hadoop-common源码分析之-WritableUtils，hadoopcommon源码

hadoop-common源码分析之-WritableUtils，hadoopcommon源码

hadoop将java的基本类型进行封装，对整型进行编码时，分为固定长度格式、可变长度格式。可变长度格式使用一种比较灵活的编码方式，对与较小的数（尤其是负数）可以节省空间存储。

VIntWritable

public class VIntWritable implements WritableComparable<VIntWritable> {
  private int value;

//getter //setter
  @Override
  public void readFields(DataInput in) throws IOException {
    value = WritableUtils.readVInt(in);
  }

  @Override
  public void write(DataOutput out) throws IOException {
    WritableUtils.writeVInt(out, value);
  }

}

WritableUtils.writeVLong

  public static void writeVInt(DataOutput stream, int i) throws IOException {
    writeVLong(stream, i);
  }
  public static void writeVLong(DataOutput stream, long i) throws IOException {
    if (i >= -112 && i <= 127) {
      stream.writeByte((byte)i);
      return;
    }

    int len = -112;
    if (i < 0) {
      i ^= -1L; // take one's complement'
      len = -120;
    }

    long tmp = i;
    while (tmp != 0) {
      tmp = tmp >> 8;
      len--;
    }

    stream.writeByte((byte)len);

    len = (len < -120) ? -(len + 120) : -(len + 112);

    for (int idx = len; idx != 0; idx--) {
      int shiftbits = (idx - 1) * 8;
      long mask = 0xFFL << shiftbits;
      System.out.println(((i & mask) >> shiftbits));
      stream.writeByte((byte)((i & mask) >> shiftbits));
    }
  }

WritableUtils.readVLong

  public static long readVLong(DataInput stream) throws IOException {
    byte firstByte = stream.readByte();
    int len = decodeVIntSize(firstByte);
    if (len == 1) {
      return firstByte;
    }
    long i = 0;
    for (int idx = 0; idx < len-1; idx++) {
      byte b = stream.readByte();
      i = i << 8;
      i = i | (b & 0xFF);
    }
    return (isNegativeVInt(firstByte) ? (i ^ -1L) : i);
  }
  public static int decodeVIntSize(byte value) {
    if (value >= -112) {
      return 1;
    } else if (value < -120) {
      return -119 - value;
    }
    return -111 - value;
  } 
  public static boolean isNegativeVInt(byte value) {
    return value < -120 || (value >= -112 && value < 0);
  }

总结：

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