rest api mashall object to json default not ignore null fileds

mashall object to Json default not ignore null fileds,but mashall to xml is ok .
"user":

{ "id": 26357, "gameId": null, "userIds": null, "userName": "testn008", "status": 1, "password": null, "payPassword": null, "qq": null, "email": null, "userType": 16, "level": 0, "subCompany": null, "majorShareholder": null, "sharehoder": null, "majorAgent": null, "agent": null, "nickName": "sdfs", "cellPhoneNumber": null, "createTime": "2015-05-08 20:32:37", "currency": "CNY", "registerIp": "127.0.0.1", "lastLoginIP": "127.0.0.1", "lastLoginTime": 1444146936000, "roles": [], "roleIds": [], "userCount": 0, "fromId": null, "fromUrl": null, "hasUpdatePwd": 1, "isSpread": 0, "spreadType": null, "parentUserName": null, "memberCnt": null, "serverTime": "2015-10-06 23:58:40" }

,