老师关于if (loginUser == null)的问题

和视频中操作一样  登陆页面没有输入任何文字。直接转到了message页面。过滤器不起作用怎么回事？

package filter;

import java.io.IOException;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class SessionFilter implements Filter {

@Override

public void destroy() {

// TODO Auto-generated method stub

}

@Override

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)

throws IOException, ServletException {

HttpServletRequest hrequest=(HttpServletRequest) request;

HttpServletResponse hresponse=(HttpServletResponse) response;

String loginUser=(String)hrequest.getSession().getAttribute("loginUser");

if(loginUser==null) {

hresponse.sendRedirect(hrequest.getContextPath()+"/index.jsp?flag=1");

return;

}else {

chain.doFilter(hrequest, hresponse);

return;

}

}

@Override

public void init(FilterConfig filterConfig) throws ServletException {

// TODO Auto-generated method stub

}

}

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">

  <display-name>FilterDemo</display-name>

  <welcome-file-list>

    <welcome-file>index.jsp</welcome-file>

  </welcome-file-list>

 <!-- 字符集编码过滤器配置 -->

<filter>

<filter-name>characterEncodingFilter</filter-name>

<filter-class>filter.CharacterEncodingFilter</filter-class>

<init-param>

<param-name>charset</param-name>

<param-value>UTF-8</param-value>

</init-param>

</filter>

<filter-mapping>

<filter-name>characterEncodingFilter</filter-name>

<url-pattern>/*</url-pattern>

</filter-mapping>

<!-- 用户登录安全控制过滤器配置 -->

<filter>

<filter-name>sessionFilter</filter-name>

<filter-class>filter.SessionFilter</filter-class>

</filter>

<filter-mapping>

<filter-name>sessionFilter</filter-name>

<url-pattern>/message.jsp</url-pattern>

</filter-mapping>

</web-app>

麻烦老师帮我看一下