ORC 讀數據源碼分析 之 createStreams

不管是 readAllDataStreams 還是 readPartialDataStreams ，最後都要 經過 createStreams

createStreams

createStreams 最後的輸出結果是這個
Map<StreamName, InStream> streams

每個 StreamName 對應的內存流

void createStreams(List<OrcProto.Stream> streamDescriptions,
      DiskRangeList ranges,
      boolean[] includeColumn,
      CompressionCodec codec,
      int bufferSize,
      Map<StreamName, InStream> streams) throws IOException {
    long streamOffset = 0;
    for (OrcProto.Stream streamDesc : streamDescriptions) {
      int column = streamDesc.getColumn();
      if ((includeColumn != null &&
          (column < includeColumn.length && !includeColumn[column])) ||
          streamDesc.hasKind() &&
              (StreamName.getArea(streamDesc.getKind()) != StreamName.Area.DATA)) {
        streamOffset += streamDesc.getLength();
        continue;
      }
      //獲得流對應的 撕裂開的 bytebuffer
      List<DiskRange> buffers = RecordReaderUtils.getStreamBuffers(
          ranges, streamOffset, streamDesc.getLength());
      StreamName name = new StreamName(column, streamDesc.getKind());
      // 輸出的結果
      streams.put(name, InStream.create(name.toString(), buffers,
          streamDesc.getLength(), codec, bufferSize));
      streamOffset += streamDesc.getLength();
    }
  }

 /**
   * Create an input stream from a list of disk ranges with data.
   * @param name the name of the stream
   * @param input the list of ranges of bytes for the stream; from disk or cache
   * @param length the length in bytes of the stream
   * @param codec the compression codec
   * @param bufferSize the compression buffer size
   * @return an input stream
   * @throws IOException
   */
  public static InStream create(String name,
                                List<DiskRange> input,
                                long length,
                                CompressionCodec codec,
                                int bufferSize) throws IOException {
    if (codec == null) {
      return new UncompressedStream(name, input, length);
    } else {
      return new CompressedStream(name, input, length, codec, bufferSize);
    }
  }```