function getXMLHttpRequest(){
  http_request = false;
  //开始初始化XMLHttpRequest对象
  if(window.XMLHttpRequest) { //Mozilla 浏览器
    http_request = new XMLHttpRequest();
    if (http_request.overrideMimeType) {  //设置MiME类别
      http_request.overrideMimeType('text/xml');
    }
  }
  else if (window.ActiveXObject) { // IE浏览器
    //try {
    //  http_request = new ActiveXObject("Msxml2.XMLHTTP");
    //}catch(e) {
      try {
        http_request = new ActiveXObject("Microsoft.XMLHTTP");
      }catch(e) {}
    //}
  }
  if (!http_request) { // 异常，创建对象实例失败
    window.alert("不能创建XMLHttpRequest对象实例.");
    return null;
  }
  return http_request;
}

function sendRequest(http_request,url,handler) {//初始化、指定处理函数、发送请求的函数
  http_request.onreadystatechange = handler;
  // 确定发送请求的方式和URL以及是否同步执行下段代码
  //http_request.setHeader("Cache-Control", "no-cache, must-revalidate");
  http_request.open("GET", url, true);
  http_request.send(null);
}

function postRequest(http_request,url,handler) {//初始化、指定处理函数、发送请求的函数
  http_request.onreadystatechange = handler;
  // 确定发送请求的方式和URL以及是否同步执行下段代码
  http_request.open("POST", url, true);
  http_request.send(null);
}


/**
* 从指定的url中获取数值来填充oSelect指定的select控件的指。
*/
function populate_select(url,oSelect,remove){
  var req=getXMLHttpRequest();
  if(remove){
    removeAll(oSelect);
  }
  var callback=function(){
    if((req.readystate==4)&&(req.status==200)){
    	var resXML=req.responseXML;
    	var properties=resXML.getElementsByTagName("parameter");
  		for(var i=0;i<properties.length;i++){
    		var name=properties[i].getElementsByTagName("name")[0].text;
    		var value=properties[i].getElementsByTagName("value")[0].text;
    		var newOption=document.createElement("option");
    		newOption.value=name;
    		newOption.text=value;
    		oSelect.add(newOption);
		  }
    }
  }
  sendRequest(req,url,callback);
}