function createXHR () {//创建XMLHttpRequest对象

         var xhr=null;

         if(window.XMLHttpRequest){

             createXHR=createStandardXHR; // Firefox, Opera 8.0+,Safari，IE7+

         }

         else{

             createXHR=createCompatibleXHR; // IE5、IE6

         }

         try{

             xhr=createXHR();

         }

         catch(e){

             createXHR = createErrorXHR;

             xhr=createXHR();

         }

         return xhr;

     }

     function createStandardXHR(){

         return new XMLHttpRequest();

     }

     function createCompatibleXHR(){

         return new ActiveXObject("Microsoft.XMLHTTP");

     }

     function createErrorXHR(){

         alert("Your browser does not support XMLHTTP.");

         return null;

     }