javaweb学习总结(四十六)——Filter(过滤器)常见应用

一、统一全站字符编码

　　通过配置参数charset指明使用何种字符编码,以处理Html Form请求参数的中文问题

 1 package me.gacl.web.filter; 2  3 import java.io.IOException; 4 import javax.servlet.Filter; 5 import javax.servlet.FilterChain; 6 import javax.servlet.FilterConfig; 7 import javax.servlet.ServletException; 8 import javax.servlet.ServletRequest; 9 import javax.servlet.ServletResponse;10 import javax.servlet.http.HttpServletRequest;11 import javax.servlet.http.HttpServletRequestWrapper;12 import javax.servlet.http.HttpServletResponse;13 14 /**15 * @ClassName: CharacterEncodingFilter16 * @Description: 此过滤器用来解决全站中文乱码问题17 * @author: 孤傲苍狼18 * @date: 2014-8-31 下午11:09:3719 *20 */ 21 public class CharacterEncodingFilter implements Filter {22 23     private FilterConfig filterConfig = null;24     //设置默认的字符编码25     private String defaultCharset = "UTF-8";26 27     public void doFilter(ServletRequest req, ServletResponse resp,28             FilterChain chain) throws IOException, ServletException {29         30         HttpServletRequest request = (HttpServletRequest) req;31         HttpServletResponse response = (HttpServletResponse) resp;32         String charset = filterConfig.getInitParameter("charset");33         if(charset==null){34             charset = defaultCharset;35         }36         request.setCharacterEncoding(charset);37         response.setCharacterEncoding(charset);38         response.setContentType("text/html;charset="+charset);39         40         MyCharacterEncodingRequest requestWrapper = new MyCharacterEncodingRequest(request);41         chain.doFilter(requestWrapper, response);42     }43 44     public void init(FilterConfig filterConfig) throws ServletException {45         //得到过滤器的初始化配置信息46         this.filterConfig = filterConfig;47     }48     49     public void destroy() {50 51     }52 }53 54 /*55 1.实现与被增强对象相同的接口 56 2、定义一个变量记住被增强对象57 3、定义一个构造器，接收被增强对象58 4、覆盖需要增强的方法59 5、对于不想增强的方法，直接调用被增强对象（目标对象）的方法60  */61  62 class MyCharacterEncodingRequest extends HttpServletRequestWrapper{63     64     private HttpServletRequest request;65     public MyCharacterEncodingRequest(HttpServletRequest request) {66         super(request);67         this.request = request;68     }69     /* 重写getParameter方法70      * @see javax.servlet.ServletRequestWrapper#getParameter(java.lang.String)71      */72     @Override73     public String getParameter(String name) {74         75         try{76             //获取参数的值77             String value= this.request.getParameter(name);78             if(value==null){79                 return null;80             }81             //如果不是以get方式提交数据的，就直接返回获取到的值82             if(!this.request.getMethod().equalsIgnoreCase("get")) {83                 return value;84             }else{85                 //如果是以get方式提交数据的，就对获取到的值进行转码处理86                 value = new String(value.getBytes("ISO8859-1"),this.request.getCharacterEncoding());87                 return value;88             }89         }catch (Exception e) {90             throw new RuntimeException(e);91         }92     }93 }

　　web.xml文件中的配置如下：

 1  <filter> 2       <filter-name>CharacterEncodingFilter</filter-name> 3       <filter-class>me.gacl.web.filter.CharacterEncodingFilter</filter-class> 4       <init-param> 5           <param-name>charset</param-name> 6           <param-value>UTF-8</param-value> 7       </init-param> 8   </filter> 9   10   <filter-mapping>11       <filter-name>CharacterEncodingFilter</filter-name>12       <url-pattern>/*</url-pattern>13   </filter-mapping>

二、禁止浏览器缓存所有动态页面

　　有3 个HTTP 响应头字段都可以禁止浏览器缓存当前页面，它们在 Servlet 中的示例代码如下：

1 response.setDateHeader("Expires",-1);2 response.setHeader("Cache-Control","no-cache");3 response.setHeader("Pragma","no-cache"); 

