MaxSecondSort 代码片段以及说明

/** * 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.hadoop.examples;import java.io.DataInput;import java.io.DataOutput;import java.io.IOException;import java.util.StringTokenizer;import org.apache.hadoop.conf.Configuration;import org.apache.hadoop.fs.Path;import org.apache.hadoop.io.IntWritable;import org.apache.hadoop.io.LongWritable;import org.apache.hadoop.io.RawComparator;import org.apache.hadoop.io.Text;import org.apache.hadoop.io.WritableComparable;import org.apache.hadoop.io.WritableComparator;import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;import org.apache.hadoop.mapreduce.Job;import org.apache.hadoop.mapreduce.Mapper;import org.apache.hadoop.mapreduce.Partitioner;import org.apache.hadoop.mapreduce.Reducer;import org.apache.hadoop.util.GenericOptionsParser;/** * This is an example Hadoop Map/Reduce application. * It reads the text input files that must contain two integers per a line. * The output is sorted by the first and second number and grouped on the  * first number. * * To run: bin/hadoop jar build/hadoop-examples.jar secondarysort *            <i>in-dir</i> <i>out-dir</i>  */public class MaxSecondarySort {   /**   * Define a pair of integers that are writable.   * They are serialized in a byte comparable format.   */  public static class IntPair                       implements WritableComparable<IntPair> {    private int first = 0;    private int second = 0;        /**     * Set the left and right values.     */    public void set(int left, int right) {      first = left;      second = right;    }    public IntPair(){}    public IntPair(int left,int right){    set(left, right);    }    public int getFirst() {      return first;    }    public int getSecond() {      return second;    }    /**     * Read the two integers.      * Encoded as: MIN_VALUE -> 0, 0 -> -MIN_VALUE, MAX_VALUE-> -1     */    @Override    public void readFields(DataInput in) throws IOException {      first = in.readInt() + Integer.MIN_VALUE;      second = in.readInt() + Integer.MIN_VALUE;    }    @Override    public void write(DataOutput out) throws IOException {      out.writeInt(first - Integer.MIN_VALUE);      out.writeInt(second - Integer.MIN_VALUE);    }    @Override    public int hashCode() {      return first * 157 + second;    }    @Override    public boolean equals(Object right) {      if (right instanceof IntPair) {        IntPair r = (IntPair) right;        return r.first == first && r.second == second;      } else {        return false;      }    }    /** A Comparator that compares serialized IntPair. */     public static class Comparator extends WritableComparator {      public Comparator() {        super(IntPair.class);      }      public int compare(byte[] b1, int s1, int l1,                         byte[] b2, int s2, int l2) {        return -compareBytes(b1, s1, l1, b2, s2, l2);//负号表示按降序排列      }    }    static {                                        // register this comparator      WritableComparator.define(IntPair.class, new Comparator());    }    @Override    public int compareTo(IntPair o) {      if (first != o.first) {        return first < o.first ? -1 : 1;      } else if (second != o.second) {        return second < o.second ? -1 : 1;      } else {        return 0;      }    }  }    /**   * Partition based on the first part of the pair.   */  public static class FirstPartitioner extends Partitioner<IntPair,IntWritable>{    @Override    public int getPartition(IntPair key, IntWritable value,                             int numPartitions) {      return Math.abs(key.getFirst() * 127) % numPartitions;    }  }  /**   * Compare only the first part of the pair, so that reduce is called once   * for each value of the first part.   */  public static class FirstGroupingComparator                 implements RawComparator<IntPair> {    @Override    public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {      return WritableComparator.compareBytes(b1, s1, Integer.SIZE/8,                                              b2, s2, Integer.SIZE/8);    }    @Override    public int compare(IntPair o1, IntPair o2) {      int l = o1.getFirst();      int r = o2.getFirst();      return l == r ? 0 : (l < r ? -1 : 1);    }  }  /**   * Read two integers from each line and generate a key, value pair   * as ((left, right), right).   */  public static class MapClass          extends Mapper<LongWritable, Text, IntPair, IntWritable> {        private final IntPair key = new IntPair();    private final IntWritable value = new IntWritable();        @Override    public void map(LongWritable inKey, Text inValue,                     Context context) throws IOException, InterruptedException {      StringTokenizer itr = new StringTokenizer(inValue.toString());      int left = 0;      int right = 0;      if (itr.hasMoreTokens()) {        left = Integer.parseInt(itr.nextToken());        if (itr.hasMoreTokens()) {          right = Integer.parseInt(itr.nextToken());        }        key.set(left, right);        value.set(right);        context.write(key, value);      }    }  }    /**   * A reducer class that just emits the sum of the input values.   */  public static class Reduce          extends Reducer<IntPair, IntWritable, Text, IntWritable> {    private static final Text SEPARATOR =       new Text("------------------------------------------------");    private final Text first = new Text();        @Override    public void reduce(IntPair key, Iterable<IntWritable> values,                       Context context                       ) throws IOException, InterruptedException {      context.write(SEPARATOR, null);      first.set(Integer.toString(key.getFirst()));      for(IntWritable value: values) {        context.write(first, value);        break;//对于每一个IntPare.first 只输出最大的second      }    }  }    public static void main(String[] args) throws Exception {    Configuration conf = new Configuration();    String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();    if (otherArgs.length != 2) {      System.err.println("Usage: secondarysrot <in> <out>");      System.exit(2);    }    Job job = new Job(conf, "secondary sort");    job.setJarByClass(MaxSecondarySort.class);    job.setMapperClass(MapClass.class);    job.setReducerClass(Reduce.class);    // group and partition by the first int in the pair    job.setPartitionerClass(FirstPartitioner.class);    job.setGroupingComparatorClass(FirstGroupingComparator.class);    // the map output is IntPair, IntWritable    job.setMapOutputKeyClass(IntPair.class);    job.setMapOutputValueClass(IntWritable.class);    // the reduce output is Text, IntWritable    job.setOutputKeyClass(Text.class);    job.setOutputValueClass(IntWritable.class);        FileInputFormat.addInputPath(job, new Path(otherArgs[0]));    FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));    System.exit(job.waitForCompletion(true) ? 0 : 1);  }}

setGroupingComparatorClass 这里会按照Map的结果对key相同的值进行聚合开成<key,value-list> 这里的key是按map出的结果对key进行排序得到的最大的key,这段程序里是按照IntPair进行的对比first，因为second存在于map输出的value里，所以这里只要保留Intpair的first即可它的second可以忽略。最终的reduce的输入的values是按照camprator进行排序的，小于：-1 等于：0 大于：1 这样的结果是升序，在compare的返回结果前加一个负号则可以让结果以降序的形式。