想学一下struts1，求指点

public class HelloForm extends ActionForm { private static final long serialVersionUID = 1L;
private String username; public String getUsername() {
return username;
} public void setUsername(String username) {
this.username = username;
} @Override
public ActionErrors validate(ActionMapping mapping,
HttpServletRequest request) {
ActionErrors ae = new ActionErrors();
System.out.println(username);
if(username==null||username.length()==0){
ae.add("hello_error",new ActionMessage("error.hello.username"));
}
if(ae.isEmpty()){
return null;
}
return ae;
}

}
public class UsernameAction extends Action { @Override
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response)
throws Exception {
System.out.println(111);
HelloForm hf = (HelloForm) form;
System.out.println(hf.getUsername());
ActionMessages am = new ActionMessages();
if(hf.getUsername().equals("god")){
am.add("action_error", new ActionMessage("error.action.username"));
this.saveErrors(request, am);

return mapping.getInputForward();
}
return mapping.findForward("success");
}
}
java.lang.ClassCastException: com.pb.action.HelloForm cannot be cast to org.apache.struts.action.Action
at org.apache.struts.chain.commands.servlet.CreateAction.createAction(CreateAction.java:98)
at org.apache.struts.chain.commands.servlet.CreateAction.getAction(CreateAction.java:68)
at org.apache.struts.chain.commands.AbstractCreateAction.execute(AbstractCreateAction.java:91)
at org.apache.struts.chain.commands.ActionCommandBase.execute(ActionCommandBase.java:51)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at org.apache.commons.chain.generic.LookupCommand.execute(LookupCommand.java:305)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at org.apache.struts.chain.ComposableRequestProcessor.process(ComposableRequestProcessor.java:283)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:449)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)
2011-7-20 19:20:55 org.apache.struts.chain.commands.ExceptionCatcher postprocess
警告: Exception from exceptionCommand 'servlet-exception'
java.lang.ClassCastException: com.pb.action.HelloForm cannot be cast to org.apache.struts.action.Action
at org.apache.struts.chain.commands.servlet.CreateAction.createAction(CreateAction.java:98)
at org.apache.struts.chain.commands.servlet.CreateAction.getAction(CreateAction.java:68)
at org.apache.struts.chain.commands.AbstractCreateAction.execute(AbstractCreateAction.java:91)
at org.apache.struts.chain.commands.ActionCommandBase.execute(ActionCommandBase.java:51)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at org.apache.commons.chain.generic.LookupCommand.execute(LookupCommand.java:305)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at org.apache.struts.chain.ComposableRequestProcessor.process(ComposableRequestProcessor.java:283)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:449)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)
2011-7-20 19:20:55 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet action threw exception
java.lang.ClassCastException: com.pb.action.HelloForm cannot be cast to org.apache.struts.action.Action
at org.apache.struts.chain.commands.servlet.CreateAction.createAction(CreateAction.java:98)
at org.apache.struts.chain.commands.servlet.CreateAction.getAction(CreateAction.java:68)
at org.apache.struts.chain.commands.AbstractCreateAction.execute(AbstractCreateAction.java:91)
at org.apache.struts.chain.commands.ActionCommandBase.execute(ActionCommandBase.java:51)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at org.apache.commons.chain.generic.LookupCommand.execute(LookupCommand.java:305)
at org.apache.commons.chain.impl.ChainBase.execute(ChainBase.java:191)
at org.apache.struts.chain.ComposableRequestProcessor.process(ComposableRequestProcessor.java:283)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1913)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:449)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)