重定向的传值问题

作业中当添加完图书点击保存的时候跳转到该分类下图书列表界面，我在Controller中的add方法中使用重定向response.sendRedirect("list.do?categoryId=");  请问老师这里的categoryId的值应该怎么获取？

public void add(HttpServletRequest request,HttpServletResponse response) throws FileUploadException, IOException {
    Book book = new Book();
    List<Book> books = new ArrayList<>();
    //创建一个磁盘文件项工厂对象
    DiskFileItemFactory diskFileItemFactory = new DiskFileItemFactory();
    //创建一个核心解析类
    ServletFileUpload servletFileUpload = new ServletFileUpload(diskFileItemFactory);
    //解析request请求，返回List集合，List集合中存放FileItem对象
    List<FileItem> list = servletFileUpload.parseRequest(request);
    //遍历集合，获得每个FileItem，判断是表单项还是文件上传项
    for(FileItem fileItem:list){
        if(fileItem.isFormField()){
            //表单项
            if(fileItem.getFieldName().equals("categoryId")){
                book.setCategoryId(Integer.parseInt(fileItem.getString("UTF-8")));
            }
            if(fileItem.getFieldName().equals("name")){
                book.setName(fileItem.getString("UTF-8"));
            }
            if(fileItem.getFieldName().equals("level")){
                book.setLevel(Integer.parseInt(fileItem.getString("UTF-8")));
            }
            if(fileItem.getFieldName().equals("price")){
                book.setPrice(Integer.parseInt(fileItem.getString("UTF-8")));
            }
            if(fileItem.getFieldName().equals("imgPath")&&book.getImgPath() != null){
                book.setImgPath(fileItem.getString("UTF-8"));
            }

        }else {
            //文件上传项
            if(fileItem.getFieldName().equals("smallImg")){
                String rootPath = request.getServletContext().getRealPath("/");
                String path = fileItem.getName();
                String type = ".jpg";
                if(path.indexOf(".") != -1){
                    type = path.substring(path.lastIndexOf("."));
                }
                path = "/download/images/"+System.currentTimeMillis()+type;
                try {
                    fileItem.write(new File(rootPath+path));
                    book.setImgPath(path);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }
    books.add(book);
    //调用Service
    bookService.add(books);
    //重定向到界面
    response.sendRedirect("list.do?categoryId=");
}