学步园

package hive.udaf;


import org.apache.hadoop.hive.ql.exec.UDAF;
import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;

/**
 * 
 * It should be very easy to follow and can be used as an example for writing
 * new UDAFs.
 * 
 * Note that Hive internally uses a different mechanism (called GenericUDAF) to
 * implement built-in aggregation functions, which are harder to program but
 * more efficient.
 * 
 */
public final class ActionCount extends UDAF {

  /**
   * The internal state of an aggregation for average.
   * 
   * Note that this is only needed if the internal state cannot be represented
   * by a primitive.
   * 
   * The internal state can also contains fields with types like
   * ArrayList<String> and HashMap<String,Double> if needed.
   */
  public static class UDAFState {
    private long mCount;
    private long mSum;
  }

  /**
   * The actual class for doing the aggregation. Hive will automatically look
   * for all internal classes of the UDAF that implements UDAFEvaluator.
   */
  public static class UDAFExampleAvgEvaluator implements UDAFEvaluator {

    UDAFState state;

    public UDAFExampleAvgEvaluator() {
      super();
      state = new UDAFState();
      init();
    }

    /**
     * Reset the state of the aggregation.
     */
    public void init() {
      state.mSum = 0;
      state.mCount = 0;
    }

    /**
     * Iterate through one row of original data.
     * 
     * The number and type of arguments need to the same as we call this UDAF
     * from Hive command line.
     * 
     * This function should always return true.
     */
    public boolean iterate(String act_code,long act_times,String act_type) // 来了一行
    { 	
      if (act_code .equals(act_type)) 
      {
        state.mSum += act_times;
        state.mCount++;
      }
      return true;
    }

    /**
     * Terminate a partial aggregation and return the state. If the state is a
     * primitive, just return primitive Java classes like Integer or String.
     */
    public UDAFState terminatePartial() {//状态传递
      // This is SQL standard - average of zero items should be null.
      return state.mCount == 0 ? null : state;
    }

    /**
     * Merge with a partial aggregation.
     * 
     * This function should always have a single argument which has the same
     * type as the return value of terminatePartial().
     */
    public boolean merge(UDAFState o) {//子任务合并
      if (o != null) {
        state.mSum += o.mSum;
        state.mCount += o.mCount;
      }
      return true;
    }

    /**
     * Terminates the aggregation and return the final result.
     */
    public long terminate() {//返回最终结果
      // This is SQL standard - average of zero items should be null.
      return state.mCount == 0 ? 0 : state.mSum;
    }
  }

  private ActionCount() {
    // prevent instantiation
  }

}

关键还是要深刻理解map-reduce工作模型，才能更好驾驭hive。