struts1.2 actionform对象为空

刚接触struts1 ,做了一个简单的登陆例子,但是报空指针错误!断点调试了下actionform对象为空!一直没找出错误,求解。login.jsp页面代码
<div id="b">
    <form action="login.do" method="post">
        用户名：<input name="username" type="text" /><br />
        密  码 ：<input name="password" type="password" /><br />
        &nbsp;&nbsp;&nbsp;&nbsp;      
       <input name="ok" type="submit" value=""  style="background-image:url(images/66.gif);height:30px;width:80px;border:0;"/>
     </form>
</div>]action类：
public class LoginAction extends Action {
@Override
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response)
throws Exception {
// TODO Auto-generated method stub

userLogin u=(userLogin)form;
if(u.getUsername().equals("mandy")&&u.getPassword().equals("123"))
{
return mapping.findForward("index.jsp");
}
return super.execute(mapping, form, request, response);
}actionform类：public class userLogin extends ActionForm {
private String username;
private String password;
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}struts-config.xml文件配置：
<struts-config>
 
  <form-beans>
    <form-bean name="userLogin" type="com.mycompany.struts"></form-bean>
  </form-beans>
  <action-mappings>
     <action path="/login" type="com.mycompany.struts.LoginAction" name="userLogin">
     <forward name="success" path="/index.jsp"></forward>
     </action>
  </action-mappings>
  <message-resources parameter="com.mycompany.struts.ApplicationResources" />
</struts-config>web.xml配置<servlet>
    <servlet-name>action</servlet-name>
    <servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
    <init-param>
      <param-name>config</param-name>
      <param-value>/WEB-INF/struts-config.xml</param-value>
    </init-param>
    <init-param>
      <param-name>debug</param-name>
      <param-value>3</param-value>
    </init-param>
    <init-param>
      <param-name>detail</param-name>
      <param-value>3</param-value>
    </init-param>
    <load-on-startup>0</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>action</servlet-name>
    <url-pattern>*.do</url-pattern>
  </servlet-mapping>