How to write a Junit test of Excel Upload?

guys.I'm a beginner learning java.And now i have to write a Junit test of Excel upload function in my company demo project.

@RequestMapping(value = "/excelImport")
public ModelAndView excelImport(@RequestParam(value = "filename")MultipartFile file,
        ModelAndView mav) {
    session.setAttribute("imported",false);//セッションのインポート記録をリセットして、次の操作を影響しないように
    List<RetirementPayAndHealthInsurance> errorList= new ArrayList<>();//エラーデータリスト
    List<RetirementPayAndHealthInsurance> importList = new ArrayList<>();//導入されたデータリスト
    int wnt = 0;//エラー数
    int cnt = 0;//正常数
    String fileName = file.getOriginalFilename();
    if(file.isEmpty()) {    //ファイル選択なしにインポートボタンを押す場合
        mav.setViewName("redirect:/retailList");
        return mav;
    }else {

        try {
            Map<String,List<RetirementPayAndHealthInsurance>> map = retirementPayAndHealthInsuranceService.batchImport(fileName, file);
            errorList = map.get("errorList");
            importList = map.get("importList");
        } catch (Exception e) {
            // TODO 自動生成された catch ブロック
            e.printStackTrace();
        }
        if(errorList!=null) {//エラーリストあるの場合
            wnt = errorList.size();//エラー数
            mav.addObject("errorList",errorList);
            session.setAttribute("errorList", errorList);
        }
        if(importList!=null) {
            cnt = importList.size();//正常数
        }
        //インポートボタン　と　導出ボタンを分けるため設定の記録
        session.setAttribute("imported",true);
        mav.addObject("wnt",wnt);//エラー数
        mav.addObject("cnt",cnt);//正常数
        mav.setViewName("/retailExcelImport");
        return mav;
    }
}

i'll explain my logic below.

The excel upload will process and show us the successful imported data number and the error either , service puts the successList,errorList into map and return to controller then count the size of them .Then i could export and reedit the error data and do this process again until it's done.

Now i want to write the junit test of this .But i can't find the similar sample of excel upload test , help me please ,thank you!