　　并不是所有的浏览器都能完全支持上面的三个响应头，因此最好是同时使用上面的三个响应头。

• Expires数据头：值为GMT时间值，为-1指浏览器不要缓存页面
• Cache-Control响应头有两个常用值：
• no-cache指浏览器不要缓存当前页面。
• max-age:xxx指浏览器缓存页面xxx秒。

 1 package me.gacl.web.filter; 2  3 import java.io.IOException; 4  5 import javax.servlet.Filter; 6 import javax.servlet.FilterChain; 7 import javax.servlet.FilterConfig; 8 import javax.servlet.ServletException; 9 import javax.servlet.ServletRequest;10 import javax.servlet.ServletResponse;11 import javax.servlet.http.HttpServletRequest;12 import javax.servlet.http.HttpServletResponse;13 14 /**15 * @ClassName: NoCacheFilter16 * @Description: 禁止浏览器缓存所有动态页面17 * @author: 孤傲苍狼18 * @date: 2014-8-31 下午11:25:4019 *20 */ 21 public class NoCacheFilter implements Filter {22 23 24     public void doFilter(ServletRequest req, ServletResponse resp,25             FilterChain chain) throws IOException, ServletException {26         //把ServletRequest强转成HttpServletRequest27         HttpServletRequest request = (HttpServletRequest) req;28         //把ServletResponse强转成HttpServletResponse29         HttpServletResponse response = (HttpServletResponse) resp;30         //禁止浏览器缓存所有动态页面31         response.setDateHeader("Expires", -1);32         response.setHeader("Cache-Control", "no-cache");33         response.setHeader("Pragma", "no-cache");34         35         chain.doFilter(request, response);36     }37 38     public void init(FilterConfig filterConfig) throws ServletException {39 40     }41     42     public void destroy() {43         44     }45 }

　　web.xml文件中的配置如下：

 1   <filter> 2       <filter-name>NoCacheFilter</filter-name> 3       <filter-class>me.gacl.web.filter.NoCacheFilter</filter-class> 4   </filter> 5    6   <filter-mapping> 7       <filter-name>NoCacheFilter</filter-name> 8         <!--只拦截Jsp请求--> 9       <servlet-name>*.jsp</servlet-name>10   </filter-mapping>

三、控制浏览器缓存页面中的静态资源

　　有些动态页面中引用了一些图片或css文件以修饰页面效果，这些图片和css文件经常是不变化的，所以为减轻服务器的压力，可以使用filter控制浏览器缓存这些文件，以提升服务器的性能。

 1 package me.gacl.web.filter; 2  3 import java.io.IOException; 4  5 import javax.servlet.Filter; 6 import javax.servlet.FilterChain; 7 import javax.servlet.FilterConfig; 8 import javax.servlet.ServletException; 9 import javax.servlet.ServletRequest;10 import javax.servlet.ServletResponse;11 import javax.servlet.http.HttpServletRequest;12 import javax.servlet.http.HttpServletResponse;13 14 /**15 * @ClassName: CacheFilter16 * @Description: 控制缓存的filter17 * @author: 孤傲苍狼18 * @date: 2014-9-1 下午9:39:3819 *20 */ 21 public class CacheFilter implements Filter {22 23     private FilterConfig filterConfig;24 25     public void doFilter(ServletRequest req, ServletResponse resp,26             FilterChain chain) throws IOException, ServletException {27     28         HttpServletRequest request = (HttpServletRequest) req;29         HttpServletResponse response = (HttpServletResponse) resp;30         31         //1.获取用户想访问的资源32         String uri = request.getRequestURI(); 33         34         //2.得到用户想访问的资源的后缀名35         String ext = uri.substring(uri.lastIndexOf(".")+1);36         37         //得到资源需要缓存的时间38         String time = filterConfig.getInitParameter(ext);39         if(time!=null){40             long t = Long.parseLong(time)*3600*1000;41             //设置缓存42             response.setDateHeader("expires", System.currentTimeMillis() + t);43         }44         45         chain.doFilter(request, response);46 47     }48 49     public void init(FilterConfig filterConfig) throws ServletException {50         this.filterConfig = filterConfig;51     }52     53     public void destroy() {54         55     }56 }

