sb.toString();
+ }
+
+ /**
+ * Convert a color to a string for comparison.
+ */
+ private static String colorToString(Color color) {
+ if (color == null) {
+ return "null";
+ }
+ if (color instanceof XSSFColor) {
+ XSSFColor xssfColor = (XSSFColor) color;
+ byte[] rgb = xssfColor.getRGB();
+ if (rgb != null) {
+ return String.format(Locale.ROOT, "#%02X%02X%02X", rgb[0] & 0xFF, rgb[1] & 0xFF, rgb[2] & 0xFF);
+ }
+ return "indexed:" + xssfColor.getIndexed();
+ }
+ return color.toString();
+ }
+}
diff --git a/poi-ooxml/src/test/java/org/apache/poi/xssf/usermodel/TestXSSFOptimiser.java b/poi-ooxml/src/test/java/org/apache/poi/xssf/usermodel/TestXSSFOptimiser.java
new file mode 100755
index 0000000000..8587cffbe3
--- /dev/null
+++ b/poi-ooxml/src/test/java/org/apache/poi/xssf/usermodel/TestXSSFOptimiser.java
@@ -0,0 +1,305 @@
+/* ====================================================================
+ Licensed to the Apache Software Foundation (ASF) under one or more
+ contributor license agreements. See the NOTICE file distributed with
+ this work for additional information regarding copyright ownership.
+ The ASF licenses this file to You under the Apache License, Version 2.0
+ (the "License"); you may not use this file except in compliance with
+ the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+==================================================================== */
+
+package org.apache.poi.xssf.usermodel;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+
+import org.apache.poi.ss.usermodel.Cell;
+import org.apache.poi.ss.usermodel.CellStyle;
+import org.apache.poi.ss.usermodel.CellType;
+import org.apache.poi.ss.usermodel.DataFormat;
+import org.apache.poi.ss.usermodel.Font;
+import org.apache.poi.ss.usermodel.HorizontalAlignment;
+import org.apache.poi.ss.usermodel.Row;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link XSSFOptimiser}.
+ */
+class TestXSSFOptimiser {
+
+ /**
+ * Test that optimiseCellStyles remaps cells with duplicate styles to canonical style.
+ */
+ @Test
+ void testOptimiseCellStylesRemapsDuplicates() throws Exception {
+ try (XSSFWorkbook workbook = new XSSFWorkbook()) {
+ XSSFSheet sheet = workbook.createSheet("Test");
+
+ // Create 10 cells, each with a "new" style that is actually identical
+ for (int i = 0; i < 10; i++) {
+ Row row = sheet.createRow(i);
+ Cell cell = row.createCell(0);
+ cell.setCellValue("Value " + i);
+
+ // Create a "new" style for each cell (causes style explosion)
+ CellStyle style = workbook.createCellStyle();
+ Font font = workbook.createFont();
+ font.setFontName("Calibri");
+ font.setFontHeightInPoints((short) 11);
+ font.setBold(false);
+ style.setFont(font);
+ cell.setCellStyle(style);
+ }
+
+ int stylesBeforeOptimize = workbook.getNumCellStyles();
+ assertTrue(stylesBeforeOptimize > 5,
+ "Should have created multiple styles before optimization, got: " + stylesBeforeOptimize);
+
+ // Optimize - returns number of cells remapped
+ int cellsRemapped = XSSFOptimiser.optimiseCellStyles(workbook);
+ assertTrue(cellsRemapped > 0,
+ "Should have remapped cells to canonical styles, got: " + cellsRemapped);
+
+ // Verify all cells now reference the same canonical style index
+ int canonicalStyleIdx = sheet.getRow(0).getCell(0).getCellStyle().getIndex();
+ for (int i = 1; i < 10; i++) {
+ Cell cell = sheet.getRow(i).getCell(0);
+ assertEquals(canonicalStyleIdx, cell.getCellStyle().getIndex(),
+ "Cell " + i + " should use canonical style index");
+ }
+
+ // Verify cells still have correct values
+ for (int i = 0; i < 10; i++) {
+ Cell cell = sheet.getRow(i).getCell(0);
+ assertEquals("Value " + i, cell.getStringCellValue());
+ }
+ }
+ }
+
+ /**
+ * Test that optimiseCellStyles handles unused styles gracefully.
