请高手帮小弟我看一下为什么这个filter没有起到作用

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:

HTML code


<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>

messageFilter.java 代码如下:

Java code


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");
    }    
}

messageServlet代码如下：

Java code


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);        
    }    
}

web.xml 代码如下:

XML code


<?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>

请高手帮小弟我看一下为什么这个filter没有起到作用

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