// 建立工作簿
XSSFWorkbook workBook = new XSSFWorkbook(inputStream);
// 建立sheet
XSSFSheet sheet = workBook.createSheet(sheetName);
XSSFDrawing draw = newSheet.createDrawingPatriarch();
// 定義註釋的大小和位置
/*
 * @param dx1  the x coordinate within the first cell.
 * @param dy1  the y coordinate within the first cell.
 * @param
 
 dx2  the x coordinate within the second cell.
 * @param dy2  the y coordinate within the second cell.
 * @param col1 the column (0 based) of the first cell.
 * @param row1 the row (0 based) of the first cell.
 * @param col2 the column (0 based) of the second cell.
 * @param row2 the row (0 based) of the second cell.
 */
 

XSSFComment comment = draw.createCellComment(new XSSFClientAnchor(0, 0, 0, 0, 4, 2, 9, 7));
XSSFRichTextString rtf = new XSSFRichTextString("新增批註內容！");
XSSFFont commentFormatter = workBook.createFont();
commentFormatter.setFontName("宋體");
//設定字型大小
commentFormatter.setFontHeightInPoints((short) 9);
rtf.applyFont(commentFormatter);
comment.setString(rtf);
comment.setAuthor("Zhihui Li"
 
);
// 將批註新增到單元格物件中
sheet.getRow(rowIndex).getCell(columnIndex).setCellComment(comment);

如果有不熟悉如何操作Excel的，可以參考前面的博文：
poi操作excel總結