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An org.apache.hadoop.mapred compatible API for using Avro Serializatin in Hadoop

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/*
 * 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
 *
 *     https://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
 * limitations under the License.
 */

package org.apache.avro.mapred;

import java.io.IOException;

import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.util.ReflectionUtils;

/**
 * An {@link Mapper} that delegates behaviour of paths to multiple other
 * mappers. Similar to {@link HadoopMapper}, but instantiates map classes in the
 * map() call instead of during configure(), as we rely on the split object to
 * provide us that information.
 *
 * @see {@link AvroMultipleInputs#addInputPath(JobConf, Path, Class, Schema)}
 */
class DelegatingMapper extends MapReduceBase
    implements Mapper, NullWritable, KO, VO> {
  AvroMapper mapper;
  JobConf conf;
  boolean isMapOnly;
  AvroCollector out;

  @Override
  public void configure(JobConf conf) {
    this.conf = conf;
    this.isMapOnly = conf.getNumReduceTasks() == 0;
  }

  @Override
  public void map(AvroWrapper wrapper, NullWritable value, OutputCollector collector, Reporter reporter)
      throws IOException {
    if (mapper == null) {
      TaggedInputSplit is = (TaggedInputSplit) reporter.getInputSplit();
      Class mapperClass = is.getMapperClass();
      mapper = (AvroMapper) ReflectionUtils.newInstance(mapperClass, conf);
    }
    if (out == null)
      out = new MapCollector(collector, isMapOnly);
    mapper.map(wrapper.datum(), out, reporter);
  }
}