hadoop Partitioner 源码

  • 2022-10-20
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haddop Partitioner 代码


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 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
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 *     http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.hadoop.mapreduce;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configurable;

 * Partitions the key space.
 * <p><code>Partitioner</code> controls the partitioning of the keys of the 
 * intermediate map-outputs. The key (or a subset of the key) is used to derive
 * the partition, typically by a hash function. The total number of partitions
 * is the same as the number of reduce tasks for the job. Hence this controls
 * which of the <code>m</code> reduce tasks the intermediate key (and hence the 
 * record) is sent for reduction.</p>
 * <p>Note: A <code>Partitioner</code> is created only when there are multiple
 * reducers.</p>
 * <p>Note: If you require your Partitioner class to obtain the Job's
 * configuration object, implement the {@link Configurable} interface.</p>
 * @see Reducer
public abstract class Partitioner<KEY, VALUE> {
   * Get the partition number for a given key (hence record) given the total 
   * number of partitions i.e. number of reduce-tasks for the job.
   * <p>Typically a hash function on a all or a subset of the key.</p>
   * @param key the key to be partioned.
   * @param value the entry value.
   * @param numPartitions the total number of partitions.
   * @return the partition number for the <code>key</code>.
  public abstract int getPartition(KEY key, VALUE value, int numPartitions);


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