ajax 异步请求

<script type="text/javascript" >
	var http_request = false;
	function send_request(url){
		http_request = false;
		if(window.XMLHttpRequest){	//Mozilla浏览器
			http_request = new  XMLHttpRequest();
			if(http_request.overrideMimeType){
				http_request.overrideMimeType("text/xml");	
			}
		}else if(window.ActiveXObject){
			try{
				http_request = new ActiveXObject("Msxml2.XMLHTTP");
			}catch(e){
				try{
					http_request = new ActiveXObject("Microsoft.XMLHTTP");
					}catch(e){
					
				}
			}
		}
		
		if(!http_request){
			window.alert("不能创建XMLHttpRequest对象实例");
			return false;	
		}
		
		http_request.onreadystatechange = processRequest;
		http_request.open("GET",url,true);
		http_request.send(null);
			
	}
	
	//处理返回信息的函数
	function  processRequest(){
		if(http_request.readyState == 4){
			if(http_request.status == 200){
				alert(http_request.responseText);	
			}else{
				alert("你请求的页面有异常");	
			}	
		}
	}
	
</script>