页面加载后的AJAX CAll

Question

我试图在使用ajax调用加载页面后填充html选择。由于某种原因，servlet帖子没有被调用。有谁看到我可能做错了什么？

    window.onload = function(){
    var localXMLHttp = getXMLHttpRequest();
    localXMLHttp.setRequestHeader("Content-Type",
                                  "application/x-www-form-urlencoded");
    localXMLHttp.open("POST","DateServlet",true);
    localXMLHttp.onreadystatechange = function() {
        if(localXMLHttp.readyState == 4 && localXMLHttp.status == 200) {
            alert(localXMLHttp.responseText);
            var datesToDisplay = localXMLHttp.responseText.split("*");
            var periodSelect = document.getElementById("period");
            periodSelect.options[periodSelect.options.length] = new Option(datesToDisplay[0], datesToDisplay[0]);
            periodSelect.options[periodSelect.options.length] = new Option(datesToDisplay[1], datesToDisplay[1]);
            periodSelect.options[periodSelect.options.length] = new Option(datesToDisplay[2], datesToDisplay[2]);
            periodSelect.options[periodSelect.options.length] = new Option(datesToDisplay[3], datesToDisplay[3]);
        }
    };
    localXMLHttp.send(null);
};

此代码发布在body标记末尾的script标记内。