//首先备份下jquery的ajax方法

    var _ajax=$.ajax;  

    //重写jquery的ajax方法

    $.ajax=function(opt){

        //备份opt中error和success方法

        var fn = {

            error:function(XMLHttpRequest, textStatus, errorThrown){},

            success:function(data, textStatus){}

        }

        if(opt.error){

            fn.error=opt.error;

        }

        if(opt.success){

            fn.success=opt.success;

        }  

        //扩展增强处理

        var _opt = $.extend(opt,{

            error:function(XMLHttpRequest, textStatus, errorThrown){

            debugger;

                erro = eval("(" + XMLHttpRequest.responseText + ")");

                if(erro.err_code == 500)

                    alert(erro.err_msg);

                //错误方法增强处理

                fn.error(XMLHttpRequest, textStatus, errorThrown);

            },

            success:function(data, textStatus){

                //成功回调方法增强处理

                fn.success(data, textStatus);

            },

            beforeSend:function(xhr){

                xhr.setRequestHeader('X-Token',$.getCookie("X-Token"));

                 xhr.setRequestHeader('X-Client',"PC");

                xhr.setRequestHeader('Content-Type',"application/json");

            }

        });

        return _ajax(_opt);

    };