xmlHttp请求在IE6/IE7/Firefox中只成功执行一次之分析

        异步调用的问题是五花八门，说碰上就碰上，这次我遇到的麻烦就是这样，同样的异步请求，在IE6SP1中运行一点问题没有，在IE6SP2中只成功执行一次后，每次的结果都和第一次一样，随后又尝试IE7和Firefox，效果和IE6SP2一样，用Fiddler监视明明显示正确的请求和返回值，可就是最终执行的效果一点都没变化，相当郁闷。这样的结果自然就想到了肯定是缓存在作怪，但始终还是要查明到底是哪段代码引起缓存的，我把关注的焦点放在如下代码：
var xmlHttp = null;
function getXmlHttp(){
    if(window.XMLHttpRequest){
        xmlHttp = new XMLHttpRequest();
    }else if(window.ActiveXObject){
        var arrXmlHttpTypes = ['MSXML2.XMLHTTP.6.0','MSXML2.XMLHTTP.3.0','Microsoft.XMLHTTP'];
        for(var i=0;i<arrXmlHttpTypes.length;i++){
            try{
                xmlHttp = new ActiveXObject(arrXmlHttpTypes[i]);
            }catch(ex){}
        }
    }
}

function getXmlDom(content){
    var _xmlDom = null;
    if (!window.DOMParser  && window.ActiveXObject){
        var arrXmlDomTypes = ['MSXML2.DOMDocument.6.0','MSXML2.DOMDocument.3.0','Microsoft.XMLDOM'];
        for(var i=0;i<arrXmlDomTypes.length;i++){
            try{
                _xmlDom = new ActiveXObject(arrXmlDomTypes[i]);
                _xmlDom.async = false;
                _xmlDom.loadXML(content);
//                _xmlDom.setProperty('SelectionLanguage', 'XPath');
            }catch(ex){}
        }
    }else{
            try{
                var domParser = new window.DOMParser();
                _xmlDom =  domParser.parseFromString(content, 'text/xml');
                Element.prototype.selectSingleNode=function(sXPath){
                    var oEvaluator = new XPathEvaluator();
                    var oResult = oEvaluator.evaluate(sXPath,this,null, XPathResult.FIRST_ORDERED_NODE_TYPE,null);
                    if(null != oResult){
                        return oResult.singleNodeValue;
                    }
                    return null;
                }
               
                Element.prototype.selectNodes = function(sXPath){
                    var oEvaluator = new XPathEvaluator();
                    var oResult = oEvaluator.evaluate(sXPath,this,null, XPathResult.ORDERED_NODE_ITERATOR_TYPE,null);
                    var aNodes = new Array();
                    if(null != oResult){
                        var oElement = oResult.iterateNext();
                        while(oElement){
                            aNodes.push(oElement);
                            oElement = oResult.iterateNext();
                        }
                    }
                    return aNodes;
                }
               
                Element.prototype.__defineGetter__("text",function(){ return this.textContent; });        }
        catch (ex)
        {
           
        }
    }
    return _xmlDom;
}              

function postRequest(url,parameters,callBack){
    if(xmlHttp !=null){
        xmlHttp.onreadystatechange = function(){
            if(xmlHttp.readyState == 4){
                if(xmlHttp.status == 200){
                    xmlDom = getXmlDom(xmlHttp.responseText);
                    if(xmlDom != null){
                        eval(callBack(xmlDom));
                    }
                }
            }
        }
        xmlHttp.open('Post',url,true);
        xmlHttp.setRequestHeader("Content-Length",parameters.length);
        xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
        xmlHttp.send(parameters);
    }
}
getXmlHttp();

        仔细一看红色标注的代码，可不是这里有问题么，用了全局的xmlHttp对象，这一定就是缓存的根本原因了，也怪一开始自己太不认真了，写这么懒的代码。赶紧改一下.

function  getXmlHttp(){
     var  _xmlHttp  =   null ;
     if (window.XMLHttpRequest){
        _xmlHttp  =   new  XMLHttpRequest();
         return  _xmlHttp;
    } else   if (window.ActiveXObject){
         var  arrXmlHttpTypes  =  [ ' MSXML2.XMLHTTP.6.0 ' , ' MSXML2.XMLHTTP.3.0 ' , ' Microsoft.XMLHTTP ' ];
         for ( var  i = 0 ;i < arrXmlHttpTypes.length;i ++ ){
             try {
                _xmlHttp  =   new  ActiveXObject(arrXmlHttpTypes[i]);
                 return  _xmlHttp;
            } catch (ex){}
        }
    }
}

function  getXmlDom(content){
     var  _xmlDom  =   null ;
     if  ( ! window.DOMParser   &&  window.ActiveXObject){
         var  arrXmlDomTypes  =  [ ' MSXML2.DOMDocument.6.0 ' , ' MSXML2.DOMDocument.3.0 ' , ' Microsoft.XMLDOM ' ];
         for ( var  i = 0 ;i < arrXmlDomTypes.length;i ++ ){
             try {
                _xmlDom  =   new  ActiveXObject(arrXmlDomTypes[i]);
                _xmlDom.async  =   false ;
                _xmlDom.loadXML(content);
//                 _xmlDom.setProperty('SelectionLanguage', 'XPath');
            } catch (ex){}
        }
    } else { 
         try {
                 var  domParser  =   new  window.DOMParser();
                _xmlDom  =   domParser.parseFromString(content,  ' text/xml ' );
                Element.prototype.selectSingleNode = function (sXPath){
                     var  oEvaluator  =   new  XPathEvaluator();
                     var  oResult  =  oEvaluator.evaluate(sXPath, this , null , XPathResult.FIRST_ORDERED_NODE_TYPE, null );
                     if ( null   !=  oResult){
                         return  oResult.singleNodeValue;
                    }
                     return   null ;
                }
                
