学习学习servlet过滤器(javax.servlet.Filter)一、过滤器

Servlet过滤器Fileter是一个小型的web组件,它们通过拦截请求和响应,以便查看、提取或以某种方式操作客户端和服务器之间交换的数据,实现“过滤”的功能。Filter通常封装了一些功能的web组件,过滤器提供了一种面向对象的模块化机制,将任务封装到一个可插入的组件中, Filter组件通过配置文件来声明,并动态的代理。

二、servlet过滤方法

过滤器是一个实现了 javax.servlet.Filter 接口的 Java 类。javax.servlet.Filter 接口定义了三个方法:

序号方法描述1public void init(FilterConfig filterConfig)web 应用程序启动时,web 服务器将创建Filter 的实例对象,并调用其init方法,读取web.xml配置,完成对象的初始化功能,从而为后续的用户请求作好拦截的准备工作(filter对象只会创建一次,init方法也只会执行一次)。开发人员通过init方法的参数,可获得代表当前filter配置信息的FilterConfig对象。2public void doFilter (ServletRequest, ServletResponse, FilterChain)该方法完成实际的过滤操作,当客户端请求方法与过滤器设置匹配的URL时,Servlet容器将先调用过滤器的doFilter方法。FilterChain用户访问后续过滤器。3public void destroy()Servlet容器在销毁过滤器实例前调用该方法,在该方法中释放Servlet过滤器占用的资源。三、写servlet过滤器

写一个servlet屏蔽负面评论。

1、在web.xml中配置好servlet和filter

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5"> <filter> <filter-name>hello</filter-name> <filter-class>filter.CommentFilter</filter-class> </filter> <filter-mapping> <filter-name>hello</filter-name> <url-pattern>/comment</url-pattern> </filter-mapping> <servlet> <servlet-name>h</servlet-name> <servlet-class>web.CommentServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>h</servlet-name> <url-pattern>/comment</url-pattern> </servlet-mapping></web-app>

2、在src/main/java下新建一个包,名为web;并且在web包下面新建一个类,名为CommentServlet;

package web;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class CommentServlet extends HttpServlet { public void service(HttpServletRequest request, HttpServletResponse response) throws IOException { request.setCharacterEncoding("UTF-8"); response.setContentType("text/html;charset=utf8"); PrintWriter out = response.getWriter(); String str = request.getParameter("comment"); out.println("<h2>您发表的评论内容为:" + str + "</h2>"); out.close(); }}

3、在src/main/java下新建一个包,名为filter;并且在filter包下面新建一个类,名为CommentFilter

package filter;import java.io.IOException;import java.io.PrintWriter;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 CommentFilter implements Filter {private String illegalWord;public void init(FilterConfig filterConfig) throws ServletException { illegalWord = filterConfig.getInitParameter("illegalWord"); System.out.println("过滤器Filter已经开始运行......" + illegalWord);}public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { HttpServletRequest req = (HttpServletRequest) request; HttpServletResponse resp = (HttpServletResponse) response; req.setCharacterEncoding("UTF-8"); resp.setContentType("text/html;charset=UTF-8"); PrintWriter out = resp.getWriter(); String comment = req.getParameter("comment"); System.out.println(comment); if (comment.indexOf("dam") == -1) { out.println("<h2>以隐藏该评论</h2>"); } else { chain.doFilter(req, resp); }} public void destroy() { System.out.println(); }}

4、在src/main/webapp下面新建一个JSP文件,名为comment.jsp;

<%@ page language="java" contentType="text/html; charset=utf8"pageEncoding="utf-8"%><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">;<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf8"> <title>Insert title here</title> </head> <body> <form action="comment" method="post"> <fieldset> <legend>评论</legend> 请输入评论:<textarea name="comment" ></textarea><br> <input type="submit" value="发表评论"> </fieldset> </form> </body></html>四、测试

1、在浏览器中输入http://localhost:8089/servlet-test55-filter/comment.jsp ,界面如下:

2、在评论框中输入不含有“dam”的字符串,

点击“发表评论”时,返回结果为

3、在评论框中输入含有“dam”的字符串,

点击“发表评论”,返回的结果为: