Filter介绍及使用

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一、Filter概述

filter功能.它使用户可以改变一个request和修改一个response. Filter 不是一个servlet，它不能产生一个response，它能够在一个request到达servlet之前预处理request，也可以在response离开servlet时处理response.换种说法，filter其实是一个“servlet chaining“（servlet 链）.
包括
1. 在servlet被调用之前截获；
2. 在servlet被调用之前检查servlet request;
3. 根据需要修改request头和request数据；
4. 根据需要修改response头和response数据；
5. 在servlet被调用之后截获.
你能够配置一个filter 到一个或多个servlet；单个servlet或servlet组能够被多个filter 使用。几个实用的filter 包括：用户辨认filter，日志filter，审核filter，加密filter，符号filter，能改变xml内容的XSLT filter等。
一个filter必须实现javax.servlet.Filter。
三个方法
1. void setFilterConfig(FilterConfig config) //设置filter 的配置对象；
2. FilterConfig getFilterConfig() //返回filter的配置对象；
3. void doFilter(ServletRequest req,ServletResponse res,FilterChain chain) //执行filter 的工作.
注：现setFilterConfig和getFilterConfig方法已取消，代之为init(FilterConfig config)和destory()方法。[3]
服务器每次只调用setFilterConfig方法一次准备filter 的处理；调用doFilter方法多次以处理不同的请求.FilterConfig接口有方法可以找到filter名字及初始化参数信息.服务器可以设置FilterConfig为空来指明filter已经终结.
每一个filter从doFilter（）方法中得到当前的request及response.在这个方法里，可以进行任何的针对request及response的操作.（包括收集数据，包装数据等）.filter调用chain.doFilter（）方法把控制权交给下一个filter.一个filter在doFilter（）方法中结束.如果一个filter想停止request处理而获得对response的完全的控制，那它可以不调用下一个filter.
一个filter可以包装request 或response以改变几个方法和提供用户定制的属性.Api2.3提供了HttpServletRequestWrapper 和HttpServletResponseWrapper来实现.它们能分派最初的request和response.如果要改变一个方法的特性，必须继承wapper和重写方法.下面是一段简单的日志filter用来记录所有request的持续时间

二、创建Filter类

package Filter;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;public class LogFilter implements Filter{    //FilterConfig用于可访问Filter的配置信息    private FilterConfig filterConfig;    //实现销毁方法    public void destroy() {        this.filterConfig=null;    }    public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) throws IOException, ServletException {        //-----------下面代码对用户请求进行预处理-----------        //获取ServletContext对象，用于记录日志        ServletContext context=this.filterConfig.getServletContext();        long before=System.currentTimeMillis();        System.out.println("开始过滤............");        //将请求转换成HttpServletRequest对象        HttpServletRequest hrequest=(HttpServletRequest)request;        //输出提示信息        System.out.println("Filter已经获取到用户请求地址："+hrequest.getServletPath());        //Filter只是链式处理，请求依然放到目的地址，这个方法决定是否放行        chain.doFilter(hrequest, response);        //------------下面代码用于对服务器响应执行后处理------------        long after=System.currentTimeMillis();        //输出提示信息        System.out.println("过滤结束、、、、、、");        System.out.println("请求被定位到 "+hrequest.getRequestURI()+"所花时间为"+(after-before));    }     //实现初始化方法    public void init(FilterConfig filterConfig) throws ServletException {        this.filterConfig=filterConfig;    }}

三、配置Filter

<?xml version="1.0" encoding="UTF-8"?><web-app version="2.5"     xmlns="http://java.sun.com/xml/ns/javaee"     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee     http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">  <welcome-file-list>    <welcome-file>index.jsp</welcome-file>  </welcome-file-list>  <!-- 定义Filter -->  <filter>     <filter-name>logFilter</filter-name>     <filter-class>Filter.LogFilter</filter-class>  </filter>  <!-- 定义Filter拦截的URL地址 -->  <filter-mapping>      <filter-name>logFilter</filter-name>      <!-- 负责拦截的URL，/*表示拦截所有的请求 -->      <url-pattern>/*</url-pattern>  </filter-mapping>  </web-app>

四、测试类

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">    <title>Login</title>    <meta http-equiv="pragma" content="no-cache">    <meta http-equiv="cache-control" content="no-cache">    <meta http-equiv="expires" content="0">        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">    <meta http-equiv="description" content="This is my page">    <!--    <link rel="stylesheet" type="text/css" href="styles.css">    -->  </head>  <body>    ${tip}<br>    <form action="login?action=service" method="post">                    名称:<input type="text" name="user"><br>                    密码:<input type="text" name="pass"><br/>      <input type="submit" value="登录"><input type="reset" value="重置">    </form>  </body></html>输出：开始过滤............Filter已经获取到用户请求地址：/index.jsp过滤结束、、、、、、请求被定位到 /webPrj/index.jsp所花时间为56开始过滤............Filter已经获取到用户请求地址：/login

