<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd" id="WebApp_ID" version="4.0">
  <display-name>filter</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  
  <filter>
  <filter-name>DeviceAdapterFilter</filter-name>
  <filter-class>com.zt.filter.DeviceAdapterFilter</filter-class>
  </filter>
  
  <filter-mapping>
  <filter-name>DeviceAdapterFilter</filter-name>
  <url-pattern>*.html</url-pattern>
  </filter-mapping>
</web-app>

package com.zt.filter;

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;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DeviceAdapterFilter implements Filter{

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		// TODO Auto-generated method stub
		
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
			throws IOException, ServletException {
		// TODO Auto-generated method stub
		HttpServletRequest req = (HttpServletRequest)request;
		HttpServletResponse res = (HttpServletResponse)response;
		/*
		 /index.html
		 PC: /desktop/index.html
		 MOBILE: /mobile/index.html
		 /test.html
		 PC: /desktop/test.html
		 MOBILE: /mobile/test.html
		 */
		String uri = req.getRequestURI();
		System.out.println("URI:" + uri);
		if(uri.startsWith("/desktop") || uri.startsWith("/mobile")) {
			chain.doFilter(request, response);
		}else {
			String userAgent = req.getHeader("user-agent").toLowerCase();
			String targetURI="";
			if(userAgent.indexOf("android")!=-1 || userAgent.indexOf("iphone") != -1) {
				targetURI = "/mobile" + uri;
				System.out.println("移动端设备正在访问,重新跳转URI:" + targetURI);
				res.sendRedirect(targetURI);
			}else {
				targetURI = "/desktop" + uri;
				System.out.println("PC端设备正在访问,重新跳转URI:" + targetURI);
				res.sendRedirect(targetURI);
			}
		}
	}

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		
	}

}

明明包名类名一模一样的，前面的也跟着老师的步骤一步步打下来，急急急