配置過濾器

		
<filter>
    <filter-name>LoginFilter</filter-name>
    <filter-class>cn.netkiller.Filter</filter-class>
    <init-param>
        <param-name>username</param-name>
        <param-value>netkiller</param-value>
    </init-param>
</filter>


<filter>
      <filter-name>LoginFilter</filter-name>
      <filter-class>cn.netkiller.Filter</filter-class>
</filter>
<filter-mapping>
      <filter-name>LoginFilter</filter-name>
      <url-pattern>/*</url-pattern>
</filter-mapping>
		
		

實現 Filter 介面

		
public class LoginFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("init LoginFilter");
        //獲取Filter初始化參數
    	String username = filterConfig.getInitParameter("username");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        //把ServletRequest和ServletResponse轉換成真正的類型
        HttpServletRequest req = (HttpServletRequest)request;
        HttpSession session = req.getSession();

        //由於web.xml中設置Filter過濾全部請求，可以排除不需要過濾的url
        String requestURI = req.getRequestURI();
        if(requestURI.endsWith("login.jsp")){
            chain.doFilter(request, response);
            return;
        }

        //判斷用戶是否登錄，進行頁面的處理
        if(null == session.getAttribute("user")){
            //未登錄用戶，重定向到登錄頁面
            ((HttpServletResponse)response).sendRedirect("login.jsp");
            return;
        } else {
            //已登錄用戶，允許訪問
            chain.doFilter(request, response);
        }
    }

    @Override
    public void destroy() {
        System.out.println("destroy!!!");
    }
}