过滤器在web.xml中的配置　

<!-- 添加系统过滤器 0914-->
<filter>
<filter-name>systemFilter</filter-name>
<filter-class>cn.itcast.elec.util.SystemFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>systemFilter</filter-name>
<url-pattern>*.do</url-pattern>
<url-pattern>*.jsp</url-pattern>
</filter-mapping>

过滤器的java代码

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

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;

import cn.itcast.elec.domain.ElecUser;

public class SystemFilter implements Filter {

private List<String> list = new ArrayList<String>();
/**初始化的时候启动init*/
public void init(FilterConfig arg0) throws ServletException {
//定义可以进行放行的连接，用List集合存储放行的连接
list.add("/index.jsp");
list.add("/image.jsp");
list.add("/system/elecMenuAction_home.do");
list.add("/error.jsp");
list.add("/system/elecMenuAction_logouterror.do");
}

/**当访问每一个连接的时候，先执行过滤器的doFilter方法*/
public void doFilter(ServletRequest req, ServletResponse res,
FilterChain chain) throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
//使用当前访问的链接与放行list的连接进行比较，如果匹配成功，就放行
//当前访问的链接
String path = request.getServletPath();
if(list!=null && list.contains(path)){
chain.doFilter(request, response);
return;
}
//主要验证系统session是否存在，如果存在则放行，不存在就直接跳转到登录页面
ElecUser elecUser = (ElecUser) request.getSession().getAttribute("globle_user");
//如果session中存在用户信息，则表示正常访问，此时需要放行
if(elecUser!=null){
chain.doFilter(request, response);
return;
}
//重定向到登录页面
//response.sendRedirect(request.getContextPath()+"/");
//使用error.jsp实现倒计时功能每隔5秒钟，自动调整到登录页面
response.sendRedirect(request.getContextPath()+"/error.jsp");
}

public void destroy() {


}

}