Struts+Hibernate 做上传图片时的错误

这个是userlist.jsp,当点击上传相册时提示一个错误。用的自己参考struts上传包写个一个上传方法。

<form action="fileUpload.do?user_id='${user.id }'" method="post" enctype="multipart/form-data">
<%
 User list = (User) request.getSession().getAttribute("user");
 if (list.getIs_admin() == 1)
 {
%>
<table width=500 border=1 align='center'>
 <tr bgcolor='#cccccc'>
  <td>登录名</td>
  <td>用户类型</td>
  <td>照片</td>
 </tr>
 <%
  UserService us = new UserService();
   for (User user : us.userlist())
   {
 %>
 <tr>
  <td><%=user.getLoginName()%></td>
  <td><%=user.getIs_admin() == 1 ? "Administrator"
       : "user"%></td>
  <td>  
  <a href="fileUpload.do?user_id=<%=user.getId() %>">上传相册</a>
  </td>
 </tr>
 <%
  }
 %>
</table>
</form>

下面的是UserListAction.java

if (user_id != null)
  {
   User user = (User) service.getById(Integer.parseInt(user_id));
   request.setAttribute("user", user);
   
   FileUpload upload = new FileUpload();
   upload.uploadByApacheCommonsFileupload(request, savePath); //此法是自己写的
   user.setName(upload.getParameter("name"));
   user.setLoginName(upload.getParameter("loginName"));
   Upload up = upload.getUploadedFile("photo");
   if (up != null)
   {
    Photo p = new Photo();
    p.setUser(user);
    p.setPath(up.getFile().getName());
    service.addPhoto(p);
    user.getPhoto().add(p);
    service.updateUser(user);
   }
  }

FileUpload

public void uploadByApacheCommonsFileupload(HttpServletRequest request,
   String saveDir) throws FileUpDownException
 {
  if (ServletFileUpload.isMultipartContent(request)) //当运行到此的时候抛出异常
  {
   // 准备工作
   File tempDir = new File(System.getProperty("java.io.tmpdir"));// 临时文件目录
   System.out.println(tempDir.getAbsolutePath());
   DiskFileItemFactory factory = new DiskFileItemFactory();
   factory.setSizeThreshold(1024 * 1024);// 缓存大小 字节
   factory.setRepository(tempDir);
   ServletFileUpload sfu = new ServletFileUpload(factory);
   sfu.setHeaderEncoding(encoding);

异常信息：

严重: Servlet.service() for servlet action threw exception
alpha.fileUpload.FileUpDownException: 表单格式不能支持文件上传
 at alpha.fileUpload.FileUpload.uploadByApacheCommonsFileupload(FileUpload.java:323)
 at action.UserListAction.execute(UserListAction.java:51)
 at org.apache.struts.chain.commands.servlet.ExecuteAction.execute(ExecuteAction.java:58)
 at org.apache.struts.chain.commands.AbstractExecuteAction.execute(AbstractExecuteAction.java:67)
 at org.apache.struts.chain.commands.ActionCommandBase.execute(ActionCommandBase.java:51)
 at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:190)
 at org.apache.commons.chain.generic.LookupCommand.execute