function postJson(){
  var json = {"username" : "imp", "password" : "123456"};
  $.ajax({
    type : "post",
    url : "<%=basePath %>ajaxRequest",
    contentType : "application/json;charset=utf-8",
    dataType : "json",
    data: JSON.stringify(json),
    success : function(data){
      alert("username："+data.username+"  id："+data.id);
    },
    error : function(){
      alert("请求失败");
    }
  })
}

<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

  @ResponseBody
  @RequestMapping(value="/ajaxRequest",method=RequestMethod.POST)
  public User ajaxRequest(@RequestBody User user){
    System.out.println(user);
    return user;
  }