ajax验证用户名是否存在

<script> 
		var xmlHttpRequest;
		function getXmlHttpRequset(){
			try{ 
				if( window.ActiveXObject ){ 
					for( var i = 5; i; i-- ){ 
						try{ 
							if( i == 2 ){ 
								xmlHttpRequest = new ActiveXObject( "Microsoft.XMLHTTP" ); 
							}else{ 
								xmlHttpRequest = new ActiveXObject( "Msxml2.XMLHTTP." + i + ".0" ); 
								xmlHttpRequest.setRequestHeader("Content-Type","text/xml"); 
								xmlHttpRequest.setRequestHeader("Charset","gbk"); }
								break;
							} 
						catch(e){ 
							xmlHttpRequest = false; 
						} 
					} 
				}else if( window.XMLHttpRequest ){ 
					xmlhttp_request = new XMLHttpRequest(); 
					if (xmlHttpRequest.overrideMimeType){ 
						xmlHttpRequest.overrideMimeType('text/xml'); 
					} 
				} 
			}catch(e){ 
				xmlHttpRequest = false; 
			} 
			return xmlHttpRequest;
		}
		
		function checkUserName(url){
			document.getElementById("checkUserName").innerHTML="正在检查用户名...";
			if(document.getElementById("userName").value==""||document.getElementById("userName").value==null){
				document.getElementById("checkUserName").innerHTML="请输入用户名！";
			}else{
				xmlHttpRequest=getXmlHttpRequset();
				xmlHttpRequest.open("POST",url+document.getElementById("userName").value,false);
				xmlHttpRequest.send(null);
				
				xmlHttpRequest.onreadystatechange=changeView(xmlHttpRequest);
			}
		}
		
		function changeView(xmlHttpRequest){
			if(xmlHttpRequest.readyState==4&&xmlHttpRequest.status==200){
				if(xmlHttpRequest.responseText==1){
					document.getElementById("checkUserName").innerHTML="该用户名可以使用！";
				}else if(xmlHttpRequest.responseText==2){
					document.getElementById("checkUserName").innerHTML="该用户名已经存在！";
				}else{
					document.getElementById("checkUserName").innerHTML="系统错误！";
				}
			}
		}
	</script>

web.xml

<servlet>
		<servlet-name>studyRoom</servlet-name>
		<servlet-class>
			hbu.hicc.crms.servlet.StudyRoomServlet
		</servlet-class>
	</servlet>

<servlet-mapping>
		<servlet-name>studyRoom</servlet-name>
		<url-pattern>/StudyRoom/studyRoomSelect</url-pattern>
	</servlet-mapping>

url="../RoomSystem/StudyRoom/studyRoomSelect";