InputFormat牛逼（2）org.apache.hadoop.mapreduce.InputSplit & DBInputSplit

@Public
@Stable

InputSplit represents the data to be processed by an individual Mapper.

Typically, it presents a byte-oriented view on the input and is the responsibility of RecordReader of the job to process this and present a record-oriented view.

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@InterfaceAudience.Public
@InterfaceStability.Stable
public abstract class InputSplit {
  /**
   * Get the size of the split, so that the input splits can be sorted by size.
   * @return the number of bytes in the split
   * @throws IOException
   * @throws InterruptedException
   */
  public abstract long getLength() throws IOException, InterruptedException;

  /**
   * Get the list of nodes by name where the data for the split would be local.
   * The locations do not need to be serialized.
   *
   * @return a new array of the node nodes.
   * @throws IOException
   * @throws InterruptedException
   */
  public abstract
    String[] getLocations() throws IOException, InterruptedException;
 
  /**
   * Gets info about which nodes the input split is stored on and how it is
   * stored at each location.
   *
   * @return list of <code>SplitLocationInfo</code>s describing how the split
   *    data is stored at each location. A null value indicates that all the
   *    locations have the data stored on disk.
   * @throws IOException
   */
  @Evolving
  public SplitLocationInfo[] getLocationInfo() throws IOException {
    return null;
  }

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/**
   * A InputSplit that spans a set of rows
   */
  @InterfaceStability.Evolving
  public static class DBInputSplit extends InputSplit implements Writable {

    private long end = 0;
    private long start = 0;

    /**
     * Default Constructor
     */
    public DBInputSplit() {
    }

    /**
     * Convenience Constructor
     * @param start the index of the first row to select
     * @param end the index of the last row to select
     */
    public DBInputSplit(long start, long end) {
      this.start = start;
      this.end = end;
    }

    /** {@inheritDoc} */
    public String[] getLocations() throws IOException {
      // TODO Add a layer to enable SQL "sharding" and support locality
      return new String[] {};
    }

    /**
     * @return The index of the first row to select
     */
    public long getStart() {
      return start;
    }

    /**
     * @return The index of the last row to select
     */
    public long getEnd() {
      return end;
    }

    /**
     * @return The total row count in this split
     */
    public long getLength() throws IOException {
      return end - start;
    }

    /** {@inheritDoc} */
    public void readFields(DataInput input) throws IOException {
      start = input.readLong();
      end = input.readLong();
    }

    /** {@inheritDoc} */
    public void write(DataOutput output) throws IOException {
      output.writeLong(start);
      output.writeLong(end);
    }
  }
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