代码如下，success获取不到返回值

var data;
var grant_type = "client_credentials";
var client_id = "rlVkXeUg6L6hVbKwBygKEq6D";//必须参数，应用的API Key；
var client_secret = "cto5Hj1ZBLUISl6os8kqiasv7sP6YQBD";//必须参数，应用的Secret Key；
app.loading(true);
$.ajax({
url: "https://aip.baidubce.com/oauth/2.0/token？" + grant_type + "&" + client_id + "&" + client_secret,
type: "POST",
dataType: "json",
async: false,
contentType: "application/json;charset=utf-8",
success: function (value) {
data = value;
app.loading(false);
},
error: function (err)
{
alert(err);
}
}).fail(function (error) {
data = error;
app.loading(false);
});