五、综合例子

1、Filter类package 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.HttpSession;public class AuthorityFilter implements Filter{    private FilterConfig config;    //销毁方法    public void destroy() {        this.config=null;    }    //Filter的核心方法    public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) throws IOException, ServletException {        // TODO Auto-generated method stub        String encode=config.getInitParameter("encode");        String indexPage=config.getInitParameter("indexPage");        String shopPage=config.getInitParameter("shopPage");        //设置编码字符集        request.setCharacterEncoding(encode);        HttpServletRequest requ=(HttpServletRequest)request;        //获取用户请求页面        String requestPath=requ.getServletPath();        HttpSession session=requ.getSession(true);        //如果session为空，则说明用户没有登录        if(session.getAttribute("user")==null && !requestPath.endsWith(indexPage) && !requestPath.endsWith(shopPage) && !requestPath.endsWith("login")){            //forward到首页            request.setAttribute("tip","你还没有登录");            request.getRequestDispatcher(indexPage).forward(request, response);        }else{//放行             chain.doFilter(request, response);        }        //chain.doFilter(request, response);    }    //初始化    public void init(FilterConfig config) throws ServletException {        this.config=config;    }}2、请求Servletpackage Login;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class LoginServlet extends HttpServlet{    public void service(HttpServletRequest request,HttpServletResponse response)throws ServletException,IOException{        System.out.println("##################################");        String user=String.valueOf(request.getAttribute("user"));        String pass=String.valueOf(request.getAttribute("pass"));        HttpSession session=request.getSession();        session.setAttribute("user", user);        session.setAttribute("pass", pass);        try {            //转发            //request.getRequestDispatcher("/shop.jsp").forward(request, response);            //重定向            response.sendRedirect("/webPrj/shop.jsp");        } catch (IOException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}3、配置文件<?xml version="1.0" encoding="UTF-8"?><web-app version="2.5"     xmlns="http://java.sun.com/xml/ns/javaee"     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee     http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">  <welcome-file-list>    <welcome-file>index.jsp</welcome-file>  </welcome-file-list>  <!-- 定义Filter -->  <filter>     <filter-name>logFilter</filter-name>     <filter-class>Filter.LogFilter</filter-class>  </filter>  <!-- 定义Filter拦截的URL地址 -->  <filter-mapping>      <filter-name>logFilter</filter-name>      <!-- 负责拦截的URL，/*表示拦截所有的请求 -->      <url-pattern>/*</url-pattern>  </filter-mapping>  <!-- 定义AuthorityFilter -->  <filter>      <filter-name>authority</filter-name>      <filter-class>Filter.AuthorityFilter</filter-class>      <init-param>          <param-name>encode</param-name>          <param-value>UTF-8</param-value>      </init-param>      <init-param>          <param-name>indexPage</param-name>          <param-value>/index.jsp</param-value>      </init-param>      <init-param>           <param-name>shopPage</param-name>           <param-value>/shop.jsp</param-value>      </init-param>  </filter>  <filter-mapping>      <filter-name>authority</filter-name>      <url-pattern>/*</url-pattern>  </filter-mapping>  <!-- 登录servlet -->  <servlet>       <servlet-name>LoginServlet</servlet-name>       <servlet-class>Login.LoginServlet</servlet-class>       <load-on-startup>1</load-on-startup>  </servlet>  <servlet-mapping>        <servlet-name>LoginServlet</servlet-name>        <url-pattern>/login</url-pattern>  </servlet-mapping></web-app>4、index.jsp页面 <%@ page language="java" import="java.util.*" pageEncoding="utf-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">    <title>Login</title>    <meta http-equiv="pragma" content="no-cache">    <meta http-equiv="cache-control" content="no-cache">    <meta http-equiv="expires" content="0">        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">    <meta http-equiv="description" content="This is my page">    <!--    <link rel="stylesheet" type="text/css" href="styles.css">    -->  </head>  <body>    ${tip}<br>    <form action="login?action=service" method="post">                    名称:<input type="text" name="user"><br>                    密码:<input type="text" name="pass"><br/>      <input type="submit" value="登录"><input type="reset" value="重置">    </form>  </body></html>5、shop.jsp<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">    <title>My JSP 'shop.jsp' starting page</title>    <meta http-equiv="pragma" content="no-cache">    <meta http-equiv="cache-control" content="no-cache">    <meta http-equiv="expires" content="0">        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">    <meta http-equiv="description" content="This is my page">    <!--    <link rel="stylesheet" type="text/css" href="styles.css">    -->  </head>  <body>    <form action="buy.jsp">              书籍：<input type="checkbox" name="item" value="book">              电脑：<input type="checkbox" name="item" value="computer">              汽车：<input type="checkbox" name="item" value="car">     <input type="submit" value="提交">    </form>  </body></html>  结果：对没有登录的用户进行相对应的拦截请求

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