You need to:
- Set your data as number (floating-point), not as text.
- Specify cell format as percentage.
Something like:
cell.setCellValue(0.123); // set value as number
CellStyle style = workbook.createCellStyle();
style.setDataFormat(workbook.createDataFormat().getFormat("0.000%"));
cell.setCellStyle(style);
Take a look at user defined formats section of POI quick guide for more details. You may also want to go through the examples which show how to use different POI capabilities.