问个文件上传的问题

运行到下面这步的时候报了错，请问是什么原因啊
try {
items = upload.parseRequest(request);
     } catch (FileUploadException e1) {
e1.printStackTrace();
}java.lang.NoClassDefFoundError: org/apache/commons/io/output/DeferredFileOutputStream
org.apache.commons.fileupload.disk.DiskFileItemFactory.createItem(DiskFileItemFactory.java:191)
org.apache.commons.fileupload.FileUploadBase.parseRequest(FileUploadBase.java:350)
org.apache.commons.fileupload.servlet.ServletFileUpload.parseRequest(ServletFileUpload.java:126)
com.ebidding.admin.servlet.UploadTest.doPost(UploadTest.java:36)
javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)