Basic strategy:
1) Apache Commons CSV is the standard library for writing CSV values.
2) But we need to loop through the Workbook ourselves, and then call Commons CSV's Printer on each cell value, with a newline at the end of each row. Unfortunately this is custom code, it's not automatically available in XSSF. But it's easy:
// In this example we construct CSVPrinter on a File, can also do an OutputStream
Reader reader = Files.newBufferedReader(Paths.get(SAMPLE_CSV_FILE_PATH));
CSVPrinter csvPrinter = new CSVPrinter(reader, CSVFormat.DEFAULT);
if (workbook != null) {
XSSFSheet sheet = workbook.getSheetAt(0); // Sheet #0
Iterator<Row> rowIterator = sheet.rowIterator();
while (rowIterator.hasNext()) {
Row row = rowIterator.next();
Iterator<Cell> cellIterator = row.cellIterator();
while (cellIterator.hasNext()) {
Cell cell = cellIterator.next();
csvPrinter.print(cell.getStringCellValue()); // Call Commons CSV here to print
}
// Newline after each row
csvPrinter.println();
}
}
// at the end, close and flush CSVPrinter
csvPrinter.flush();
csvPrinter.close();