import org.apache.hadoop.hdfs.server.namenode.FSEditLogLoader.PositionTrackingInputStream; //导入依赖的package包/类
/**
 * Refresh, preferably to a known position
 * @see EditLogInputStream#refresh(long)
 * @see BookKeeperJournalInputStream#position(long)
 */
@Override
public void refresh(long position, long skippedUntilTxid) throws IOException {
  checkInitialized();

  if (isInProgress()) {
    // If a ledger is in progress, re-open it for reading in order
    // to determine the correct bounds of the ledger.
    LedgerHandle ledger = ledgerProvider.openForReading(ledgerId);
    journalInputStream.resetLedger(ledger);
  }
  // Try to set the underlying stream to the specified position
  journalInputStream.position(position);
  // Reload the position tracker and log reader to adjust to the newly
  // refreshed position
  bin = new BufferedInputStream(journalInputStream);
  tracker = new PositionTrackingInputStream(bin, position);
  DataInputStream in = new DataInputStream(tracker);
  if (position == 0) { // If we are at the beginning, re-read the version
    logVersion = readLogVersion(in);
  }
  reader = new Reader(in, logVersion);
}
 

import org.apache.hadoop.hdfs.server.namenode.FSEditLogLoader.PositionTrackingInputStream; //导入依赖的package包/类
public void init() throws IOException {
  LedgerHandle ledger = ledgerProvider.openForReading(ledgerId);
  if (!isInProgress() && firstBookKeeperEntry > ledger.getLastAddConfirmed()) {
    // ledger.getLastAddConfirmed() returns the last quorum-acknowledged entry
    // id in the ledger. Unless the segment is in-progress, we should throw
    // an exception if this last entry is lower than the first expected entry id.
    throw new IllegalArgumentException(
        "Invalid first BookKeeper entry to read: " + firstBookKeeperEntry +
            " > last confirmed entry id(" + ledger.getLastAddConfirmed() +
            ")");
  }

  journalInputStream = new BookKeeperJournalInputStream(ledger,
      firstBookKeeperEntry);
  bin = new BufferedInputStream(journalInputStream);
  tracker = new PositionTrackingInputStream(bin, 0);
  DataInputStream in = new DataInputStream(tracker);
  try {
    logVersion = readLogVersion(in);
  } catch (EOFException e) {
    throw new LedgerHeaderCorruptException("No header file in the ledger");
  }
  reader = new Reader(in, logVersion);
  LOG.info("Reading from ledger id " + ledgerId +
      ", starting with book keeper entry id " + firstBookKeeperEntry +
      ", log version " + logVersion +
      ", first txn id " +
      firstTxId +  (isInProgress() ?
                        ", in-progress log segment"
                        : (" last txn id " + lastTxId))
      + ".");
}