Workbook wb = new XSSFWorkbook();   //or new HSSFWorkbook();  
Sheet sheet = wb.createSheet();

Row row = sheet.createRow(2);
Cell cell = row.createCell(2);
cell.setCellValue("Use \n with word wrap on to create a new line");


CellStyle cs = wb.createCellStyle();
cs.setWrapText(true); //关键
cell.setCellStyle(cs); //单元格应用设置的style