lang/java/mapred/src/main/java/org/apache/avro/mapred/HadoopCombiner.java

/**
 * 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
 * limitations under the License.
 */

package org.apache.avro.mapred;

import java.io.IOException;

import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.util.ReflectionUtils;

/** Bridge between a {@link org.apache.hadoop.mapred.Reducer} and an {@link
 * AvroReducer} used when combining.  When combining, map output pairs must be
 * split before they're collected. */
class HadoopCombiner<K,V>
  extends HadoopReducerBase<K,V,Pair<K,V>,AvroKey<K>,AvroValue<V>> {

  @Override @SuppressWarnings("unchecked")
  protected AvroReducer<K,V,Pair<K,V>> getReducer(JobConf conf) {
    return ReflectionUtils.newInstance
      (conf.getClass(AvroJob.COMBINER, AvroReducer.class, AvroReducer.class),
       conf);
  }

  private class PairCollector extends AvroCollector<Pair<K,V>> {
    private final AvroKey<K> keyWrapper = new AvroKey<K>(null);
    private final AvroValue<V> valueWrapper = new AvroValue<V>(null);
    private OutputCollector<AvroKey<K>,AvroValue<V>> collector;

    public PairCollector(OutputCollector<AvroKey<K>,AvroValue<V>> collector) {
      this.collector = collector;
    }

    public void collect(Pair<K,V> datum) throws IOException {
      keyWrapper.datum(datum.key());              // split the Pair
      valueWrapper.datum(datum.value());
      collector.collect(keyWrapper, valueWrapper);
    }
  }

  @Override
  protected AvroCollector<Pair<K,V>>
    getCollector(OutputCollector<AvroKey<K>,AvroValue<V>> collector) {
    return new PairCollector(collector);
  }

}