org.apache.hadoop.hdfs.DFSInputStream

read 接口的关键逻辑在以下 pread 接口。

  private int pread(long position, ByteBuffer buffer)
      throws IOException {
    // sanity checks
    dfsClient.checkOpen();
    if (closed.get()) {
      throw new IOException("Stream closed");
    }
    failures = 0;
    long filelen = getFileLength();
    if ((position < 0) || (position >= filelen)) {
      return -1;
    }
    int length = buffer.remaining();
    int realLen = length;
    if ((position + length) > filelen) {
      realLen = (int)(filelen - position);
    }

    // determine the block and byte range within the block
    // corresponding to position and realLen
    List<LocatedBlock> blockRange = getBlockRange(position, realLen);
    int remaining = realLen;
    CorruptedBlocks corruptedBlocks = new CorruptedBlocks();
    for (LocatedBlock blk : blockRange) {
      long targetStart = position - blk.getStartOffset();
      int bytesToRead = (int) Math.min(remaining,
          blk.getBlockSize() - targetStart);
      long targetEnd = targetStart + bytesToRead - 1;
      try {
        if (dfsClient.isHedgedReadsEnabled() && !blk.isStriped()) {
          hedgedFetchBlockByteRange(blk, targetStart,
              targetEnd, buffer, corruptedBlocks);
        } else {
          fetchBlockByteRange(blk, targetStart, targetEnd,
              buffer, corruptedBlocks);
        }
      } finally {
        // Check and report if any block replicas are corrupted.
        // BlockMissingException may be caught if all block replicas are
        // corrupted.
        reportCheckSumFailure(corruptedBlocks, blk.getLocations().length,
            false);
      }

      remaining -= bytesToRead;
      position += bytesToRead;
    }
    assert remaining == 0 : "Wrong number of bytes read.";
    return realLen;
  }

遇到临界位置， 这边起主要作用

    if ((position + length) > filelen) {
      realLen = (int)(filelen - position);
    }

hadoop 中 H1SeekableInputStream 和 H2SeekableInputStream

• H1SeekableInputStream

/**
 * SeekableInputStream implementation that implements read(ByteBuffer) for
 * Hadoop 1 FSDataInputStream.
 */
class H1SeekableInputStream extends DelegatingSeekableInputStream {

H1SeekableInputStream 直接使用父类 DelegatingSeekableInputStream 中的 readFully 方法。

@Override
  public int read(ByteBuffer buf) throws IOException {
    if (buf.hasArray()) {
      return readHeapBuffer(stream, buf);
    } else {
      return readDirectBuffer(stream, buf, temp);
    }
  }

最后到这里：

  // Visible for testing
  static void readFullyHeapBuffer(InputStream f, ByteBuffer buf) throws IOException {
    readFully(f, buf.array(), buf.arrayOffset() + buf.position(), buf.remaining());
    buf.position(buf.limit());
  }

巧妙的转成了 bytes 数组，进行读写。

• H2SeekableInputStream

/**
 * SeekableInputStream implementation for FSDataInputStream that implements
 * ByteBufferReadable in Hadoop 2.
 */
class H2SeekableInputStream extends DelegatingSeekableInputStream {

H2SeekableInputStream 覆写了 父类 DelegatingSeekableInputStream 中的 readFully 方法。

@Override
  public void readFully(ByteBuffer buf) throws IOException {
    readFully(reader, buf);
  }

org.apache.parquet.hadoop.util

• org.apache.parquet.hadoop.util.HadoopInputFile

  public SeekableInputStream newStream() throws IOException {
    return HadoopStreams.wrap(fs.open(stat.getPath()));
  }

• org.apache.parquet.hadoop.util.HadoopStreams

相关返回哪个 SeekableInputStream， 逻辑在这里。

  public static SeekableInputStream wrap(FSDataInputStream stream) {
    Objects.requireNonNull(stream, "Cannot wrap a null input stream");
    if (byteBufferReadableClass != null && h2SeekableConstructor != null &&
        byteBufferReadableClass.isInstance(stream.getWrappedStream())) {
      try {
        return h2SeekableConstructor.newInstance(stream);
      } catch (InstantiationException | IllegalAccessException e) {
        LOG.warn("Could not instantiate H2SeekableInputStream, falling back to byte array reads", e);
        return new H1SeekableInputStream(stream);
      } catch (InvocationTargetException e) {
        throw new ParquetDecodingException(
            "Could not instantiate H2SeekableInputStream", e.getTargetException());
      }
    } else {
      return new H1SeekableInputStream(stream);
    }
  }