public String build() {
        sealed = true;
        if (builtID == null) {
            builtID = Joiner.on(Const.ID_CONNECTOR).join(fragments);
        }
        return builtID;
    }
@return the string ID concatenating the values of {@link #fragments} by the underline(_).
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/storage/StorageID.java