ajax session过期问题的几个解决方案

;}
        if((this.openURL==null)||(this.openURL=="")){
            Alert("请指定目标URL");
            return false;
        }
        try{
            this.HttpRequest.open(this.openMethod, this.openURL, this.openAsync, username, psw);
        }catch(e){
            if(e instanceof Error){
                Alert("无法建立HTTP连接");
                return false;
            }
        }

        if(this.openMethod.toUpperCase()=="POST"){
            if(!this.setRequestHeader("application/x-www-form-urlencoded")){
                Alert("修改MIME类别失败");
                return false;
            }
        }

        if(this.openAsync){ //异步模式，程序继续执行
            if(this.ProcessRequestFunction==null){
                Alert("请指定处理返回信息的函数");
                return false;
            }
            var _http_request_ajax = this.HttpRequest;
            var _this_ajax = this;
            this.HttpRequest.onreadystatechange = function(){
                if(_http_request_ajax.readyState==4) {
                    if(_http_request_ajax.status==200) {
                        _this_ajax.ProcessRequestFunction(_http_request_ajax, _this_ajax.ProcessReque