1. 我们可以在web.xml中配置filter来对指定的URL进行过滤，进行一些特殊操作如权限验证等。

<!– session过滤filter –>
<filter>
    <filter-name>SessionFilter</filter-name>
    <filter-class>com.xm.chris.SessionFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SessionFilter</filter-name>
    <url-pattern>/resources/*</url-pattern>
</filter-mapping>
public class SessionFilter implements Filter {
    private static final String CONTENT_TYPE = "text/html; charset=UTF-8";
    private FilterConfig _filterConfig = null;

    public void init(FilterConfig filterConfig) throws ServletException {
        _filterConfig = filterConfig;
    }

    public void destroy() {
        _filterConfig = null;
    }

    public void doFilter(ServletRequest request, ServletResponse response,
                         FilterChain chain) throws IOException,
            ServletException {
        HttpServletRequest rq = (HttpServletRequest) request;
        HttpSession httpSession = rq.getSession();
        Long userId = (Long) httpSession.getAttribute("userId");
        if (userId == null) {
            response.setContentType(CONTENT_TYPE);
            PrintWriter out = response.getWriter();
            out.println("<html>");
            out.println("<head><title>Error</title></head>");
            out.println("<body>");
            out.println("<p id='Message'>错误.</p>");
            out.println("</body></html>");
            out.close();
        } else {
            chain.doFilter(request, response);
        }

    }
}

 这时所有请求了contextPath/resources/*路径的request都会被SessionFilter验证是否登录。

2. 但是我们有一些特定的url不想验证登录，想要直接能够访问，怎么办呢？

这时可以配置一个参数，告诉Filter哪些url不想验证。

<filter>
    <filter-name>SecurityFilter</filter-name>
    <filter-class>com.oracle.ccsc.jcs.sx.filter.SecurityFilter</filter-class>
    <init-param>
        <param-name>excludedPages</param-name>
        <param-value>/xm/portal/notice</param-value>
    </init-param>
</filter>

 然后在Filter中就可以根据参数判断是否需要过滤。

public class SecurityFilter implements Filter {
    private static final String CONTENT_TYPE = "text/html; charset=UTF-8";
    private FilterConfig _filterConfig = null;

    private String excludedPages;
    private String[] excludedPageArray;

    public void init(FilterConfig filterConfig) throws ServletException {
        _filterConfig = filterConfig;

        excludedPages = filterConfig.getInitParameter("excludedPages");
        if (StringUtils.isNotEmpty(excludedPages)) {
            excludedPageArray = excludedPages.split(",");
        }
    }

    public void destroy() {
        _filterConfig = null;
    }

    public void doFilter(ServletRequest request, ServletResponse response,
                         FilterChain chain) throws IOException,
            ServletException {
        HttpServletRequest rq = (HttpServletRequest) request;
  
        boolean isExcludedPage = false;
        for (String page : excludedPageArray) { //判断是否在过滤url之外if (rq.getPathInfo().equals(page)) {
                isExcludedPage = true;
                break;
            }
        }
        if (isExcludedPage) { //在过滤url之外
            chain.doFilter(request, response);
        } else { //不在过滤url之外，判断登录
            HttpSession httpSession = rq.getSession();
            Long userId = (Long) httpSession.getAttribute("userId");
            if (userId == null) {
                response.setContentType(CONTENT_TYPE);
                PrintWriter out = response.getWriter();
                out.println("<html>");
                out.println("<head><title>Error</title></head>");
                out.println("<body>");
                out.println("<p id='Message'>错误.</p>");
                out.println("</body></html>");
                out.close();
            } else {
                chain.doFilter(request, response);
            }
        }
    }
}

3. 关于用Servlet获取URL地址。

在HttpServletRequest类里，有以下六个取URL的函数

getContextPath 取得项目名 
getServletPath 取得Servlet名 
getPathInfo 取得Servlet后的URL名，不包括URL参数 
getRequestURL 取得不包括参数的URL 
getRequestURI 取得不包括参数的URI，即去掉协议和服务器名的URL 

具体如下图：

相对应的函数的值如下： 

getContextPath：/ServletTest 
getServletPath：/main 
getPathInfo：/index/testpage/test 
getRequestURL：http://localhost:8080/ServletTest/main/index/testpage/test 
getRequestURI：/ServletTest/main/index/testpage/test