// 1.导入excel模板
String targetPath = rs.getSession().getServletContext()
.getRealPath("/excelModel/" + fileName + ".xlsx");
// 2.创建一个workbook，对应一个Excel文件
File fi = new File(targetPath);

FileInputStream is = new FileInputStream(fi);

XSSFWorkbook wb = new XSSFWorkbook(is);

int lastRowNum=wb.getSheetAt(0).getLastRowNum();

if(fileName.contains("横向明细")){
wb.getSheetAt(0).getRow(3).getCell(0).setCellValue("分配期号："+hxmxFpqh);
}
SXSSFWorkbook sxssfWorkbook = new SXSSFWorkbook(wb,1000);

Sheet sheet = sxssfWorkbook.getSheetAt(0);
// 开始填写查找出来的数据
writeExcel(list, sheet ，lastRowNum);
// 下载
tkDownload2(sheetName, sxssfWorkbook, res);

-------------2-------------------

public static void writeExcel(List<?> list, Sheet sheet, int lastRowNum)
throws IOException, IllegalArgumentException,
IllegalAccessException {

Row row;
for (int i = 0; i < list.size(); i++) {
row = sheet.createRow((int) i + lastRowNum + 1);
row.createCell(0).setCellValue(i + 1);
Object tjb = list.get(i);
Class class1 = (Class) tjb.getClass();
Field[] fs = class1.getDeclaredFields();
for (int j = 1; j < fs.length; j++) {
Field f = fs[j];
f.setAccessible(true);
Object v = f.get(tjb);
String type = f.getType().toString();
if (v == null || v.toString() == "") {
row.createCell(j).setCellValue("");
} else {

if (type.endsWith("Double") || type.endsWith("double")) {
row.createCell(j).setCellValue(
Double.parseDouble(v.toString()));
} else if (type.endsWith("Integer") || type.endsWith("int")) {
row.createCell(j).setCellValue(
Double.parseDouble(v.toString()));
}

else if (type.endsWith("String")) {
row.createCell(j).setCellValue(v.toString());
}
}
}
}
}

-------------3---------------------

public static void tkDownload2(String sheetName, SXSSFWorkbook wb,
HttpServletResponse res) throws IOException {
String fileName = sheetName;
ByteArrayOutputStream os = new ByteArrayOutputStream();
wb.write(os);

byte[] content = os.toByteArray();
InputStream is = new ByteArrayInputStream(content);
// 设置response参数，可以打开下载页面
res.reset();
res.setContentType("application/vnd.ms-excel;charset=utf-8");

res.addHeader("Content-Disposition", "attachment;filename="
+ URLEncoder.encode(fileName + ".xlsx", "UTF-8"));// 设置文件名

try (ServletOutputStream out = res.getOutputStream();
BufferedInputStream bis = new BufferedInputStream(is);
BufferedOutputStream bos = new BufferedOutputStream(out)){

byte[] buff = new byte[2048];
int bytesRead;
// Simple read/write loop.
while (-1 != (bytesRead = bis.read(buff, 0, buff.length))) {
bos.write(buff, 0, bytesRead);
}
} catch (Exception e) {
//e.printStackTrace();
logger.error("tkDownload failed |{}",()->e.toString());
logger.debug("failed {}", e);
}
}