急！！！dwr框架做的ajax点击无反应！！

JSP页面
<%@ page language="java" import="java.util.*" pageEncoding="GB2312"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>验证用户</title>
    <script type="text/javascript" src="dwr/util.js"></script>
    <script type="text/javascript" src="dwr/engine.js"></script>
    <script type="text/javascript" src="dwr/interface/studentdao.js"></script>
    <script type="text/javascript">
    
    function checkName()
    {
        var sname=$("text").value;        alert(sname);        studentdao.isExists(sname,show);
    }
    function show(boolean)
    {
        if(boolean)
        {
            dwr.util.setValue("result","用户名已经存在");        }else        {
            dwr.util.setValue("result","用户名可以使用");
        }    }
    </script>
  </head>
  <body>
<form action="" name="f1">    请输入用户名：<input id="text"><div id="result"></div><br>
<input type="button" value="验证" onclick="checkName()">
    </form>     </body>
</html>--------------------------------------------------
java文件package dao;import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;public class StudentDao {
private Connection con;
    public StudentDao(){
       try {        Class.forName("com.mysql.jdbc.Driver");
        
        con=DriverManager.getConnection("jdbc:mysql://localhost:3306/test?user=root&password=myhero");           System.out.println(con.isClosed());       } catch (Exception e) {
           // TODO Auto-generated catch block
           e.printStackTrace();
       }
    }    public boolean isExists(String sname){       String sql="select * from student where sname=?";
       try {
           PreparedStatement pstmt=con.prepareStatement(sql);           pstmt.setString(1, sname);           ResultSet rst=pstmt.executeQuery();
           if(rst.next()){
              return true;
           }
       } catch (SQLException e) {
           e.printStackTrace();
       }
       return false;
    }
}----------------------------------------------
web.xml<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" 
xmlns="http://java.sun.com/xml/ns/j2ee" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
       <servlet-name>dwr-Invoke</servlet-name>
    <servlet-class>org.directwebremoting.servlet.DwrServlet</servlet-class>
    </servlet>
    <servlet-mapping>
       <servlet-name>dwr-Invoke</servlet-name>       <url-pattern>/dwr/*</url-pattern></servlet-mapping>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>----------------------------------------
dwr.xml<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE dwr PUBLIC    "-//GetAhead Limited//DTD Direct Web Remoting 2.0//EN"    "http://getahead.org/dwr/dwr20.dtd">
<dwr>
<allow>
    <create javascript="studentdao" creator="new">
       <param name="class" value="dao.StudentDao"></param>      
    </create></allow>
</dwr>
----------------------本人新手。。不知道是哪里的问题 点击按钮不会调用JS 帮帮忙啊 重分