Here is a reliable method for creating an Excel spreadsheet using the WebService (Resteasy Implementation) that has been thoroughly tested and proven to work:

@POST
    @Path("/excelpost")
    @Produces(MediaType.APPLICATION_OCTET_STREAM)
    public Response downloadFilePost( ) throws IOException {

         XSSFWorkbook workbook = new XSSFWorkbook();
            XSSFSheet sheet = workbook.createSheet("Datatypes in Java");
            Object[][] datatypes = {
                    {"Datatype", "Type", "Size(in bytes)"},
                    {"int", "Primitive", 2},
                    {"float", "Primitive", 4},
                    {"double", "Primitive", 8},
                    {"char", "Primitive", 1},
                    {"String", "Non-Primitive", "No fixed size"}
            };

            int rowNum = 0;
            System.out.println("Creating excel");

            for (Object[] datatype : datatypes) {
                Row row = sheet.createRow(rowNum++);
                int colNum = 0;
                for (Object field : datatype) {
                    Cell cell = row.createCell(colNum++);
                    if (field instanceof String) {
                        cell.setCellValue((String) field);
                    } else if (field instanceof Integer) {
                        cell.setCellValue((Integer) field);
                    }
                }
            }

                ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
                workbook.write(outputStream);
                workbook.close();


            return Response.status(200).header("Access-Control-Allow-Origin", "*")
                    .header("Content-Disposition", "attachment;filename='fileName.xlsx'")
                    .header("Access-Control-Allow-Headers", "origin, content-type, accept, authorization")
                    .header("Access-Control-Allow-Credentials", "true")
                    .header("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS, HEAD")
                    .header("Access-Control-Max-Age", "1209600").entity( new ByteArrayInputStream(outputStream.toByteArray() )).build();


    }

Here is the maven dependency needed to manage Excel files:

<dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi</artifactId>
    <version>3.17</version>
</dependency>

If you want to handle file input from the initial post of your service, you can easily add annotations and webparameters like this:

@POST
@Path("/excelpost")
@Consumes(MediaType.MULTIPART_FORM_DATA)
@Produces(MediaType.APPLICATION_OCTET_STREAM)
public Response downloadFilePost(MultipartFormDataInput input)
   .......

Hope this information proves helpful to you...

@Blackjack, here is the script for handling file upload and calling a REST Endpoint function:

1) The following function is triggered on click of a button. It involves an AJAX call to send an Excel file to a REST Endpoint:

function fileUploadFunction() {

    var file = $('input[name="file"').get(0).files[0];
    var formData = new FormData();

    if(file.name != null) {
        document.getElementById("btnUpload").disabled = false;

        formData.append('file', file);
        $.ajax({
            url : "<%=request.getContextPath()%>/rest/upload/bulkSearch",
            type : "POST",
            data : formData,
            cache : false,
            contentType : false,
            processData : false,
            success : function(response) {
                //Store result in Session and Enable Download button
                var cacheString = JSON.stringify(response, null, 2);
                console.log("-----------------> cacheString is: " + cacheString);
                if(cacheString != null && cacheString != "[]") {
                    document.getElementById("download").disabled = false;
                }
                var sessionresponse = sessionStorage.setItem("i98779", cacheString); 

                console.log("response is: " + response);
                console.log("cacheString is: " + cacheString);
                //excelDownload(cacheString);
                //createTable(response);
                //https://stackoverflow.com/questions/47330520/how-to-export-json-object-into-excel-using-javascript-or-jquery

            },

            error : function(jqXHR, textStatus, errorThrown) {
                console.log(errorThrown);
                alert("Error: " + errorThrown);
            }

        });
    }

}

2) Here's the code for the REST End-Point function:

@POST 
@Consumes(MediaType.MULTIPART_FORM_DATA) 
@Produces(MediaType.APPLICATION_OCTET_STREAM)
@Path("/bulkSearch") 
public Response bulkSearch( 
    @FormDataParam("file") InputStream uploadedInputStream, 
    @FormDataParam("file") FormDataContentDisposition fileDetail) throws IOException { 

    System.out.println("Entered uploadFile method");
    System.out.println("uploadedInputStream is: " + uploadedInputStream);
    System.out.println("fileDetail is: " + fileDetail.toString());

    String returnJSON = null;
    List<User> usersList = null;
    ObjectMapper uploadMapper = new ObjectMapper();

    // Check if all form parameters are provided 
    if (uploadedInputStream == null || fileDetail == null) 
        return Response.status(400).entity("Invalid form data").build();

    System.out.println("Checked Input file is ok");

    System.out.println("----------------------------------------------------------------");
    XSSFWorkbook workbook = new XSSFWorkbook();
    XSSFSheet sheet = workbook.createSheet("Datatypes in Java");
    Object[][] datatypes = {
            {"Datatype", "Type", "Size(in bytes)"},
            {"int", "Primitive", 2},
            {"float", "Primitive", 4},
            {"double", "Primitive", 8},
            {"char", "Primitive", 1},
            {"String", "Non-Primitive", "No fixed size"}
    };

    int rowNum = 0;
    System.out.println("Creating excel");

    for (Object[] datatype : datatypes) {
        Row row = sheet.createRow(rowNum++);
        int colNum = 0;
        for (Object field : datatype) {
            org.apache.poi.ss.usermodel.Cell cell = row.createCell(colNum++);
            if (field instanceof String) {
                cell.setCellValue((String) field);
            } else if (field instanceof Integer) {
                cell.setCellValue((Integer) field);
            }
        }
    }

    System.out.println("For loop done");

    ByteArrayOutputStream outputStream = new ByteArrayOutputStream(); 
    System.out.println("outputstream done");
    workbook.write(outputStream);
    System.out.println("workbook.write done");
    workbook.close();
    System.out.println("workbook closed");


    return Response.status(200).header("Access-Control-Allow-Origin", "*")
            .header("Content-Disposition", "attachment;filename='fileName.xlsx'")
            .header("Access-Control-Allow-Headers", "origin, content-type, accept, authorization")
            .header("Access-Control-Allow-Credentials", "true")
            .header("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS, HEAD")
            .header("Access-Control-Max-Age", "1209600").entity(new ByteArrayInputStream(outputStream.toByteArray())).build();



}