若发送带参数的post请求时必须加上下面这段代码告知服务器将会有参数传过来，不然后台用request.getParameter()将无法获取。

要用getreader()或getInputStream()获取了。

xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

发送同步的Ajax具体代码如下：

function checkRegistedTime() {
		var xmlhttp;
		if (window.XMLHttpRequest) {
			xmlhttp = new XMLHttpRequest;
		} else {
			xmlhttp = new ActiveObject("Microsoft.XMLHTTP");
		}
		var registedday = document.getElementById("registeddayDataBase").value;
		var registedtime = document.getElementById("registedtime").value;
		/* var url = "registedTime_validate.jsp?registedday=" + registedday
				+ "&registedtime=" + registedtime; */
		var url = "registedTime_validate.jsp";
		var data = "registedday=" + registedday + "&registedtime="
				+ registedtime;
		xmlhttp.open("post",url,false);
		xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
		xmlhttp.send(data);
		return xmlhttp.responseText;
	}

后台获取数据代码registedTime_validate.jsp：

<%
      String registeddayDataBase = request.getParameter("registeddayDataBase");
      String registedtime = request.getParameter("registedtime");
%>