调用DAO的问题，大家帮忙解决呀，

Question

我在jsp页面中这样调用的:
Page page1 = userlist.getUserList(1,10);
但是出现下列错误：
java.lang.NullPointerException
at org.struts.manage.beanUser.getUserList(beanUser.java:20)
at _pages._manage._user_0list__jsp._jspService(E:\strtus_hftax\webapps\hftax\pages\manage\user_list.jsp:14)
at com.caucho.jsp.JavaPage.service(JavaPage.java:75)
at com.caucho.jsp.Page.subservice(Page.java:497)
at com.caucho.server.http.FilterChainPage.doFilter(FilterChainPage.java:182)
at com.caucho.server.http.Invocation.service(Invocation.java:315)
at com.caucho.server.http.CacheInvocation.service(CacheInvocation.java:135)
at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:246)
at com.caucho.server.http.HttpRequest.handleConnection(HttpRequest.java:163)
at com.caucho.server.TcpConnection.run(TcpConnection.java:139)
at java.lang.Thread.run(Thread.java:536)
大家帮忙看看怎么回事呀，这里先谢了

Answer 1

忘了说明userlist是Bean：beanUser

Answer 2

UserDAOFactory 代码：
public class UserDAOFactory { public static UserDAO getUserDAO() {
UserDAO userDao = null;
String driverClasses = "com.microsoft.jdbc.sqlserver.SQLServerDriver";
try {
  Class.forName(driverClasses);
                                       userDao = new GenericUserDAO();
} catch (ClassNotFoundException cnfe) {
} 
}
return userDao;
}}

Answer 3

最好把GenericUserDAO的构造方法定义成protected