@Override
  public void evaluate(VectorizedRowBatch batch) throws HiveException {

    if (childExpressions != null) {
      super.evaluateChildren(batch);
    }

    ColumnVector inputColVector = batch.cols[inputColumnNum[0]];
    int[] sel = batch.selected;
    boolean[] inputIsNull = inputColVector.isNull;
    int n = batch.size;
    LongColumnVector outputColVector = (LongColumnVector) batch.cols[outputColumnNum];
    long[] outputVector = outputColVector.vector;
    boolean[] outputIsNull = outputColVector.isNull;

    if (n <= 0) {
      // Nothing to do, this is EOF
      return;
    }

    // We do not need to do a column reset since we are carefully changing the output.
    outputColVector.isRepeating = false;

    if (inputColVector.noNulls) {
      outputColVector.isRepeating = true;
      outputIsNull[0] = false;
      outputVector[0] = 0;
    } else if (inputColVector.isRepeating) {
      outputColVector.isRepeating = true;
      outputIsNull[0] = false;
      outputVector[0] = inputIsNull[0] ? 1 : 0;
    } else {

      /*
       * Since we have a result for all rows, we don't need to do conditional NULL maintenance or
       * turn off noNulls..
       */

      if (batch.selectedInUse) {
        for (int j = 0; j != n; j++) {
          int i = sel[j];
          outputIsNull[i] = false;
          outputVector[i] = inputIsNull[i] ? 1 : 0;
        }
      } else {
        Arrays.fill(outputIsNull, 0, n, false);
        for (int i = 0; i != n; i++) {
          outputVector[i] = inputIsNull[i] ? 1 : 0;
        }
      }
    }
  }
This expression evaluates to true if the given input columns is null.
The boolean output is stored in the specified output column.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/IsNull.java