Filter(过滤器)常见应用

朴灿烈づ我的快乐病毒、 2022-07-13 00:28 170阅读 0赞

## 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

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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: CharacterEncodingFilter
    16 * @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11:09:37
    19 *
    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     @Override
    73     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 }

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web.xml文件中的配置如下：

![复制代码][copycode.gif]

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>

![复制代码][copycode.gif]

## 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。

![复制代码][copycode.gif]

web.xml文件中的配置如下：

![复制代码][copycode.gif]

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         
     9       <servlet-name>*.jsp</servlet-name>
    10   </filter-mapping>

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## 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。

![复制代码][copycode.gif]

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: CacheFilter
    16 * @Description: 控制缓存的filter
    17 * @author: 孤傲苍狼
    18 * @date: 2014-9-1 下午9:39:38
    19 *
    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 }

![复制代码][copycode.gif]

web.xml文件中的配置如下：

![复制代码][copycode.gif]

1  
     2    <filter>
     3       <filter-name>CacheFilter</filter-name>
     4       <filter-class>me.gacl.web.filter.CacheFilter</filter-class>
     5        
     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   
    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>

![复制代码][copycode.gif]

## 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

![复制代码][copycode.gif]

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: sendAutoLoginCookie
    38     * @Description: 发送自动登录cookie给客户端浏览器
    39     * @Anthor:孤傲苍狼
    40     *
    41     * @param request
    42     * @param response
    43     * @param user
    44     */ 
    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 }

![复制代码][copycode.gif]

处理用户自动登录的过滤器：AutoLoginFilter

![复制代码][copycode.gif]

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cookie
    33         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 }

![复制代码][copycode.gif]

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：

![复制代码][copycode.gif]

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user
    16         request.getSession().removeAttribute("user");
    17         //移除自动登录的cookie
    18         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: removeAutoLoginCookie
    25     * @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 request
    31     * @param response
    32     */ 
    33     private void removeAutoLoginCookie(HttpServletRequest request, HttpServletResponse response) {
    34         //创建一个名字为autologin的cookie
    35         Cookie cookie = new Cookie("autologin","");
    36          //将cookie的有效期设置为0，命令浏览器删除该cookie
    37         cookie.setMaxAge(0);
    38         //设置要删除的cookie的path
    39         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 }

![复制代码][copycode.gif]

以上就是过滤器的几个常见应用场景。

原文博客地址

http://www.cnblogs.com/xdp-gacl/p/3948422.html

[copycode.gif]: /images/20220713/c59ba4b05861473e8644e23f8407e0e8.png