判断用户是否登录，如果登录了放行，否则跳转到登陆页面

package elkw.action;
import javax.servlet.Filter;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.FilterChain;
import java.io.IOException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpServletResponse;
import elkw.po.system.User;

public class RightFilter
implements Filter {
public void init(FilterConfig filterConfig) throws ServletException
{
}

public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException
{
HttpServletRequest req = (HttpServletRequest) request;
HttpServletResponse res = (HttpServletResponse) response;
HttpSession session = req.getSession(true);
//从session里取的用户名信息
User username = (User) session.getAttribute("user");
String reuris = req.getRequestURI(); // 获取访问路径
String reuri = reuris.substring(0, 17); //截取0-17的字符
String loginAction = "/backserver/login";//定义登陆的地址
//判断如果没有取到用户信息,就跳转到登陆页面
String ipad = "/backserver/jsond";

if (username == null || "".equals(username)) {
//ipad请求数据和登陆页面不过滤
if (reuri.equals(loginAction) || reuri.equals(ipad))
{
chain.doFilter(request,response);
return;
}
//跳转到登陆页面
res.sendRedirect("http://"+req.getHeader("Host")+"/backserver");
}
else {
//已经登陆,继续此次请求
chain.doFilter(request,response);
}
}

public void destroy() {
}
}

<!-- 判断用户是否登录 ,过滤action的请求(你也可以写 (*.jsp) ,(*.html),(/*)全局的 ,)。。。。 -->
<filter>
<filter-name>right</filter-name>
<filter-class>你的类名</filter-class>
</filter>
<filter-mapping>
<filter-name>right</filter-name>
<url-pattern>*.action</url-pattern><!-- 拦截-->
</filter-mapping>