Workbook workbook = new Workbook(file);
Sheet sheet = workbook.getSheetAt(0);  // get first sheet
Iterator<Row> iterator = sheet.iterator();

while (iterator.hasNext()) {
    Iterator<Cell> cellIterator = iterator.next().iterator();
    while (cellIterator.hasNext()) {
        Cell c = cellIterator.next();
        // add to data structure
    }
}

String[][] data; // loaded from XLSX
CSVPrinter printer = new CSVPrinter(new File("output.csv"), CSVFormat.DEFAULT);
printer.printRecord("Column 1", "Column 2", "Column 3");  // print header

for (String[] strings : data) {
    printer.printRecord(strings[0], strings[1], strings[2]);
}