+ */
+ @Test
+ void testOptimiseCellStylesHandlesUnusedStyles() throws Exception {
+ try (XSSFWorkbook workbook = new XSSFWorkbook()) {
+ XSSFSheet sheet = workbook.createSheet("Test");
+
+ // Create a cell with one style
+ Row row = sheet.createRow(0);
+ Cell cell = row.createCell(0);
+ cell.setCellValue("Used style");
+ CellStyle usedStyle = workbook.createCellStyle();
+ usedStyle.setAlignment(HorizontalAlignment.CENTER);
+ cell.setCellStyle(usedStyle);
+
+ // Create several unused styles
+ for (int i = 0; i < 5; i++) {
+ CellStyle unusedStyle = workbook.createCellStyle();
+ unusedStyle.setAlignment(HorizontalAlignment.LEFT);
+ // Never apply this style to any cell
+ }
+
+ int stylesBeforeOptimize = workbook.getNumCellStyles();
+ assertTrue(stylesBeforeOptimize >= 6,
+ "Should have at least 6 styles (1 default + 1 used + 5 unused), got: " + stylesBeforeOptimize);
+
+ // Optimize - should not throw, returns 0 (no duplicates to remap)
+ int cellsRemapped = XSSFOptimiser.optimiseCellStyles(workbook);
+ assertEquals(0, cellsRemapped, "No duplicate styles to remap");
+
+ // Verify the used cell still has correct alignment
+ CellStyle cellStyle = cell.getCellStyle();
+ assertEquals(HorizontalAlignment.CENTER, cellStyle.getAlignment(),
+ "Cell should still have CENTER alignment after optimization");
+ }
+ }
+
+ /**
+ * Test that optimiseCellStyles preserves different styles.
+ */
+ @Test
+ void testOptimiseCellStylesPreservesDifferentStyles() throws Exception {
+ try (XSSFWorkbook workbook = new XSSFWorkbook()) {
+ XSSFSheet sheet = workbook.createSheet("Test");
+
+ // Cell with bold style
+ Row row1 = sheet.createRow(0);
+ Cell cell1 = row1.createCell(0);
+ cell1.setCellValue("Bold");
+ CellStyle boldStyle = workbook.createCellStyle();
+ Font boldFont = workbook.createFont();
+ boldFont.setBold(true);
+ boldStyle.setFont(boldFont);
+ cell1.setCellStyle(boldStyle);
+
+ // Cell with italic style
+ Row row2 = sheet.createRow(1);
+ Cell cell2 = row2.createCell(0);
+ cell2.setCellValue("Italic");
+ CellStyle italicStyle = workbook.createCellStyle();
+ Font italicFont = workbook.createFont();
+ italicFont.setItalic(true);
+ italicStyle.setFont(italicFont);
+ cell2.setCellStyle(italicStyle);
+
+ // Cell with number format
+ Row row3 = sheet.createRow(2);
+ Cell cell3 = row3.createCell(0);
+ cell3.setCellValue(1234.56);
+ CellStyle numStyle = workbook.createCellStyle();
+ DataFormat format = workbook.createDataFormat();
+ numStyle.setDataFormat(format.getFormat("#,##0.00"));
+ cell3.setCellStyle(numStyle);
+
+ // Optimize
+ XSSFOptimiser.optimiseCellStyles(workbook);
+
+ // Verify each cell retains its distinct formatting
+ Font font1 = workbook.getFontAt(cell1.getCellStyle().getFontIndex());
+ assertTrue(font1.getBold(), "Cell 1 should still be bold");
+
+ Font font2 = workbook.getFontAt(cell2.getCellStyle().getFontIndex());
+ assertTrue(font2.getItalic(), "Cell 2 should still be italic");
+
+ String numFormat = cell3.getCellStyle().getDataFormatString();
+ assertTrue(numFormat.contains("#,##0"),
+ "Cell 3 should still have number format, got: " + numFormat);
+ }
+ }
+
+ /**
+ * Test that optimiseFonts remaps style font references to canonical fonts.
