//Instantiating a Workbook object
Workbook workbook = new Workbook();

//Adding a new worksheet to the Workbook object
Worksheet worksheet = workbook.getWorksheets().get(0);

//Adding a comment to cell
int commentIndex = worksheet.getComments().add("A1");
Comment comment = worksheet.getComments().get(commentIndex);

//Setting the comment note
comment.setNote("Hello Aspose!");

public static void main(String[] args) {

    try { 

        String outputfilepath = System.getProperty("user.dir") + "/data/xlsx4j/AddComments-Xlsx4j.xlsx";

        SpreadsheetMLPackage pkg = SpreadsheetMLPackage.createPackage();

        WorksheetPart sheet = pkg.createWorksheetPart(new PartName("/xl/worksheets/sheet1.xml"), "Sheet1", 1);
        addContent(sheet);

        pkg.save(new File(outputfilepath));

        System.out.println("\n\n done .. " + outputfilepath);   

    } 
    catch (Exception e) 
    { 
        e.printStackTrace();
    }
}

private static void addContent(WorksheetPart sheet) throws JAXBException, Docx4JException {

    // Minimal content already present
    SheetData sheetData = sheet.getContents().getSheetData();

    // Now add
    Row row = Context.getsmlObjectFactory().createRow();
    Cell cell = Context.getsmlObjectFactory().createCell();
    cell.setV("1234");
    row.getC().add(cell);

    row.getC().add(createCell("hello world!"));
    sheetData.getRow().add(row);

    // ADD A COMMENT TO CELL A1
    CommentsPart cp = new CommentsPart();
    cp.setContents(createComment("A1"));
    sheet.addTargetPart(cp);

    // Add <legacyDrawing r:id="rId1"/>
    VMLPart vmlPart = new VMLPart();
    vmlPart.setContents(getVml(0,0));  // corresponds to A1
      // you'll need extra VML for each comment
    Relationship rel = sheet.addTargetPart(vmlPart);
    CTLegacyDrawing legacyDrawing = Context.getsmlObjectFactory().createCTLegacyDrawing();
    legacyDrawing.setId(rel.getId());
    sheet.getContents().setLegacyDrawing(legacyDrawing);

}

private static Cell createCell(String content) {

    Cell cell = Context.getsmlObjectFactory().createCell();

    CTXstringWhitespace ctx = Context.getsmlObjectFactory().createCTXstringWhitespace();
    ctx.setValue(content);

    CTRst ctrst = new CTRst();
    ctrst.setT(ctx);

    cell.setT(STCellType.INLINE_STR);
    cell.setIs(ctrst); // add ctrst as inline string

    return cell;

}

public static CTComments createComment(String cell) throws JAXBException { 
    String openXML = "<comments xmlns=\"http://schemas.openxmlformats.org/spreadsheetml/2006/main\">"
            + "<authors>"
                + "<author>Author</author>"
            +"</authors>"
            + "<commentList>"
                + "<comment authorId=\"0\" ref=\"" + cell + "\">"
                    + "<text>"
                        + "<r>"
                            + "<rPr>"
                                + "<b/>"
                                + "<sz val=\"9\"/>"
                                + "<color indexed=\"81\"/>"
                                + "<rFont val=\"Tahoma\"/>"
                                + "<charset val=\"1\"/>"
                            +"</rPr>"
                            + "<t>Thomas: hello world!</t>"
                        +"</r>"
                    +"</text>"
                +"</comment>"
            +"</commentList>"
        +"</comments>";
    CTComments comments = (CTComments)XmlUtils.unwrap(
        XmlUtils.unmarshalString(openXML, Context.jcSML));
    return comments;
}

public static org.docx4j.vml.root.Xml getVml(int row, int col) throws JAXBException {

    String openXML = "<xml xmlns:v=\"urn:schemas-microsoft-com:vml\" xmlns:o=\"urn:schemas-microsoft-com:office:office\" xmlns:x=\"urn:schemas-microsoft-com:office:excel\">"

        + "<o:shapelayout v:ext=\"edit\" >"
        + "<o:idmap data=\"1\" v:ext=\"edit\"/>"

        + "</o:shapelayout>"

        + "<v:shapetype coordsize=\"21600,21600\" id=\"_x0000_t202\" o:spt=\"202\" path=\"m,l,21600r21600,l21600,xe\" >"
        + "<v:stroke joinstyle=\"miter\"/>"

        + "<v:path gradientshapeok=\"t\" o:connecttype=\"rect\"/>"

        + "</v:shapetype>"

        // The VML part must have a <v:shape> element for each comment
        + "<v:shape fillcolor=\"#ffffe1\" id=\"_x0000_s1025\" o:insetmode=\"auto\" style=\"position:absolute;   margin-left:107.25pt;margin-top:7.5pt;width:108pt;height:59.25pt;z-index:1;   visibility:visible\" type=\"#_x0000_t202\" >"
        + "<v:fill color2=\"#ffffe1\"/>"

        + "<v:shadow color=\"black\" obscured=\"t\" on=\"t\"/>"

        + "<v:path o:connecttype=\"none\"/>"

        + "<v:textbox style=\"mso-direction-alt:auto\">"
        + "<div style=\"text-align:left\"/>"

        + "</v:textbox>"

        + "<x:ClientData ObjectType=\"Note\">" + "<x:MoveWithCells/>"

        + "<x:SizeWithCells/>"

        + "<x:Anchor>2, 15, 0, 10, 4, 31, 4, 9</x:Anchor>"

        + "<x:AutoFill>False</x:AutoFill>"

        + "<x:Row>" + row + "</x:Row>"

        + "<x:Column>" + col + "</x:Column>"

        + "</x:ClientData>"

        + "</v:shape></xml>";

        return (org.docx4j.vml.root.Xml) XmlUtils.unmarshalString(openXML, org.docx4j.jaxb.Context.jc);

}