protected long getFileNumFromFileName(Path fileName) {
    checkNotNull(fileName, "file name can't be null");
    if (!ourFiles.accept(fileName)) {
      throw new IllegalArgumentException(
        "The log file " + fileName + " doesn't belong to this WAL. (" + toString() + ")");
    }
    final String fileNameString = fileName.toString();
    String chompedPath = fileNameString.substring(prefixPathStr.length(),
      (fileNameString.length() - walFileSuffix.length()));
    return Long.parseLong(chompedPath);
  }
A log file has a creation timestamp (in ms) in its file name ({@link #filenum}. This helper
method returns the creation timestamp from a given log file. It extracts the timestamp assuming
the filename is created with the {@link #computeFilename(long filenum)} method.
@return timestamp, as in the log file name.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java