File upload problem using Apache commans File upload API in JSP/Servlet-Collection of common programming errors

im trying to upload a file using org.apache.commons.fileupload. but i dont no, what mistake i have made im getting the following error in my servlet. please any one help me on this..this is the error im getting.

     javax.servlet.ServletException: Servlet execution threw an exception


 root cause 

 java.lang.NoClassDefFoundError: org/apache/commons/io/output/DeferredFileOutputStream
org.apache.commons.fileupload.disk.DiskFileItemFactory.createItem(DiskFileItemFactory.java:199)
org.apache.commons.fileupload.FileUploadBase.parseRequest(FileUploadBase.java:361)
org.apache.commons.fileupload.servlet.ServletFileUpload.parseRequest(ServletFileUpload.java:126)
upload1.doPost(upload1.java:34)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
     javax.servlet.http.HttpServlet.service(HttpServlet.java:717)

this is my servlet code

   if (ServletFileUpload.isMultipartContent(req)) {

         // Create a factory for disk-based file items
         FileItemFactory factory = new DiskFileItemFactory();

         // Create a new file upload handler
         ServletFileUpload upload = new ServletFileUpload(factory);

         // Parse the request
         try {
             List items = upload.parseRequest(req);
             for (FileItem item : items) {
                 // process only file upload - discard other form item types
                 if (item.isFormField()) continue;

                 String fileName = item.getName();
                 // get only the file name not whole path
                 if (fileName != null) {
                    // fileName = FilenameUtils. getName(fileName);
                 }


             }
         } catch (Exception e) {
             res.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
                     "An error occurred while creating the file : " + e.getMessage());
         }

     } else {
         res.sendError(HttpServletResponse.SC_UNSUPPORTED_MEDIA_TYPE,
                         "Request contents type is not supported by the servlet.");
     }

and form

thank u