Writing UDTF's

GenericUDTF Interface

可以通过扩展 GenericUDTF 抽象类，然后实现initialize，process以及可能的close方法来创建自定义 UDTF。 Hive 调用initialize方法以通知 UDTF 期望的参数类型。然后，UDTF 必须返回与 UDTF 将生成的行对象相对应的对象检查器。 initialize()被调用后，Hive 将使用process()方法将行分配给 UDTF。在process()中时，UDTF 可以通过调用forward()来生成行并将行转发给其他运算符。最后，当所有行都传递给 UDTF 时，Hive 将调用close()方法。

UDTF Example:

package org.apache.hadoop.hive.contrib.udtf.example;

import java.util.ArrayList;

import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
import org.apache.hadoop.hive.ql.metadata.HiveException;
import org.apache.hadoop.hive.ql.udf.generic.GenericUDTF;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorFactory;
import org.apache.hadoop.hive.serde2.objectinspector.StructObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.primitive.PrimitiveObjectInspectorFactory;

/**
 * GenericUDTFCount2 outputs the number of rows seen, twice. It's output twice
 * to test outputting of rows on close with lateral view.
 *
 */
public class GenericUDTFCount2 extends GenericUDTF {

  Integer count = Integer.valueOf(0);
  Object forwardObj[] = new Object[1];

  @Override
  public void close() throws HiveException {
    forwardObj[0] = count;
    forward(forwardObj);
    forward(forwardObj);
  }

  @Override
  public StructObjectInspector initialize(ObjectInspector[] argOIs) throws UDFArgumentException {
    ArrayList<String> fieldNames = new ArrayList<String>();
    ArrayList<ObjectInspector> fieldOIs = new ArrayList<ObjectInspector>();
    fieldNames.add("col1");
    fieldOIs.add(PrimitiveObjectInspectorFactory.javaIntObjectInspector);
    return ObjectInspectorFactory.getStandardStructObjectInspector(fieldNames,
        fieldOIs);
  }

  @Override
  public void process(Object[] args) throws HiveException {
    count = Integer.valueOf(count.intValue() + 1);
  }

}

作为参考，下面是抽象类：

package org.apache.hadoop.hive.ql.udf.generic;

import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
import org.apache.hadoop.hive.ql.metadata.HiveException;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.StructObjectInspector;

/**
 * A Generic User-defined Table Generating Function (UDTF)
 * 
 * Generates a variable number of output rows for a single input row. Useful for
 * explode(array)...
 */

public abstract class GenericUDTF {
  Collector collector = null;

  /**
 * Initialize this GenericUDTF. This will be called only once per instance.
 * 
 * @param args
 *          An array of ObjectInspectors for the arguments
 * @return A StructObjectInspector for output. The output struct represents a
 *         row of the table where the fields of the stuct are the columns. The
 *         field names are unimportant as they will be overridden by user
 *         supplied column aliases.
   */
  public abstract StructObjectInspector initialize(ObjectInspector[] argOIs)
      throws UDFArgumentException;

  /**
 * Give a set of arguments for the UDTF to process.
 * 
 * @param o
 *          object array of arguments
   */
  public abstract void process(Object[] args) throws HiveException;

  /**
 * Called to notify the UDTF that there are no more rows to process.
 * Clean up code or additional forward() calls can be made here.
   */
  public abstract void close() throws HiveException;

  /**
 * Associates a collector with this UDTF. Can't be specified in the
 * constructor as the UDTF may be initialized before the collector has been
 * constructed.
 * 
 * @param collector
   */
  public final void setCollector(Collector collector) {
    this.collector = collector;
  }

  /**
 * Passes an output row to the collector.
 * 
 * @param o
 * @throws HiveException
   */
  protected final void forward(Object o) throws HiveException {
    collector.collect(o);
  }

}