采用ajax实现无刷新文件上传

现自己做个ajax的jquery组件实现无刷新文件上传，现在遇到一个问题，贴出如下代码，
Java code:

package servlet;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.*;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import org.apache.commons.fileupload.*;
import org.apache.commons.fileupload.disk.*;
import org.apache.commons.fileupload.servlet.*;
import entity.User;

public class FileUploadServlet extends HttpServlet {
/**
* Constructor of the object.
*/
public FileUploadServlet() {
super();
}
/**
* Destruction of the servlet. <br>
*/
public void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=gbk");
PrintWriter out = response.getWriter();
boolean flag=false;String file="";
String msg="";String error="";String filename="";
HttpSession session=request.getSession();
User u=new User();
if(session.getAttribute("user")!