有关ajax send()的一个小问题

客户端：
<script type="text/javascript">
            var xmlHttp;
            function createXMLHttpRequest(){
            
                try {
                    // Firefox, Opera 8.0+, Safari   
                    xmlHttp = new XMLHttpRequest();
                } 
                catch (e) {
                    // Internet Explorer   
                    try {
                        xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
                    } 
                    catch (e) {
                        try {
                            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
                        } 
                        catch (e) {
                            alert("Your browser does not support AJAX!");
                            return false;
                        }
                    }
                }
                
            }            
            function startRequest(){
               var text=document.getElementById("msgText").value;
              
                createXMLHttpRequest();
                xmlHttp.onreadystatechange = function(){
                    if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                        alert(xmlHttp.responseText);
                    }
                }
  xmlHttp.open("POST","sendMsg.do",true);
                  xmlHttp.send("msgText="+text);

                
            }
        </script>
</head>
<body>
<input type="text" name="msgText" />
<input type="button" value="Send" onclick="startRequest()"/>
</body>
</html>
服务器端：String msg=request.getParameter("msgText");
问题：
为什么我得到的msg的值是null，而不是我在页面上输入的值？我该怎么获取页面上输入的值？