+ */
+ @Test
+ void testOptimiseFontsRemapsDuplicates() throws Exception {
+ try (XSSFWorkbook workbook = new XSSFWorkbook()) {
+ XSSFSheet sheet = workbook.createSheet("Test");
+
+ // Create cells with duplicate fonts
+ for (int i = 0; i < 10; i++) {
+ Row row = sheet.createRow(i);
+ Cell cell = row.createCell(0);
+ cell.setCellValue("Value " + i);
+
+ // Create a "new" font for each cell (identical fonts)
+ CellStyle style = workbook.createCellStyle();
+ Font font = workbook.createFont();
+ font.setFontName("Arial");
+ font.setFontHeightInPoints((short) 12);
+ font.setBold(true);
+ style.setFont(font);
+ cell.setCellStyle(style);
+ }
+
+ int fontsBeforeOptimize = workbook.getNumberOfFonts();
+ assertTrue(fontsBeforeOptimize > 5,
+ "Should have created multiple fonts before optimization, got: " + fontsBeforeOptimize);
+
+ // Optimize fonts - returns number of duplicates found
+ int duplicatesFound = XSSFOptimiser.optimiseFonts(workbook);
+ assertTrue(duplicatesFound > 0,
+ "Should have found duplicate fonts, got: " + duplicatesFound);
+
+ // Verify cells still have bold Arial font (formatting preserved)
+ for (int i = 0; i < 10; i++) {
+ Cell cell = sheet.getRow(i).getCell(0);
+ Font font = workbook.getFontAt(cell.getCellStyle().getFontIndex());
+ assertEquals("Arial", font.getFontName(), "Font name should be preserved");
+ assertTrue(font.getBold(), "Font should still be bold");
+ }
+ }
+ }
+
+ /**
+ * Test that workbook is saveable after optimization.
+ */
+ @Test
+ void testWorkbookSaveableAfterOptimization() throws Exception {
+ try (XSSFWorkbook workbook = new XSSFWorkbook()) {
+ XSSFSheet sheet = workbook.createSheet("Test");
+
+ // Create style explosion
+ for (int i = 0; i < 20; i++) {
+ Row row = sheet.createRow(i);
+ Cell cell = row.createCell(0);
+ cell.setCellValue("Value " + i);
+
+ CellStyle style = workbook.createCellStyle();
+ Font font = workbook.createFont();
+ font.setFontName("Calibri");
+ style.setFont(font);
+ cell.setCellStyle(style);
+ }
+
+ // Optimize
+ XSSFOptimiser.optimiseCellStyles(workbook);
+ XSSFOptimiser.optimiseFonts(workbook);
+
+ // Save to byte array (should not throw XmlValueDisconnectedException)
+ ByteArrayOutputStream baos = new ByteArrayOutputStream();
+ workbook.write(baos);
+ byte[] savedBytes = baos.toByteArray();
+
+ assertTrue(savedBytes.length > 0, "Saved workbook should have content");
+
+ // Verify we can read it back
+ try (XSSFWorkbook reloaded = new XSSFWorkbook(new ByteArrayInputStream(savedBytes))) {
+ XSSFSheet reloadedSheet = reloaded.getSheetAt(0);
+ assertEquals("Value 0", reloadedSheet.getRow(0).getCell(0).getStringCellValue());
+ assertEquals("Value 19", reloadedSheet.getRow(19).getCell(0).getStringCellValue());
+ }
+ }
+ }
+
+ /**
+ * Test that optimization preserves formulas.
+ */
+ @Test
+ void testOptimizationPreservesFormulas() throws Exception {
+ try (XSSFWorkbook workbook = new XSSFWorkbook()) {
+ XSSFSheet sheet = workbook.createSheet("Test");
+
+ // Create cells with values
+ for (int i = 0; i < 5; i++) {
+ Row row = sheet.createRow(i);
+ Cell cell = row.createCell(0);
+ cell.setCellValue(i + 1);
+
+ CellStyle style = workbook.createCellStyle();
+ style.setAlignment(HorizontalAlignment.RIGHT);
+ cell.setCellStyle(style);
+ }
+
+ // Create formula cell
+ Row sumRow = sheet.createRow(5);
+ Cell sumCell = sumRow.createCell(0);
+ sumCell.setCellFormula("SUM(A1:A5)");
+ CellStyle sumStyle = workbook.createCellStyle();
+ Font boldFont = workbook.createFont();
+ boldFont.setBold(true);
+ sumStyle.setFont(boldFont);
+ sumCell.setCellStyle(sumStyle);
+
+ // Optimize
+ XSSFOptimiser.optimiseCellStyles(workbook);
+
+ // Verify formula is preserved
+ Cell formulaCell = sheet.getRow(5).getCell(0);
+ assertEquals(CellType.FORMULA, formulaCell.getCellType());
+ assertEquals("SUM(A1:A5)", formulaCell.getCellFormula());
+
+ // Verify bold style is preserved on formula cell
+ Font font = workbook.getFontAt(formulaCell.getCellStyle().getFontIndex());
+ assertTrue(font.getBold(), "Formula cell should still be bold");
+ }
+ }
+}