a)Filter是SUN公司提供的一个资源过滤器接口，不同的Web容器有着不同的实现
b)Filter位于Web服务器和Web资源（Servlet/Jsp/Html）之间
c)过滤请求和响应二者
d)Filter可以进行简单判段，是否将请求放行给Web资源
e)Filter的开发过程：

>类 implements javax.servlet.Filter接口
>在web.xml文件配置Filter过滤器，告之Web服务器有过滤器的存在
web.xml中的配置信息如下：
<filter>
<filter-name>FilterDemo1</filter-name>(过滤器，可以随意，但要和filter-mapping中的name一致)
<filter-class>cn.web.filter.FilterDemo1</filter-class>(过滤器全路径)
</filter>
<filter-mapping>
<filter-name>FilterDemo1</filter-name>(过滤器名，同上)
<url-pattern>/*</url-pattern>(过滤器能够过滤的资源路径，不是用户在URL中访问的路径)
</filter-mapping>

ShowServlet

import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ShowServlet extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        response.setContentType("test/html; charset=UTF-8");        response.getWriter().write("欢迎学习servlet下篇");    }}

FilterDemo1

import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;//自定义Filter过渡器public class FilterDemo1 implements Filter {    public void destroy() {    }    public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {        System.out.println("过滤器+++doFilter()");        //放行请求[类似于函数调用]        chain.doFilter(request,response);        System.out.println("放行请求[类似于函数调用]");    }    public void init(FilterConfig filterConfig) throws ServletException {    }}

web.xml配置

<?xml version="1.0" encoding="UTF-8"?><web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee"    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">    <filter>        <filter-name>FilterDemo1</filter-name>        <filter-class>cn.web.servlet.filter.FilterDemo1</filter-class>    </filter>    <filter-mapping>        <filter-name>FilterDemo1</filter-name>        <url-pattern>/*</url-pattern>    </filter-mapping>    </web-app>