java.lang.IllegalArgumentException: No bean specified

[束建荣]

Hi, 
I am writing a web application based on Struts and encountered a problem. When I ran 
it on Tomcat, I got the following exception:
java.lang.IllegalArgumentException: No bean specified
        
org.apache.commons.beanutils.PropertyUtils.getPropertyDescriptor(PropertyUtils.java:837)
        org.apache.commons.beanutils.BeanUtils.setProperty(BeanUtils.java:934)
        org.apache.commons.beanutils.BeanUtils.populate(BeanUtils.java:808)
        org.apache.struts.util.RequestUtils.populate(RequestUtils.java:1252)
        
org.apache.struts.action.RequestProcessor.processPopulate(RequestProcessor.java:821)
        org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:254)
        org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
        org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
        org.apache.struts.action.RequestProcessor.doForward(RequestProcessor.java:1069)
        
org.apache.struts.action.RequestProcessor.processForwardConfig(RequestProcessor.java:455)
        org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:279)
        org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
        org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:810)I have correctly 
set the action mappings.
What are the possible causes of this exception?Any help is highly appreciated!