@Override
    public String toString() {
        if (id == null) {
            return null;
        }
        //长度必须是固定的
        assert (id.length == MESSAGE_ID_LEN);

        StringBuffer buffer = new StringBuffer();

        for (int i = MESSAGE_ID_LEN - 1; i >= 0; i--) {
            String hex = Integer.toHexString(id[i]);
            hex = hex.length() > 1 ? hex : ("0" + hex);
            hex = hex.substring(0, 2);
            buffer.append(hex);
        }
        return buffer.toString();
    }