java web的html中调用servlet怎么显示空白页，点击登录后跳转过LoginServlet,但是页面显示空白。怎么回事？

package servlets;
import java.io.IOException;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.*;
public class LoginServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

doPost(request,response);

}
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String user=req.getParameter("user");
RequestDispatcher rd=null;
if(user!=null&&user.equals("admin")){
req.getSession().setAttribute("login", "true");
rd=req.getRequestDispatcher("/admin.html");
rd.forward(req, resp);}
else if(user!=null&&user.equals("common")){
req.getSession().setAttribute("login", "true");
rd=req.getRequestDispatcher("/common.html");
rd.forward(req, resp);
}
else{

resp.sendRedirect("/ch08/login.html");

}

}

}
web的配置
<servlet>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>servlets.LoginServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/servlet/LoginServlet</url-pattern>
</servlet-mapping>
<filter>
<filter-name>LoginValidationFilter</filter-name>
<filter-class>servlets.LoginValidationFilter</filter-class>
</filter>
<filter-mapping>
login.html
<form method="post" action="/ch08/servlet/LoginServlet">
<table bordercolor="blue">
<tr align="center"><td colspan="2">选择用户</td></tr>
<tr align="center"><td width="75%">
<input type="radio" name="user" value="admin">管理员
<input type="radio" name="user" value="common">普通用户
</td>
<td>
<input type="submit" name="login" value="登录">
</td>
</tr>
</table>
</form>