/**
 * 用于检测用户是否登陆的过滤器，如果未登录，则重定向到指的登录页面 
 * 配置参数 
 * checkSessionKey 需检查的在 Session 中保存的关键字
 * redirectURL 如果用户未登录，则重定向到指定的页面，URL不包括 ContextPath 
 * notCheckURLList 不做检查的URL列表，以分号分开，并且 URL 中不包括 ContextPath
 */

public class LoginFilter implements Filter {

    protected FilterConfig filterConfig = null;
    private String redirectURL = null;
    private List notCheckURLList = new ArrayList();
    private String sessionKey = null;

    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
        redirectURL = filterConfig.getInitParameter("redirectURL");
        sessionKey = filterConfig.getInitParameter("checkSessionKey");

        String notCheckURLListStr = filterConfig.getInitParameter("notCheckURLList");

        if (notCheckURLListStr != null) {
            StringTokenizer st = new StringTokenizer(notCheckURLListStr, ";");
            notCheckURLList.clear();
            while (st.hasMoreTokens()) {
                notCheckURLList.add(st.nextToken());
            }
        }
    }

    public void doFilter(ServletRequest servletRequest,
            ServletResponse servletResponse, FilterChain filterChain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        HttpSession session = request.getSession();
        if (sessionKey == null) {
            filterChain.doFilter(request, response);
            return;
        }
        if ((!checkRequestURIIntNotFilterList(request))
                && session.getAttribute(sessionKey) == null) {
            response.sendRedirect(request.getContextPath() + redirectURL);
            return;
        }
        filterChain.doFilter(servletRequest, servletResponse);
    }

    public void destroy() {
        notCheckURLList.clear();
    }

    private boolean checkRequestURIIntNotFilterList(HttpServletRequest request) {
        String uri = request.getServletPath()
                + (request.getPathInfo() == null ? "" : request.getPathInfo());
        return notCheckURLList.contains(uri);
    }
}

    <filter>
        <filter-name>Set Character Encoding</filter-name>
        <filter-class>com.yj