　　web.xml文件中的配置如下：

 1  <!-- 配置缓存过滤器 --> 2    <filter> 3       <filter-name>CacheFilter</filter-name> 4       <filter-class>me.gacl.web.filter.CacheFilter</filter-class> 5        <!-- 配置要缓存的web资源以及缓存时间，以小时为单位 --> 6       <init-param> 7           <param-name>css</param-name> 8           <param-value>4</param-value> 9       </init-param>10       <init-param>11           <param-name>jpg</param-name>12           <param-value>1</param-value>13       </init-param>14       <init-param>15           <param-name>js</param-name>16           <param-value>4</param-value>17       </init-param>18       <init-param>19           <param-name>png</param-name>20           <param-value>4</param-value>21       </init-param>22   </filter>23   <!-- 配置要缓存的web资源的后缀-->24   <filter-mapping>25       <filter-name>CacheFilter</filter-name>26       <url-pattern>*.jpg</url-pattern>27   </filter-mapping>28   29   <filter-mapping>30       <filter-name>CacheFilter</filter-name>31       <url-pattern>*.css</url-pattern>32   </filter-mapping>33   34   <filter-mapping>35       <filter-name>CacheFilter</filter-name>36       <url-pattern>*.js</url-pattern>37   </filter-mapping>38    <filter-mapping>39       <filter-name>CacheFilter</filter-name>40       <url-pattern>*.png</url-pattern>41   </filter-mapping>

四、实现用户自动登陆

　　思路是这样的：

　　1、在用户登陆成功后，发送一个名称为user的cookie给客户端，cookie的值为用户名和md5加密后的密码。
　　2、编写一个AutoLoginFilter，这个filter检查用户是否带有名称为user的cookie来，如果有，则调用dao查询cookie的用户名和密码是否和数据库匹配，匹配则向session中存入user对象（即用户登陆标记），以实现程序完成自动登陆。

　　核心代码如下：

　　处理用户登录的控制器：LoginServlet

 1 package me.gacl.web.controller; 2  3 import java.io.IOException; 4  5 import javax.servlet.ServletException; 6 import javax.servlet.http.Cookie; 7 import javax.servlet.http.HttpServlet; 8 import javax.servlet.http.HttpServletRequest; 9 import javax.servlet.http.HttpServletResponse;10 11 import me.gacl.dao.UserDao;12 import me.gacl.domain.User;13 import me.gacl.util.WebUtils;14 15 public class LoginServlet extends HttpServlet {16 17     public void doGet(HttpServletRequest request, HttpServletResponse response)18             throws ServletException, IOException {19 20         String username = request.getParameter("username");21         String password = request.getParameter("password");22         23         UserDao dao = new UserDao();24         User user = dao.find(username, password);25         if(user==null){26             request.setAttribute("message", "用户名或密码不对！！");27             request.getRequestDispatcher("/message.jsp").forward(request, response);28             return;29         }30         request.getSession().setAttribute("user", user);31         //发送自动登陆cookie给客户端浏览器进行存储32         sendAutoLoginCookie(request,response,user);33         request.getRequestDispatcher("/index.jsp").forward(request, response);34     }35 36     /**37     * @Method: sendAutoLoginCookie38     * @Description: 发送自动登录cookie给客户端浏览器39     * @Anthor:孤傲苍狼40     *41     * @param request42     * @param response43     * @param user44     */ 45     private void sendAutoLoginCookie(HttpServletRequest request, HttpServletResponse response, User user) {46         if (request.getParameter("logintime")!=null) {47             int logintime = Integer.parseInt(request.getParameter("logintime"));48             //创建cookie,cookie的名字是autologin，值是用户登录的用户名和密码，用户名和密码之间使用.进行分割，密码经过md5加密处理49             Cookie cookie = new Cookie("autologin",user.getUsername() + "." + WebUtils.md5(user.getPassword()));50             //设置cookie的有效期51             cookie.setMaxAge(logintime);52             //设置cookie的有效路径53             cookie.setPath(request.getContextPath());54             //将cookie写入到客户端浏览器55             response.addCookie(cookie);56         }57     }58     59     public void doPost(HttpServletRequest request, HttpServletResponse response)60             throws ServletException, IOException {61 62         doGet(request, response);63     }64 65 }

