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;
import javax.servlet.http.HttpSession;
public class LoginFilter implements Filter {
 @Override
 public void init(FilterConfig filterConfig) throws ServletException {
  // TODO Auto-generated method stub
 }
 @Override
 public void doFilter(ServletRequest request, ServletResponse response,
   FilterChain chain) throws IOException, ServletException {
  // 获得在下面代码中要用的request,response,session对象
  HttpServletRequest servletRequest = (HttpServletRequest) request;
  HttpServletResponse servletResponse = (HttpServletResponse) response;
  HttpSession session = servletRequest.getSession();
  // 获得用户请求的URI
  String path = servletRequest.getRequestURI();
  //System.out.println(path);
  // 从session里取员工工号信息
  String empId = (String) session.getAttribute("empId");
  /*创建类Constants.java，里面写的是无需过滤的页面
  for (int i = 0; i < Constants.NoFilter_Pages.length; i++) {
   if (path.indexOf(Constants.NoFilter_Pages[i]) > -1) {
    chain.doFilter(servletRequest, servletResponse);
    return;
   }
  }*/
  // 登陆页面无需过滤
  if(path.indexOf("/login.jsp") > -1) {
   chain.doFilter(servletRequest, servletResponse);
   return;
  }
  // 判断如果没有取到员工信息,就跳转到登陆页面
  if (empId == null || "".equals(empId)) {
   // 跳转到登陆页面
   servletResponse.sendRedirect("/JingXing_OA/login.jsp");
  } else {
   // 已经登陆,继续此次请求
   chain.doFilter(request, response);
  }
 }
 @Override
 public void destroy() {
  // TODO Auto-generated method stub
 }
}

<!-- 配置登陆过滤器 -->
<filter>
  <filter-name>login</filter-name>
  <filter-class>com.jingxing.oa.filter.LoginFilter</filter-class>
 </filter>
 <filter-mapping>
  <filter-name>login</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

<!-- 配置登陆过滤器 -->
 <filter>
  <filter-name>login</filter-name>
  <filter-class>com.jingxing.oa.filter.LoginFilter</filter-class>
 </filter>
 <filter-mapping>
  <filter-name>login</filter-name>
  <url-pattern>/a/*</url-pattern>
 </filter-mapping>