UDAF:oneof(任意获得待聚合集中的一个)

```
package cn.cqc3073.hive.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.typeinfo.TypeInfo;

/**
 * 任意获得待聚合集中的一个
 * 支持复杂数据结构(map, array)
 * Create by chenqichuan on 2017/7/11.
 *
 */
@Description(
        name = "oneof",
        value = "_FUNC_(expr) - Returns one of value of expr"
)
public class OneOfCollect extends AbstractGenericUDAFResolver{
    @Override
    public GenericUDAFEvaluator getEvaluator(TypeInfo[] parameters) throws SemanticException {
        if(parameters.length != 1) {
            throw new UDFArgumentTypeException(parameters.length - 1, "Exactly one argument is expected.");
        }
        return new OneOfCollectEvaluator();
    }

public static class OneOfCollectEvaluator extends GenericUDAFEvaluator{
        private ObjectInspector inputOI;
        private ObjectInspector outputOI;
        static class OneOfCollectAgg implements AggregationBuffer{
            Object o;
        }

@Override
        public ObjectInspector init(Mode m, ObjectInspector[] parameters) throws HiveException {
            super.init(m, parameters);
            assert parameters.length == 1;
            inputOI = parameters[0];
            outputOI = ObjectInspectorUtils.getStandardObjectInspector(inputOI);
            return outputOI;
        }

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

@Override
        public void reset(AggregationBuffer aggregationBuffer) throws HiveException {
            ((OneOfCollectAgg)aggregationBuffer).o = null;
        }

@Override
        public void iterate(AggregationBuffer aggregationBuffer, Object[] objects) throws HiveException {
            merge(aggregationBuffer, objects[0]);
        }

private Object readValue(Object o){
            return ObjectInspectorUtils.copyToStandardObject(o, inputOI);
        }

@Override
        public Object terminatePartial(AggregationBuffer aggregationBuffer) throws HiveException {
            return terminate(aggregationBuffer);
        }

@Override
        public void merge(AggregationBuffer aggregationBuffer, Object o) throws HiveException {
            if (o == null) {
                return;
            }
            OneOfCollectAgg agg = (OneOfCollectAgg)aggregationBuffer;
            if (agg.o == null){
                agg.o =  readValue(o);
            }
        }

@Override
        public Object terminate(AggregationBuffer aggregationBuffer) throws HiveException {
            return ((OneOfCollectAgg)aggregationBuffer).o;
        }
    }
}

```

阿川CH