　　处理用户自动登录的过滤器：AutoLoginFilter

 1 package me.gacl.web.filter; 2  3 import java.io.IOException; 4  5 import javax.servlet.Filter; 6 import javax.servlet.FilterChain; 7 import javax.servlet.FilterConfig; 8 import javax.servlet.ServletException; 9 import javax.servlet.ServletRequest;10 import javax.servlet.ServletResponse;11 import javax.servlet.http.Cookie;12 import javax.servlet.http.HttpServletRequest;13 import javax.servlet.http.HttpServletResponse;14 15 import me.gacl.dao.UserDao;16 import me.gacl.domain.User;17 import me.gacl.util.WebUtils;18 19 public class AutoLoginFilter implements Filter {20 21     public void doFilter(ServletRequest req, ServletResponse resp,22             FilterChain chain) throws IOException, ServletException {23         24         HttpServletRequest request = (HttpServletRequest) req;25         HttpServletResponse response = (HttpServletResponse) resp;26         //如果已经登录了，就直接chain.doFilter(request, response)放行27         if(request.getSession().getAttribute("user")!=null){28             chain.doFilter(request, response);29             return;30         }31         32         //1.得到用户带过来的authlogin的cookie33         String value = null;34         Cookie cookies[] = request.getCookies();35         for(int i=0;cookies!=null && i<cookies.length;i++){36             if(cookies[i].getName().equals("autologin")){37                 value = cookies[i].getValue();38             }39         }40         41         //2.得到 cookie中的用户名和密码 42         if(value!=null){43             String username = value.split("\\.")[0];44             String password = value.split("\\.")[1];45             46             //3.调用dao获取用户对应的密码47             UserDao dao = new UserDao();48             User user = dao.find(username);49             String dbpassword = user.getPassword();50             51             //4.检查用户带过来的md5的密码和数据库中的密码是否匹配,如匹配则自动登陆52             if(password.equals(WebUtils.md5(dbpassword))){53                 request.getSession().setAttribute("user", user);54             }55         }56         57         chain.doFilter(request, response);58     }59     60     public void destroy() {61         62     }63 64     public void init(FilterConfig filterConfig) throws ServletException {65 66     }67 }

　　如果想取消自动登录，那么可以在用户注销时删除自动登录cookie，核心代码如下：

 1 package me.gacl.web.controller; 2  3 import java.io.IOException; 4  5 import javax.servlet.ServletException; 6 import javax.servlet.http.Cookie; 7 import javax.servlet.http.HttpServlet; 8 import javax.servlet.http.HttpServletRequest; 9 import javax.servlet.http.HttpServletResponse;10 11 public class CancelAutoLoginServlet extends HttpServlet {12 13     public void doGet(HttpServletRequest request, HttpServletResponse response)14             throws ServletException, IOException {15         //移除存储在session中的user16         request.getSession().removeAttribute("user");17         //移除自动登录的cookie18         removeAutoLoginCookie(request,response);19         //注销用户后跳转到登录页面20         request.getRequestDispatcher("/login.jsp").forward(request, response);21     }22 23     /**24     * @Method: removeAutoLoginCookie25     * @Description: 删除自动登录cookie，26     *     JavaWeb中删除cookie的方式就是新创建一个cookie，新创建的cookie与要删除的cookie同名，27     *     设置新创建的cookie的cookie的有效期设置为0，有效路径与要删除的cookie的有效路径相同28     * @Anthor:孤傲苍狼29     *30     * @param request31     * @param response32     */ 33     private void removeAutoLoginCookie(HttpServletRequest request, HttpServletResponse response) {34         //创建一个名字为autologin的cookie35         Cookie cookie = new Cookie("autologin","");36          //将cookie的有效期设置为0，命令浏览器删除该cookie37         cookie.setMaxAge(0);38         //设置要删除的cookie的path39         cookie.setPath(request.getContextPath());40         response.addCookie(cookie);41     }42     43     public void doPost(HttpServletRequest request, HttpServletResponse response)44             throws ServletException, IOException {45         doGet(request, response);46     }47 }

　　以上就是过滤器的几个常见应用场景。