Unable to append rows in excel using POI api in java

I am trying to append rows to an existing excel using POI api following the steps mentioned below:

try {

    lFout = new FileOutputStream(lOutputFileName);

    lFin = new FileInputStream(lOutputFileName);
    lPOIfs = new POIFSFileSystem(lFin);
    lWorkBook = new HSSFWorkbook(lPOIfs);
    lWorkSheet = lWorkBook.getSheet("Sheet1");
    System.out.println("hello3");

    for (Iterator<String> iter = aOutputLines.iterator(); iter.hasNext();) {        
        String lValue = iter.next();

        if (lValue == null) {
            System.out.println("=>NULL VALUE => " + lValue);
        } else {
            // StringBuffer lSqlLine = (StringBuffer) lValue;

            HSSFRow lRow = lWorkSheet.createRow(lWorkSheet.getLastRowNum() + 1);

            System.out.println("lWorkSheet.getLastRowNum() " + lWorkSheet.getLastRowNum());

            HSSFCell lCell = lRow.createCell(0);

            lCell.setCellValue(new HSSFRichTextString((lValue)));

        }
    }

    lWorkBook.write(lFout);
    lFout.flush();

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

The above code snippet is not able to append the rows and also i found that after running the above snippet, the file size get increased and if I open the file and close it again it remains the same size as it was earlier. Some urgent help would be really apppriciable.