Ajax表单无法第二次提交

Question

我想将文件及其详细信息从jsp页面上传到服务器，并使用对servlet的 ajax 请求将其存储在数据库中。我的代码正常工作，并且仅在第一次提交数据。但是它无法为后续的提交操作提交数据。有人可以帮我弄清楚这里的错误。 我的 jsp 文件中有一个表单，它看起来像：

<form method="POST" id="form_id">
    <label for="header">Tip Header:</label><input type="text" id="header" name="header"><br>
    <label for="msg">Tip Message:</label><input type="text" id="msg" name="msg"><br>
    <label for="file_upload">Tip Image:</label><input type="file" name="file_upload" id="file_upload" style="width:500px; font-size: 15px;"><br><br>
    <label for="link">Tip Link:</label><input type="text" id="link" name="link"><br>
</form>
<button id="add">ADD</button>
<button type="reset" id="clear">Clear</button>

Ajax调用：

document.getElementById("add").onclick=function()
    {
        var header=document.getElementById('header').value;
        var msg=document.getElementById('msg').value;
        var file_upload=document.getElementById('file_upload').files[0];
        var link=document.getElementById('link').value;
        var formData=new FormData();
        formData.append("header",header);
        formData.append("msg",msg);
        formData.append("file_upload",file_upload);
        formData.append("link",link);
        var xhtp=new XMLHttpRequest();
        xhtp.onreadystatechange=function()
        {
            if(this.readyState==4 && this.status==200)
            {
                alert(xhtp.responseText);
            }
        };
        xhtp.open("POST","/TipsTab/addData",true);
        xhtp.send(formData);
    }

Servlet：

public class UploadImg extends HttpServlet
{
   private String filepath;
   private int maxFileSize = 500 * 1024;
   private int maxMemSize = 400 * 1024;
   private File file ;
   private static String URL="jdbc:mysql://localhost/pictures",className="com.mysql.jdbc.Driver",user="root",pass="";
   private static Connection con;
   private static PreparedStatement ins=null;
   private static String ins_query="insert into tips values(null,?,?,?,?,?)";
   private int i=1;

    public void doPost(HttpServletRequest request,HttpServletResponse response)throws IOException,ServletException
    {
        filepath=getServletContext().getInitParameter("file-upload");
        if(ServletFileUpload.isMultipartContent(request))
        {
            DiskFileItemFactory factory=new DiskFileItemFactory();
            factory.setSizeThreshold(maxMemSize);
            factory.setRepository(new File("/Users/test/Documents/OutputImages/extra"));
            ServletFileUpload upload=new ServletFileUpload(factory);
            upload.setSizeMax(maxFileSize);
            try
            {
               Class.forName(className);
               con=DriverManager.getConnection(URL,user,pass);
               ins=con.prepareStatement(ins_query);
               List<FileItem> items=upload.parseRequest(request);
               Iterator<FileItem> iter = items.iterator();
               while (iter.hasNext()) {
                  FileItem item = iter.next();
                  if (item.isFormField()) {
                     String name = item.getFieldName();
                     String value = item.getString();
                     ins.setString(i++,value);
                  }
                  else {
                     String fieldName = item.getFieldName();
                     ins.setString(i++,item.getName());
                     String fileName = new File(item.getName()).getName();
                     String contentType = item.getContentType();
                     boolean isInMemory = item.isInMemory();
                     long sizeInBytes = item.getSize();
                     String path=filepath+File.separator+fileName;
                     ins.setString(i++,path);
                     file=new File(path);
                     item.write(file) ;
                  }
               }
               int n=ins.executeUpdate();
               if(n>0)
               {
                  response.getWriter().write(n+" record entered");
               }
               if(con!=null)
               {
                  con.close();
               }
            }
            catch(Exception e)
            {
                System.out.println(e);
            }
        }
    }
}