jsonp跨域请求问题

$.ajax({
url: 'http://192.168.3.49:8080/PORTAL/authCode',
type: 'GET',
dataType:'jsonp',
jsonp: "callback",
success:function(data){
alert(data.ret)
},
error:function(data){
$('#mstr_ck').html("获取验证码失败，请重试！");
$("#error_ck").show();
}
});

@RequestMapping(value = "authCode")
@ResponseBody
public String getMobileAuthCode( HttpServletRequest request, String callback)
throws Exception {
String result =  "{'ret':'true'}";
//加上返回参数
result=callback+"("+result+")";
return result;
}