[Hadoop Wiki] Update of "Hive/DeveloperGuide/UDTF" by PaulYang

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The "Hive/DeveloperGuide/UDTF" page has been changed by PaulYang.
http://wiki.apache.org/hadoop/Hive/DeveloperGuide/UDTF

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New page:
#pragma section-numbers 2
= Writing UDTF's =
<<TableOfContents>>

== GenericUDTF Interface ==

To create a UDTF, you must extend the GenericUDTF abstract class and implement the initialize() and process() methods. The initialize method is called by Hive to notify the UDTF the argument types to expect. The UDTF must then return an object inspector corresponding to the row objects that the UDTF will generate.

For reference, here is the abstract class:

{{{
/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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 */

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. Note that
   * forward() should not be called in this function. Only clean up code should
   * be run.
   */
  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);
  }

}

}}}