Java 对象映射程序序列化在序列化字符串中创建其他字段

我正在使用

ObjectMapper writeValueAsString

序列化java对象，我的java pojo如下所示-

编辑-将注释移动到getter方法

public class RestrictRecharge121SMS {
   
    public String MSG;
    public String Timestamp;

    public RestrictRecharge121SMS(String msg,String timestamp) {
        this.Timestamp = timestamp;
        this.MSG=msg;
    }

    @JsonProperty("MSG")
    public String getMSG() {
        return MSG;
    }

    public void setMSG(String MSG) {
        this.MSG = msg;
    }

    @JsonProperty("Timestamp")
    public String getTimestamp() {
        return Timestamp;
    }

    public void setTimestamp(String timestamp) {
        Timestamp = timestamp;
    }

}

下面是将其序列化的代码-

private static final ObjectMapper mapper = new ObjectMapper();
RestrictRecharge121SMS sms=new RestrictRecharge121SMS(notification.getRTN(),getTransactionTime());
String message = mapper.writeValueAsString(sms);

在序列化时，我得到如下输出字符串-

{"MSG":"Hi..","msg":"Hi..","Timestamp":"2020-07-04 11:11:36","timestamp":"2020-07-04 11:11:36"}

如何强制使用对象映射器来限制在camel情况下创建的其他键

期望值-

{"MSG":"Hi..","Timestamp":"2020-07-04 11:11:36"}