fileOut = new FileOutputStream("C:/workbook.xlsx");
wb.write(fileOut);
fileOut.close();

document.getElementById('download').click();

<a href="files/workbook.xlsx" download id="download" ></a>

<a href="files/workbook.xlsx" download id="download" >скачать</a>

 FileOutputStream fileOut;
            try {

                fileOut = new FileOutputStream("C:/workbook.xlsx");
                wb.write(fileOut);
        
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }

            System.out.println("File created!");

try(OutputStream out = new BufferedOutputStream(new FileOutputStream(filename))){
   out.write(data); 
}

 FileOutputStream fileOut;
            try {

                fileOut = new FileOutputStream("C:/workbook.xlsx");
                wb.write(fileOut);
                fileOut.close();
                fileOut.flush();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }

            System.out.println("File created!");
            webSocket_.send("xxx");вот тут идет сообщение клиенту о готовности сохранения файла, в ответ на это 

клиент делает
document.getElementById('download').click();
<a href="files/workbook.xlsx" download id="download" ></a>

            ByteArrayOutputStream outByteStream = new ByteArrayOutputStream();
            wb.write(outByteStream);
            byte[] outArray = outByteStream.toByteArray();

            StringBuilder sb = new StringBuilder();
            sb.append("data:application/bin;base64,");
            sb.append(StringUtils.newStringUtf8(Base64.encodeBase64(outArray, false)));

            webSocket_.send(sb.toString());

 $('#download').attr('href', p);
 document.getElementById('download').click();

<a href="" download="xxx.xlsx" id="download" ></a>