SecondarySort代码的注释

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; 

/** 
* hadoop的map/reduce自带的例子代码，目的是演示二次排序， 
* 输入是文本文件，文本的每行是两个用空格分隔的整数，程序的输出结果是 
* 按前一个整数排序，然后按后一个整数排序。应用场景是：数据join的reduce 
* 端的算法。 
* 
* To run: bin/hadoop jar build/hadoop-examples.jar secondarysort 
*            in-dir out-dir 
*/ 
public class SecondarySort { 

  /** 
   * 自定义map的输出key类型，而不是默认的Text类型，他必须实现接口comparable ，作用是输出中对key自定义排序（倒序），实现中必须提供数据的
读写方法（readFields，write）。等于（equals）和比较方法（compareTo）。 
   * 
   */ 
  public static class IntPair 
                      implements WritableComparable { 
    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 int getFirst() { 
      return first; 
    } 
    public int getSecond() { 
      return second; 
    } 
    /** 
     * 按编码读两个整数，编码是：读第一个整数+ Integer.MIN_VALUE+读第二个整数 +  Integer.MIN_VALUE 

     * 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; 
    } 
   /*按编码写两个整数。编码是：写第一个整数-Integer.MIN_VALUE +写第二个整数 -  Integer.MIN_VALUE 
*/   
    @Override 
    public void write(DataOutput out) throws IOException { 
      out.writeInt(first - Integer.MIN_VALUE); 
      out.writeInt(second - Integer.MIN_VALUE); 
    } 

  /*哈希方法，估计用于对象比较 
*/    @Override 
    public int hashCode() { 
      return first * 157 + second; 
    } 

/*等于方法，和compareTo方法用于排序，而且两者必须一致，不然，排序的结果可能不对。 
*/    @Override 
    public boolean equals(Object right) { 
      if (right instanceof IntPair) { 
        IntPair r = (IntPair) right; 
        return r.first == first && r.second == second; 
      } else { 
        return false; 
      } 
    } 
    /** 内置的静态类，用于对象的排序比较。如果没有这个类，排序的话 
   用 IntPair类自身的等于和比较函数来排序*/ 
    public static class Comparator extends WritableComparator { 
      public Comparator() { 
        super(IntPair.class); 
      } 

     /*排序方法，b1第一个对象的字节数组第一个元素，s1首个字节的长度， 
    l1整个字节数组的长度， 
    b2第二个对象的字节数组第一个元素，s2首个字节的长度， 
    l2整个字节数组的长度， 
*/      public int compare(byte[] b1, int s1, int l1, 
                         byte[] b2, int s2, int l2) { 
        return compareBytes(b1, s1, l1, b2, s2, l2);//比较对象的字节数组，遇到第一个不同立即返回 
      } 
    } 

    static {                                        // 登记比较器comparator，对象     IntPair按比较器的定义排序 
      WritableComparator.define(IntPair.class, new Comparator()); 
    } 
/*对象的比较方法 
*/    @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;
      }
    }
  }
 /** 
   * pair对象的Partitioner方法。默认是hashpartition方法。 
   目的是按前一个整数（firest）来决定让那个reducer来处理   */ 
  public static class FirstPartitioner extends Partitioner{ 
    @Override 
    public int getPartition(IntPair key, IntWritable value, 
                            int numPartitions) { 
      return Math.abs(key.getFirst() * 127) % numPartitions; 
    } 
  } 

  /** 
   *  reducer的输出按第一个整数来输出，自定义的比较器   */ 
  public static class FirstGroupingComparator 
                implements RawComparator { 

/*比较字节数组，注释掉该函数会报错 
*/    @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); 
    } 

/*作用不明，注释掉改函数，不会报错。 
*/   @Override 
   public int compare(IntPair o1, IntPair o2) {
      int l = o1.getFirst();
      int r = o2.getFirst();
      return l == r ? 0 : (l < r ? -1 : 1);
    }
  }
/**mapper类，输出的key是自定义的IntPair 
   * Read two integers from each line and generate a key, value pair 
   * as ((left, right), right). 
   */ 
  public static class MapClass 
         extends Mapper { 
    
    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); 
      } 
    } 
  } 
  
  /**reducer类 
   * A reducer class that just emits the sum of the input values. 
   */ 
  public static class Reduce 
         extends Reducer { 
    private static final Text SEPARATOR = 
      new Text(&quot;------------------------------------------------&quot;); 
    private final Text first = new Text(); 
    
    @Override 
    public void reduce(IntPair key, Iterable values, 
                       Context context 
                       ) throws IOException, InterruptedException { 
      context.write(SEPARATOR, null); 
      first.set(Integer.toString(key.getFirst())); 
      for(IntWritable value: values) { 
        context.write(first, value); 
      } 
    } 
  } 
  
  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(&quot;Usage: secondarysrot &quot;); 
      System.exit(2); 
    } 
     Job job = new Job(conf, "secondary sort");//生成job对象 
    job.setJarByClass(SecondarySort.class);//指定jar包的名字 
    job.setMapperClass(MapClass.class);//设置mapper类 
    job.setReducerClass(Reduce.class);//设置reducer类 

    // group and partition by the first int in the pair 
    job.setPartitionerClass(FirstPartitioner.class);//设置自定义的partition类 
    job.setGroupingComparatorClass(FirstGroupingComparator.class); 
    //设置分组比较器。（按前一个整数来输出） 
    // the map output is IntPair, IntWritable 
    job.setMapOutputKeyClass(IntPair.class);//设置mapper输出的key类型 
    job.setMapOutputValueClass(IntWritable.class);//设置mapper输出的value类型 

    // the reduce output is Text, IntWritable 
    job.setOutputKeyClass(Text.class); //设置reducer输出的key的类型 
    job.setOutputValueClass(IntWritable.class);//设置reuducer输出的value的类型 
    
    FileInputFormat.addInputPath(job, new Path(otherArgs[0]));//设置输入路径 
    FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));//设置输出路径 
    System.exit(job.waitForCompletion(true) ? 0 : 1); 
  } 

}