Filter作用 ：
 每當使用應用服務器頁面或servlet的時候，應用服務器就會根據web.xml里面配置的filter 去進行過濾理。
 
調用處理過程就是：
 1.接受request->去處理第一個filter.init-> 處理第一個filter.doFilter
 2.如果不想繼續使用filter過濾就在filter.doFilter中使用 return
  3.如果要繼續使用filter過濾就filter.doFilter中 chain.doFilter(servletRequest, servletResponse);

使用例子：
  1.web.xml配置
  <filter>
        <filter-name>Filter_name</filter-name>
        <filter-class>Filter_class_name</filter-class>
        <init-param>
            <description>set 'true' to enable the filter</description>
            <param-name>enabled</param-name>
            <param-value>true</param-value>
        </init-param>
  <init-param>
         <param-name>excepts</param-name>
   <param-value>Excepts_Path</param-value>
        </init-param>
    </filter>
 <filter-mapping>
     <filter-name>Filter_name</filter-name>
     <url-pattern>filter_path</url-pattern>
    </filter-mapping>
    <filter-mapping>
        <filter-name>Filter_name</filter-name>
        <url-pattern>filter_path</url-pattern>
    </filter-mapping>

   2、java代碼
 
 2.1 init 函數
 public void init(FilterConfig filterConfig) throws ServletException {

        //檢查是否打開SSOFilter功能
        String _enable = filterConfig.getInitParameter("enabled");
        if (_enable != null && "true".equalsIgnoreCase(_enable)) {
            enabled = true;
        }

        //初始化例外列表，即不經過SSOFilter的請求地址
        String _exceptStr = filterConfig.getInitParameter("excepts");
        if (_exceptStr != null) {
            String[] _excepts = _exceptStr.split(",");
            for (int i = 0; i < _excepts.length; i ++) {
                excepts.add(_excepts[i]);
            }
        }
      }
 
 2.2 doFilter函數
 public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse,
                         FilterChain chain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        String path = request.getServletPath();

        /**
         * 當WEB程序啟了SSOFilter，并且請求的路徑沒被配置為例外時，
         * 檢查用戶是否已通過統一用戶平臺登錄，已登錄的話創建會話，否則轉到統一登錄頁面。
         */
        if (enabled && !excepts.contains(path)) {

            if (logger.isDebugEnabled()) {
                logger.debug("check member has signoned ...");
            }

            boolean signOned = SSOHelper.hasSignOned(request, response);

            if (logger.isDebugEnabled()) {
                logger.debug("Signoned? " + signOned);
            }

            if (!signOned) {
                String SSOContextUri = SysParamHelper.getSysParam(SysParamConstants.CONTEXT_URI_SSO);
                StringBuffer url = new StringBuffer(SSOContextUri);
                url.append("/Login");
                //記住入口地址
                url.append("?entrance=");
                url.append(request.getRequestURL());
                if (request.getQueryString() != null) {
                    url.append("|");
                    url.append(request.getQueryString());
                }
                response.sendRedirect(url.toString());
                return;
            }
        }
         chain.doFilter(servletRequest, servletResponse);
       }