深入分析JavaWeb Item37 -- 过滤器Filter典型應用

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

package me.gacl.web.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.HttpServletRequestWrapper;import javax.servlet.http.HttpServletResponse;/*** @ClassName: CharacterEncodingFilter* @Description: 此过滤器用来解决全站中文乱码问题* @author: 孤傲苍狼* @date: 2014-8-31 下午11:09:37**/ public class CharacterEncodingFilter implements Filter {    private FilterConfig filterConfig = null;    //设置默认的字符编码    private String defaultCharset = "UTF-8";    public void doFilter(ServletRequest req, ServletResponse resp,            FilterChain chain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        String charset = filterConfig.getInitParameter("charset");        if(charset==null){            charset = defaultCharset;        }        request.setCharacterEncoding(charset);        response.setCharacterEncoding(charset);        response.setContentType("text/html;charset="+charset);        MyCharacterEncodingRequest requestWrapper = new MyCharacterEncodingRequest(request);        chain.doFilter(requestWrapper, response);    }    public void init(FilterConfig filterConfig) throws ServletException {        //得到过滤器的初始化配置信息        this.filterConfig = filterConfig;    }    public void destroy() {    }}/*1.实现与被增强对象相同的接口 2、定义一个变量记住被增强对象3、定义一个构造器，接收被增强对象4、覆盖需要增强的方法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 */class MyCharacterEncodingRequest extends HttpServletRequestWrapper{    private HttpServletRequest request;    public MyCharacterEncodingRequest(HttpServletRequest request) {        super(request);        this.request = request;    }    /* 重写getParameter方法     * @see javax.servlet.ServletRequestWrapper#getParameter(java.lang.String)     */    @Override    public String getParameter(String name) {        try{            //获取参数的值            String value= this.request.getParameter(name);            if(value==null){                return null;            }            //如果不是以get方式提交数据的，就直接返回获取到的值            if(!this.request.getMethod().equalsIgnoreCase("get")) {                return value;            }else{                //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                value = new String(value.getBytes("ISO8859-1"),this.request.getCharacterEncoding());                return value;            }        }catch (Exception e) {            throw new RuntimeException(e);        }    }}

　　web.xml文件中的配置如下：

<filter>      <filter-name>CharacterEncodingFilter</filter-name>      <filter-class>me.gacl.web.filter.CharacterEncodingFilter</filter-class>      <init-param>          <param-name>charset</param-name>          <param-value>UTF-8</param-value>      </init-param>  </filter>  <filter-mapping>      <filter-name>CharacterEncodingFilter</filter-name>      <url-pattern>/*</url-pattern>  </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：

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

package me.gacl.web.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;/*** @ClassName: NoCacheFilter* @Description: 禁止浏览器缓存所有动态页面* @author: 孤傲苍狼* @date: 2014-8-31 下午11:25:40**/ public class NoCacheFilter implements Filter {    public void doFilter(ServletRequest req, ServletResponse resp,            FilterChain chain) throws IOException, ServletException {        //把ServletRequest强转成HttpServletRequest        HttpServletRequest request = (HttpServletRequest) req;        //把ServletResponse强转成HttpServletResponse        HttpServletResponse response = (HttpServletResponse) resp;        //禁止浏览器缓存所有动态页面        response.setDateHeader("Expires", -1);        response.setHeader("Cache-Control", "no-cache");        response.setHeader("Pragma", "no-cache");        chain.doFilter(request, response);    }    public void init(FilterConfig filterConfig) throws ServletException {    }    public void destroy() {    }}

　　web.xml文件中的配置如下：

<filter>      <filter-name>NoCacheFilter</filter-name>      <filter-class>me.gacl.web.filter.NoCacheFilter</filter-class>  </filter>  <filter-mapping>      <filter-name>NoCacheFilter</filter-name>        <!--只拦截Jsp请求-->      <servlet-name>*.jsp</servlet-name>  </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。

package me.gacl.web.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;/*** @ClassName: CacheFilter* @Description: 控制缓存的filter* @author: 孤傲苍狼* @date: 2014-9-1 下午9:39:38**/ public class CacheFilter implements Filter {    private FilterConfig filterConfig;    public void doFilter(ServletRequest req, ServletResponse resp,            FilterChain chain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        //1.获取用户想访问的资源        String uri = request.getRequestURI();         //2.得到用户想访问的资源的后缀名        String ext = uri.substring(uri.lastIndexOf(".")+1);        //得到资源需要缓存的时间        String time = filterConfig.getInitParameter(ext);        if(time!=null){            long t = Long.parseLong(time)*3600*1000;            //设置缓存            response.setDateHeader("expires", System.currentTimeMillis() + t);        }        chain.doFilter(request, response);    }    public void init(FilterConfig filterConfig) throws ServletException {        this.filterConfig = filterConfig;    }    public void destroy() {    }}

