Apache POI centering all cells in all sheets

Question

I'm trying to center every cell of every column of every sheet in my XSSFWorkbook:

 XSSFCellStyle cellStyle =  workbook.createCellStyle();
  cellStyle.setVerticalAlignment(VerticalAlignment.CENTER);
 
  for (int i = 0; i < workbook.getNumberOfSheets(); i++)
    {
      XSSFSheet sheet = workbook.getSheetAt(i);
      if(sheet.getRow(0)!=null)
       for(int j=0; j<sheet.getRow(0).getLastCellNum(); j++)
       {
         sheet.autoSizeColumn(j);
         sheet.getColumnHelper().setColDefaultStyle(j, cellStyle);
       }    
    }

Columns are now autosized but the text is not centered.