Grep源代码注释

package org.apache.hadoop.examples;

import java.util.Random;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.mapred.lib.*;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

/* 对输入文件按正则表达式查找，把结果写到输出文件上。
查找用到了RegexMapper，LongSumReducer，InverseMapper系统自带的
工具类。这个源代码运行了两个job，一个查找，一个是排序。
Extracts matching regexs from input files and counts them. */
public class Grep extends Configured implements Tool {
  private Grep() {}                               // 单例模式singleton

  public int run(String[] args) throws Exception {
    if (args.length < 3) {
      System.out.println("Grep <inDir> <outDir> <regex> [<group>]");
      ToolRunner.printGenericCommandUsage(System.out);
      return -1;
    }

    Path tempDir =
      new Path("grep-temp-"+
          Integer.toString(new Random().nextInt(Integer.MAX_VALUE)));

    JobConf grepJob = new JobConf(getConf(), Grep.class);
    
    try {
      
      grepJob.setJobName("grep-search");

      FileInputFormat.setInputPaths(grepJob, args[0]);

      grepJob.setMapperClass(RegexMapper.class);//设置系统自带的mapper类来查找
      grepJob.set("mapred.mapper.regex", args[2]);
      if (args.length == 4)
        grepJob.set("mapred.mapper.regex.group", args[3]);

      grepJob.setCombinerClass(LongSumReducer.class);//设置系统自带的reducer来做合并
      grepJob.setReducerClass(LongSumReducer.class);
     //设置系统自带的reducer。
      FileOutputFormat.setOutputPath(grepJob, tempDir);
      grepJob.setOutputFormat(SequenceFileOutputFormat.class);//设置输出格式是二进制文件
      grepJob.setOutputKeyClass(Text.class);//输出的key是Text类型
      grepJob.setOutputValueClass(LongWritable.class);//输出的value是long类型

      JobClient.runJob(grepJob);

      JobConf sortJob = new JobConf(Grep.class);
      sortJob.setJobName("grep-sort");

      FileInputFormat.setInputPaths(sortJob, tempDir);
      sortJob.setInputFormat(SequenceFileInputFormat.class);//设置输入的文件格式二进制文件

      sortJob.setMapperClass(InverseMapper.class);//设置自带的排序mapper

      sortJob.setNumReduceTasks(1);                 // write a single file
      FileOutputFormat.setOutputPath(sortJob, new Path(args[1]));
      sortJob.setOutputKeyComparatorClass           // 输出结果是降序排列sort by decreasing freq
      (LongWritable.DecreasingComparator.class);

      JobClient.runJob(sortJob);
    }
    finally {
      FileSystem.get(grepJob).delete(tempDir, true);
    }
    return 0;
  }

  public static void main(String[] args) throws Exception {
    int res = ToolRunner.run(new Configuration(), new Grep(), args);
    System.exit(res);
  }

}