$.getJSON返回无数据

function getJsonData(attr,val) {
    $.ajaxSettings.async = false;//必须加的，若不加返回的是""
    var ret;
    $.getJSON('http://localhost:8008/js/modules/jobInterface/shareFiles/data.json', function(rollback){
        ret= rollback[attr][val];
    });
       return ret;
}