请高手帮我看一下为什么这个filter没有起到作用？
在练习写servlet和filter， 但是下面写的filter 为什么没有起到做作， 请专家们帮忙看一下！
开发环境: Eclipse Javaee + Tomcat 6
我在filter中的doFilter() 方法中加放了System.out.println("do Filter"),
在提交表单的时候，要终端里可以打印出这行代码，
但是filter的就是没有起来作用。
filter的作用:
当用输入"hello"的时候，把它替换为"world",
第一个message.jsp 代码如下:
<body> <h1>留言薄</h1> <form action="messageServlet" method="post"> 姓名:<br /> <input type="text" name="username" /><br /> 留言:<br /> <textarea rows="10" cols="20" name="message" ></textarea><br /> <input type="submit" value="Submit" /> </form> </body>
package com.shy.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;public class messageFilter implements Filter{ @Override public void destroy() { } @Override public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { //HttpServletRequest req = (HttpServletRequest)request; //HttpServletResponse resp = (HttpServletResponse)response; String username = request.getParameter("username"); String message = request.getParameter("message"); if(username.equals("hello")) { username = "world"; } System.out.println("Do Filter"); chain.doFilter(request,response); } @Override public void init(FilterConfig arg0) throws ServletException { System.out.println("Filter start"); } }package com.shy.servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class messageServlet extends HttpServlet { @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { req.setCharacterEncoding("utf-8"); resp.setContentType("text/html;charset=utf-8"); String username = req.getParameter("username"); String message = req.getParameter("message"); req.setAttribute("username", username); req.setAttribute("message", message); RequestDispatcher rd = req.getRequestDispatcher("messageDisplay.jsp"); rd.forward(req,resp); } }<?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" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" 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"> <display-name>web</display-name> <welcome-file-list> <welcome-file>index.html</welcome-file> <welcome-file>index.htm</welcome-file> <welcome-file>index.jsp</welcome-file> <welcome-file>default.html</welcome-file> <welcome-file>default.htm</welcome-file> <welcome-file>default.jsp</welcome-file> </welcome-file-list> <filter> <filter-name>messageFilter</filter-name> <filter-class>com.shy.filter.messageFilter</filter-class> </filter> <filter-mapping> <filter-name>messageFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> <servlet> <servlet-name>messageServlet</servlet-name> <servlet-class>com.shy.servlet.messageServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>messageServlet</servlet-name> <url-pattern>/messageServlet</url-pattern> </servlet-mapping> </web-app>
<body> <%=request.getAttribute("username").toString() %><br /> <%=request.getAttribute("message").toString() %></body>