UDAF在Hive中实现主要有两种，类似于UDF：

• 继承org.apache.hadoop.hive.ql.exec.UDAF.udaf: 实现比较简单，但在Hive中已经deperated;
• 继承org.apache.hadoop.hive.ql.udf.generic.AbstractGenericUDAFResolver: 实现比较繁琐，但功能强大；

示例：实现UDAF，当有多行时，选择某一列中包含最多字符的值；

基于AbstractGenericUDAFResolver方式实现

package com.alipay.mbaprod.spark.udaf;

import org.apache.hadoop.hive.ql.exec.Description;
import org.apache.hadoop.hive.ql.exec.UDFArgumentTypeException;
import org.apache.hadoop.hive.ql.metadata.HiveException;
import org.apache.hadoop.hive.ql.parse.SemanticException;
import org.apache.hadoop.hive.ql.udf.generic.AbstractGenericUDAFResolver;
import org.apache.hadoop.hive.ql.udf.generic.GenericUDAFEvaluator;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorUtils;
import org.apache.hadoop.hive.serde2.objectinspector.primitive.StringObjectInspector;
import org.apache.hadoop.hive.serde2.typeinfo.TypeInfo;
import org.apache.hadoop.hive.serde2.typeinfo.TypeInfoUtils;


@Description(name = "longest_record", value = "_FUNC_(expr) - Returns the maximum's length row of expr")
public class LongestRecordUDAF extends AbstractGenericUDAFResolver {

  @Override
  public GenericUDAFEvaluator getEvaluator(TypeInfo[] parameters) throws SemanticException {
    // Now only support one parameter.
    if (parameters.length != 1) {
      throw new UDFArgumentTypeException(parameters.length - 1,
          "Exactly one argument is expected.");
    }

    ObjectInspector oi = TypeInfoUtils.getStandardJavaObjectInspectorFromTypeInfo(parameters[0]);
    if (!ObjectInspectorUtils.compareSupported(oi)) {
      throw new UDFArgumentTypeException(0, "Cannot support comparison of map<> type or complex type containing map<>.");
    }

    return new GenericUDAFMaxLenRowEvaluator();
  }

  // @UDFType(distinctLike=true)
  public static class GenericUDAFMaxLenRowEvaluator extends GenericUDAFEvaluator {
    ObjectInspector inputOI;
    ObjectInspector outputOI;

    @Override
    public ObjectInspector init(Mode mode, ObjectInspector[] parameters) throws HiveException {
      super.init(mode, parameters);

      inputOI = parameters[0];
      outputOI = ObjectInspectorUtils.getStandardObjectInspector(inputOI, ObjectInspectorUtils.ObjectInspectorCopyOption.JAVA);
      return outputOI;
    }

    static class MaxLenAgg implements AggregationBuffer {
      Object o;
    }

    @Override
    public AggregationBuffer getNewAggregationBuffer() throws HiveException {
      MaxLenAgg result = new MaxLenAgg();
      return result;
    }

    @Override
    public void reset(AggregationBuffer agg) throws HiveException {
      MaxLenAgg maxagg = (MaxLenAgg) agg;
      maxagg.o = null;
    }

    @Override
    public void iterate(AggregationBuffer agg, Object[] parameters) throws HiveException {
      System.out.println("iterate, len:" + parameters.length);
      merge(agg, parameters[0]);
    }

    @Override
    public Object terminatePartial(AggregationBuffer agg) throws HiveException {
      System.out.println("terminal:");
      return terminate(agg);
    }

    @Override
    public void merge(AggregationBuffer agg, Object partial) throws HiveException {
      if (partial != null) {
        MaxLenAgg maxagg = (MaxLenAgg) agg;

        if (maxagg.o != null) {
          String oldV = ((StringObjectInspector) inputOI).getPrimitiveJavaObject(maxagg.o);
          String newV = ((StringObjectInspector) inputOI).getPrimitiveJavaObject(partial);
          if (newV.length() > oldV.length()) {
            System.out.println("newV is greater, new:" + newV + ", old:" + oldV);
            maxagg.o = ObjectInspectorUtils.copyToStandardObject(partial, inputOI,
                ObjectInspectorUtils.ObjectInspectorCopyOption.JAVA);
          }
        } else {
          maxagg.o = ObjectInspectorUtils.copyToStandardObject(partial, inputOI,
              ObjectInspectorUtils.ObjectInspectorCopyOption.JAVA);
        }
      }
    }

    @Override
    public Object terminate(AggregationBuffer agg) throws HiveException {
      System.out.println("ter:");
      MaxLenAgg maxagg = (MaxLenAgg) agg;
      return maxagg.o;
    }
  }
}

基于udaf的方式实现

package com.alipay.mbaprod.spark.udaf;

import com.alipay.mbaprod.spark.udtf.CollectCardUDTF;
import org.apache.hadoop.hive.ql.exec.UDAF;
import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;
import org.apache.hadoop.hive.ql.metadata.HiveException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * @author shuming.lsm
 * @version 2019/03/29
 */
public class MaxLenRowUDAF extends UDAF {
  // Define Logging
  private static final Logger LOG = LoggerFactory.getLogger(CollectCardUDTF.class);

  public static class MaxLenRowUDAFEvaluator implements UDAFEvaluator {
    private String result;

    // A - Initalize evaluator - indicating that no values have been
    // aggregated yet.
    public void init() {
      LOG.debug("Initialize evaluator");
      result = null;
    }

    // B- Iterate every time there is a new value to be aggregated
    public boolean iterate(String value) throws HiveException {
      LOG.debug("Iterating over each value for aggregation: " + value);
      if (result == null) {
        result = value;
        return true;
      }

      if (value != null && value.length() > result.length()) {
        result = value;
      }
      return true;
    }

    // C - Called when Hive wants partially aggregated results.
    public String terminatePartial() {
      LOG.debug("Return partially aggregated results");
      return result;
    }

    // D - Called when Hive decides to combine one partial aggregation with another
    public boolean merge(String other) {
      LOG.debug("merging by combining partial aggregation");
      if(other == null) {
        return true;
      }

      if (result == null) {
        result = other;
        return true;
      }

      if (other.length() > result.length()) {
        result = other;
      }

      return true;
    }

    // E - Called when the final result of the aggregation needed.
    public String terminate(){
      LOG.debug("At the end of last record of the group - returning final result");
      return result;
    }
  }
}