java.lang.NullPointerException由于com.prepareStatement

Question

我希望将文件发送到数据库并能够取回，但是当我上传文件时，在servlet中由于com.prepareStatement而出现错误

我尝试强制转换，但失败了，我尝试jdbc(prepareStatement)失败，因为我正在使用java.sql中的PreparedStatement

public class UploadServlet extends HttpServlet {

PrintWriter out = null;
Connection com = null;
PreparedStatement ps = null;
HttpSession session = null;

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("text/plain/charset=UTF-8");
    try {
        out = response.getWriter();
        session = request.getSession();
        String folderName = "resources";
        String uploadPath = request.getServletContext().getRealPath("") + File.separator + folderName;
        File dir = new File(uploadPath);
        if (!dir.exists()) {
            dir.mkdirs();
        }
        Part filePart = request.getPart("file"); // textbox value fo name file
        String firstName = request.getParameter("firstname");
        String lastName = request.getParameter("lastname");
        String fileName = filePart.getName();
        String path = folderName + File.separator + fileName;
        Timestamp added_date = new Timestamp(System.currentTimeMillis());
        System.out.println("fileName: " + fileName);
        System.out.println("path: " + uploadPath);
        System.out.println("fristName: " + firstName);
        InputStream is = filePart.getInputStream();
        Files.copy(is, Paths.get(uploadPath + File.separator + fileName), StandardCopyOption.REPLACE_EXISTING);
        try {
            com = DB.getConnectiion();
            System.out.println("connection Done.");
            String sql = "insert into employee1(firstname,lastname,filename,path,added_date) values(?,?,?,?,?)";
            ps = com.prepareStatement(sql);
            ps.setString(1, firstName);
            ps.setString(2, lastName);
            ps.setString(3, fileName);
            ps.setString(4, path);
            ps.setTimestamp(5, added_date);
            int status = ps.executeUpdate();
            if (status > 0) {
                session.setAttribute("fileName", fileName);
                String msg = "" + fileName + " file uploads Successfully...";
                request.setAttribute("msg", msg);
                RequestDispatcher rd = request.getRequestDispatcher("/sucess.jsp");
                rd.forward(request, response);
                System.out.println("file uploaded Successfully...");
                System.out.println("uploaded Path: " + uploadPath);
            }
        } catch (SQLException e) {
            out.println("the Error is here " + e);
            System.out.println("Exception1 is " + e);
        } finally {
            try {
                if (ps != null) {
                    ps.close();
                }
                if (com != null) {
                    com.close();
                }
            } catch (SQLException e) {
                out.println(e);
            }
        }
    } catch (IOException | ServletException | NullPointerException e) {
        out.println("Exception: " + e);
        System.out.println("Exception2: " + e);
    }
}

}