springmvc接收参数老是为null

后台代码:
@RequestMapping(value="/login",method=RequestMethod.POST)
@ResponseBody
public Map login(String username,String password,HttpServletRequest request,HttpServletResponse response){

    return null;
}
username 和 password一直为null

前台参数：(是用jemeter提交的)
Content-Type:application/json;charset=UTF-8
{
'username':'admin2',
'password':'123'
}