function getXHR() {
  // 该方法用于获取Ajax通信对象
  var xhr = null;
  if (window.XMLHttpRequest != null 
  window.XMLHttpRequest != undefined) {
  xhr = new XMLHttpRequest();
  } else {
  / 兼容早期的IE浏览器
  xhr = new ActiveXObject("Microsoft.XMLHTTP");
  }
  return xhr;
}