Java Rest微服务中的模拟HSSF工作簿

我正在处理一个案例，其中我需要为处理HSSF工作簿的控制器编写单元测试。 但是，即使在测试类中创建了对象并通过Mockito

when/thenReturn

传递了它，我仍然在

wb.write（outByteStream）得到空指针异常。
以下是我的控制器方法：

@PostMapping(value = "/recordsToExcel")
public void exportModelsHistoryDataToExcel(HttpServletRequest request,
        @RequestBody ModelsDto modelsDto) {
    int minimum = 10;

    int rand = new Random().nextInt(1000);

    int randomNum = minimum + rand;
    String excelFileName = "Revisions" + randomNum + ".xls";

    HSSFWorkbook wb = service.getModelSearchExcel(modelsDto);

    try {
        ByteArrayOutputStream outByteStream = new ByteArrayOutputStream();
        wb.write(outByteStream);
        byte[] outArray = outByteStream.toByteArray();
        response.setContentType("application/vnd.ms-excel");
        response.setContentLength(outArray.length);
        response.setHeader("Expires:", "0"); 
        response.setHeader("Content-Disposition", "attachment; filename=" + excelFileName);
        OutputStream outStream = response.getOutputStream();
        outStream.write(outArray);
        outStream.flush();
        outStream.close();

        wb.close();
    } catch (Exception e) {
        logger.error("ModelController - exportModelsHistoryDataToExcel() - exception::", e);
    }

}

下面是我的测试方法，我也尝试过在测试方法中创建一个工作表，但没有成功。
不确定我错过了什么或做错了什么。请帮忙

@Test
public void testExportModelsHistoryDataToExcel() throws Exception {
     ModelsDto modelsDto = new ModelsDto();
    modelsDto.setBirthFrom("9/9/1989");
//TODO Mock objects instead of creating
    HSSFWorkbook wb = new HSSFWorkbook(); //final class can't mock
    ByteArrayOutputStream outByteStream = new ByteArrayOutputStream();
    wb.write(outByteStream);
    when(service.getModelSearchExcel(modelsDto)).thenReturn(wb);
    MockHttpServletRequestBuilder builder = MockMvcRequestBuilders.post("/recordsToExcel")
            .contentType(MediaType.APPLICATION_JSON).content(convertToJson(modelsDto));
    this.mockMvc.perform(builder).andExpect(MockMvcResultMatchers.status().isOk());
}