a)一个Web应用可以有0个或多个Filter，多个Filter的组合就是过滤器链
b)多个Filter的执行先后顺序，与web.xml文件中配置的顺序有关
c)chain.doFilter(request,response)具有二义性：

>如果有下一个Filter时，将请求转发给下一个Filter
>如果无下一个Filter时，将请求转发给Web资源(serlvet/jsp/html)
d)可以将web资源中的一些公共代码，提取出来，放入Filter中

`<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><filter>    <filter-name>FilterDemo2</filter-name>    <filter-class>cn.web.servlet.filter.FilterDemo2</filter-class></filter><filter-mapping>    <filter-name>FilterDemo2</filter-name>    <url-pattern>/*</url-pattern></filter-mapping>``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("text/html; charset=UTF-8");    response.getWriter().write("欢迎学习Servlet下篇");}

}`

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;public class FilterDemo2 implements Filter {    public void destroy() {    }    public void doFilter(ServletRequest request, ServletResponse response,FilterChain chain) throws IOException, ServletException {        response.setContentType("text/html;charset=UTF-8");        System.out.println("第二个拦截器：Filter2");        //放行请求[调用]        chain.doFilter(request,response);    }    public void init(FilterConfig filterConfig) throws ServletException {    }}