登陆时的问题

用strusts框架做的系统，comcat容器，配置都写好了，为什么在登陆提交后出这样的错误，问题在哪里呢？type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException: Servlet execution threw an exception
filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:123)
root cause java.lang.NoClassDefFoundError
action.LoginAction.execute(LoginAction.java:81)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:123)
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.9 logs.