                Element.prototype.selectNodes  =   function (sXPath){
                     var  oEvaluator  =   new  XPathEvaluator();
                     var  oResult  =  oEvaluator.evaluate(sXPath, this , null , XPathResult.ORDERED_NODE_ITERATOR_TYPE, null );
                     var  aNodes  =   new  Array();
                     if ( null   !=  oResult){
                         var  oElement  =  oResult.iterateNext();
                         while (oElement){
                            aNodes.push(oElement);
                            oElement  =  oResult.iterateNext();
                        }
                    }
                     return  aNodes;
                }
                
                Element.prototype.__defineGetter__( " text " , function (){  return   this .textContent; });
        }
         catch  (ex) 
        {
            
        }
    }
     return  _xmlDom;
}               

function  postRequest(url,parameters,callBack){
     var  xmlHttp  =  getXmlHttp();
     if (xmlHttp  != null ){
        xmlHttp.onreadystatechange  =   function (){
             if (xmlHttp.readyState  ==   4 ){
                 if (xmlHttp.status  ==   200 ){
                    xmlDom  =  getXmlDom(xmlHttp.responseText);
                     if (xmlDom  !=   null ){
                        eval(func(xmlDom));
                    }
                }
            }
        }
        xmlHttp.open( ' Post ' ,url, true );
        xmlHttp.setRequestHeader( " Content-Length " ,parameters.length);
        xmlHttp.setRequestHeader( " Content-Type " , " appli也cation/x-www-form-urlencoded " );
        xmlHttp.send(parameters);
    }
}

        最后在网上也搜索了一下关于这个问题的解决办法，有不少文章介绍了另一种办法，就是对异步调用的参数加上一个随机数，我想，这个办法也太不厚道了吧，另外有一点值得借鉴的就是使用JSON实例化xmlHttp，将属性和方法用键值对形式完全包装，从可读性和扩展性上说，绝对是个好主意。以后我也经常写写这样的例子。先放legend的版本如下，摘自（ http://www.ugia.cn/?p=85），带对象池的封装：

/* *
 * XMLHttpRequest Object Pool
 *
 * @author    legend <legendsky@hotmail.com>
 * @link      http://www.ugia.cn/?p=85
 * @Copyright www.ugia.cn
  */  

var  XMLHttp  =  {
    _objPool: [],

    _getInstance:  function  ()
    {
         for  ( var  i  =   0 ; i  <   this ._objPool.length; i  ++ )
        {
             if  ( this ._objPool[i].readyState  ==   0   ||   this ._objPool[i].readyState  ==   4 )
            {
                 return   this ._objPool[i];
            }
        }

         //  IE5中不支持push方法
         this ._objPool[ this ._objPool.length]  =   this ._createObj();

         return   this ._objPool[ this ._objPool.length  -   1 ];
    },

    _createObj:  function  ()
    {
         if  (window.XMLHttpRequest)
        {
             var  objXMLHttp  =   new  XMLHttpRequest();

        }
         else
        {
             var  MSXML  =  [ ' MSXML2.XMLHTTP.5.0 ' ,  ' MSXML2.XMLHTTP.4.0 ' ,  ' MSXML2.XMLHTTP.3.0 ' ,  ' MSXML2.XMLHTTP ' ,  ' Microsoft.XMLHTTP ' ];
             for ( var  n  =   0 ; n  <  MSXML.length; n  ++ )
            {
                 try
                {
                     var  objXMLHttp  =   new  ActiveXObject(MSXML[n]);
                     break ;
                }
                 catch (e)
                {
                }
            }
         }          

         //  mozilla某些版本没有readyState属性
         if  (objXMLHttp.readyState  ==   null )
        {
            objXMLHttp.readyState  =   0 ;

            objXMLHttp.addEventListener( " load " ,  function  ()
                {
                    objXMLHttp.readyState  =   4 ;

                     if  ( typeof  objXMLHttp.onreadystatechange  ==   " function " )
                    {
                        objXMLHttp.onreadystatechange();
                    }
                },   false );
        }

         return  objXMLHttp;
    },

     //  发送请求(方法[post,get], 地址, 数据, 回调函数)
    sendReq:  function  (method, url, data, callback)
    {
         var  objXMLHttp  =   this ._getInstance();

         with (objXMLHttp)
        {
             try
            {
                 //  加随机数防止缓存
                 if  (url.indexOf( " ? " )  >   0 )
                {
                    url  +=   " &randnum= "   +  Math.random();
                }
                 else
                {
                    url  +=   " ?randnum= "   +  Math.random();
                }

                open(method, url,  true );

                 //  设定请求编码方式
                setRequestHeader( ' Content-Type ' ,  ' application/x-www-form-urlencoded; charset=UTF-8 ' );
                send(data);
                onreadystatechange  =   function  ()
                {
                     if  (objXMLHttp.readyState  ==   4   &&  (objXMLHttp.status  ==   200   ||  objXMLHttp.status  ==   304 ))
                    {
                        callback(objXMLHttp);
                    }
                }
            }
             catch (e)
            {
                alert(e);
            }
        }
    }
};  

转载于:https://www.cnblogs.com/BeanHsiang/archive/2008/03/15/1107131.html