<!--拦截请求-->
    <filter>
        <filter-name>ManagerFilter</filter-name>
        <filter-class>com.white.filter.MangerFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>ManagerFilter</filter-name>
        <url-pattern>/pages/*</url-pattern>
        <url-pattern>/clientByueServlet</url-pattern>
        <url-pattern>/byueServlet</url-pattern>
    </filter-mapping>
</web-app>

package com.white.filter;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import java.io.IOException;


public class MangerFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException  {
        HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
        Object user = httpServletRequest.getSession().getAttribute("username");
        if (user == null){
            httpServletRequest.getRequestDispatcher("index.jsp").forward(servletRequest,servletResponse);
            System.out.println("已拦截请求");
        }else{
            filterChain.doFilter(servletRequest,servletResponse);
        }

    }

    @Override
    public void destroy() {

    }
}