var http_request;
function makeRequest(url) 
{
    if (window.XMLHttpRequest) 
    {
        http_request = new XMLHttpRequest(); 
    } 
    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) 
    {
        alert("您的浏览器不支持当前操作，请使用 IE 5.0 以上版本!");
        return false;
    }
    http_request.onreadystatechange = init; 
    http_request.open('GET', url, true); 
    http_request.setRequestHeader("If-Modified-Since","0");
    http_request.send(null);
}

function init() 
{
    if (http_request.readyState == 4) 
    {
        if (http_request.status == 0 || http_request.status == 200) 
        {
            var result = http_request.responseText;
            if(result=="")
            {
                alert("服务器繁忙，读取失败！");
            }
        } 
        else 
        {
            alert("请求失败!");
        }
    }
}