我的过滤器哪里写错了？

<filter>

        <filter-name>SetCharacterEncodingFilter</filter-name>

        <filter-class>

            tools.SetCharacterEncodingFilter

        </filter-class>

        <init-param>

            <param-name>encoding</param-name>

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

        </init-param>

    </filter>



    <filter-mapping>

        <filter-name>SetCharacterEncodingFilter</filter-name>

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

    </filter-mapping>



    <filter-mapping>

        <filter-name>SetCharacterEncodingFilter</filter-name>

        <url-pattern>*.do</url-pattern>

    </filter-mapping>

public class SetCharacterEncodingFilter implements Filter {



    protected FilterConfig filterConfig;

    protected String encodingName;

    protected boolean enable;



    public SetCharacterEncodingFilter() {

        encodingName = "UTF-8";

        enable = false;

    }



    // 初始化

    public void init(FilterConfig filterConfig) throws ServletException {

        this.filterConfig = filterConfig;

    }



    // 每个请求设置UTF-8

    public void doFilter(ServletRequest request, ServletResponse response,

            FilterChain chain) throws IOException, ServletException {

        request.setCharacterEncoding("UTF-8");



        HttpServletRequest httprequest = (HttpServletRequest) request;

          HttpServletResponse httpresponse = (HttpServletResponse) response;

          HttpSession session = httprequest.getSession();  

          AdminUser user=new AdminUser();

          try { //获得在session中所记录的isLogin属性，该属性由登录部分的代码写入   

           user = (ActingAdminUser) session.getAttribute("user");

           if (user!=null) //验证成功，继续处理

           {

            chain.doFilter(request, response);

           } else //验证不成功，让用户登录。

           {            

            String targetURL = httprequest.getContextPath()+"/login.jsp";

            httpresponse.sendRedirect(targetURL);        

           }

          } catch (Exception e) {

           e.printStackTrace();

          }

          

    }

    public void destroy() {

    }

}

我想实现判断session用户是否登录了
我配了这个，是在编码过滤器那里顺便加点session判断，但是我开个网页都卡在那里，好像在反复重定向，左下角闪烁正在打开"http://localhost:8080/test/login.jsp" 和 "正在重新定向到http://localhost:8080/test/login.jsp" 我写错哪里了？