Servlet 出错: java.lang.NoClassDefFoundError

sylssgw

环境：JBuilder2006+tomcat5.5+JDK1.5
编译能通过
但是访问网页是报错，出错信息如下：
type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Servlet execution threw an exception


root cause

java.lang.NoClassDefFoundError: javax/servlet/ServletInputStream
        org.apache.commons.fileupload.FileUploadBase.parseRequest(FileUploadBase.java:26
        sgwweb.upload.doPost(upload.java:27)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


note The full stack trace of the root cause is available in the Apache Tomcat/5.5.17 logs.

为什么会类找不到？