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