缺省Null值是不输出的，这个不能满足要求么？

缺省 Null 值是不输出的，但是我配置了一些 Write Feature 之后就不行了，但是这个 Feature 和 String 无关啊，可能是个 Bug?
看看是什么原理 @wenshao

Response.java

public class Response implements Serializable {
    private static final long serialVersionUID = 1L;

    /**

* successful
* /

    private boolean success;

    /**

* error code
* /

    private String errCode;

    /**

* error message
* /

    private String errMessage;

    public boolean isSuccess() {
        return success;
    }

    public void setSuccess(boolean success) {
        this.success = success;
    }

    public String getErrCode() {
        return errCode;
    }

    public void setErrCode(String errCode) {
        this.errCode = errCode;
    }

    public String getErrMessage() {
        return errMessage;
    }

    public void setErrMessage(String errMessage) {
        this.errMessage = errMessage;
    }
}

public static void main(String[] args) {
    Response response = new Response();
    response.setSuccess(true);

    // {"success":true}
    System.out.println(JSON.toJSONString(response));

    // {"errCode":null,"errMessage":null,"success":true}
    System.out.println(JSON.toJSONString(response, JSONWriter.Feature.WriteNullListAsEmpty));

    // {"success":true}
    System.out.println(JSON.toJSONString(response, JSONWriter.Feature.BrowserCompatible));
}

如何，能否确定是什么问题？ @wenshao