求助： request.getRequestDispatcher().forward(request, response) 无法跳转

小弟问个菜鸟问题：

在Servlet里用request.getRequestDispatcher().forward(request, response) 无法跳转到指定的页面。
这是一个用于登录验证用的，但是密码和用户名都输入正确时，还是跳转到 Login.jsp,希望高手帮忙解决问题

源文件：

package ernst.MVCBBS.control;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@SuppressWarnings("serial")
public class LoginServlet extends HttpServlet {


/**
* Constructor of the object.
*/
public LoginServlet() {
super();
}

/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}

/**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

String action = request.getParameter("action");

if(action != null && action.equals("login")) {
String userName = request.getParameter("userName");
String userPwd = request.getParameter("userPwd");
if(userName == null || !userName.equals("admin")) {
request.setAttribute("error", "NOUSER");
request.getRequestDispatcher("Login.jsp").forward(request, response);
} else if(userPwd == null || !userPwd.equals("admin")) {
request.setAttribute("error", "PWD");
request.getRequestDispatcher("Login.jsp").forward(request, response);
} else {
request.getSession().setAttribute("user", userName);
request.getRequestDispatcher("ArticleTreeShow.jsp").forward(request, response);
}
}
}

/**
* Initialization of the servlet. <br>
*
* @throws ServletException if an error occurs
*/
public void init() throws ServletException {
// Put your code here
}

}