　　web.xml文件中的配置如下：

<!-- 配置缓存过滤器 -->   <filter>      <filter-name>CacheFilter</filter-name>      <filter-class>me.gacl.web.filter.CacheFilter</filter-class>       <!-- 配置要缓存的web资源以及缓存时间，以小时为单位 -->      <init-param>          <param-name>css</param-name>          <param-value>4</param-value>      </init-param>      <init-param>          <param-name>jpg</param-name>          <param-value>1</param-value>      </init-param>      <init-param>          <param-name>js</param-name>          <param-value>4</param-value>      </init-param>      <init-param>          <param-name>png</param-name>          <param-value>4</param-value>      </init-param>  </filter>  <!-- 配置要缓存的web资源的后缀-->  <filter-mapping>      <filter-name>CacheFilter</filter-name>      <url-pattern>*.jpg</url-pattern>  </filter-mapping>  <filter-mapping>      <filter-name>CacheFilter</filter-name>      <url-pattern>*.css</url-pattern>  </filter-mapping>  <filter-mapping>      <filter-name>CacheFilter</filter-name>      <url-pattern>*.js</url-pattern>  </filter-mapping>   <filter-mapping>      <filter-name>CacheFilter</filter-name>      <url-pattern>*.png</url-pattern>  </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

package me.gacl.web.controller;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.Cookie;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import me.gacl.dao.UserDao;import me.gacl.domain.User;import me.gacl.util.WebUtils;public class LoginServlet extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        String username = request.getParameter("username");        String password = request.getParameter("password");        UserDao dao = new UserDao();        User user = dao.find(username, password);        if(user==null){            request.setAttribute("message", "用户名或密码不对！！");            request.getRequestDispatcher("/message.jsp").forward(request, response);            return;        }        request.getSession().setAttribute("user", user);        //发送自动登陆cookie给客户端浏览器进行存储        sendAutoLoginCookie(request,response,user);        request.getRequestDispatcher("/index.jsp").forward(request, response);    }    /**    * @Method: sendAutoLoginCookie    * @Description: 发送自动登录cookie给客户端浏览器    * @Anthor:孤傲苍狼    *    * @param request    * @param response    * @param user    */     private void sendAutoLoginCookie(HttpServletRequest request, HttpServletResponse response, User user) {        if (request.getParameter("logintime")!=null) {            int logintime = Integer.parseInt(request.getParameter("logintime"));            //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            Cookie cookie = new Cookie("autologin",user.getUsername() + "." + WebUtils.md5(user.getPassword()));            //设置cookie的有效期            cookie.setMaxAge(logintime);            //设置cookie的有效路径            cookie.setPath(request.getContextPath());            //将cookie写入到客户端浏览器            response.addCookie(cookie);        }    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        doGet(request, response);    }}

　　处理用户自动登录的过滤器：AutoLoginFilter

package me.gacl.web.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.Cookie;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import me.gacl.dao.UserDao;import me.gacl.domain.User;import me.gacl.util.WebUtils;public class AutoLoginFilter implements Filter {    public void doFilter(ServletRequest req, ServletResponse resp,            FilterChain chain) throws IOException, ServletException {        HttpServletRequest request = (HttpServletRequest) req;        HttpServletResponse response = (HttpServletResponse) resp;        //如果已经登录了，就直接chain.doFilter(request, response)放行        if(request.getSession().getAttribute("user")!=null){            chain.doFilter(request, response);            return;        }        //1.得到用户带过来的authlogin的cookie        String value = null;        Cookie cookies[] = request.getCookies();        for(int i=0;cookies!=null && i<cookies.length;i++){            if(cookies[i].getName().equals("autologin")){                value = cookies[i].getValue();            }        }        //2.得到 cookie中的用户名和密码         if(value!=null){            String username = value.split("\\.")[0];            String password = value.split("\\.")[1];            //3.调用dao获取用户对应的密码            UserDao dao = new UserDao();            User user = dao.find(username);            String dbpassword = user.getPassword();            //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            if(password.equals(WebUtils.md5(dbpassword))){                request.getSession().setAttribute("user", user);            }        }        chain.doFilter(request, response);    }    public void destroy() {    }    public void init(FilterConfig filterConfig) throws ServletException {    }}

　　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：

package me.gacl.web.controller;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.Cookie;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class CancelAutoLoginServlet extends HttpServlet {    public void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        //移除存储在session中的user        request.getSession().removeAttribute("user");        //移除自动登录的cookie        removeAutoLoginCookie(request,response);        //注销用户后跳转到登录页面        request.getRequestDispatcher("/login.jsp").forward(request, response);    }    /**    * @Method: removeAutoLoginCookie    * @Description: 删除自动登录cookie，    *     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，    *     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    * @Anthor:孤傲苍狼    *    * @param request    * @param response    */     private void removeAutoLoginCookie(HttpServletRequest request, HttpServletResponse response) {        //创建一个名字为autologin的cookie        Cookie cookie = new Cookie("autologin","");         //将cookie的有效期设置为0，命令浏览器删除该cookie        cookie.setMaxAge(0);        //设置要删除的cookie的path        cookie.setPath(request.getContextPath());        response.addCookie(cookie);    }    public void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        doGet(request, response);    }}

　　以上就是过滤器的几个常见应用场景。