Add borders to cells in POI generated Excel File

Setting up borders in the style used in the cells will accomplish this. Example:

style.setBorderBottom(HSSFCellStyle.BORDER_MEDIUM);
style.setBorderTop(HSSFCellStyle.BORDER_MEDIUM);
style.setBorderRight(HSSFCellStyle.BORDER_MEDIUM);
style.setBorderLeft(HSSFCellStyle.BORDER_MEDIUM);