Join源代码注释

package org.apache.hadoop.examples;

import java.io.IOException;
import java.util.*;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.BytesWritable;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.mapred.join.*;
import org.apache.hadoop.mapred.lib.IdentityMapper;
import org.apache.hadoop.mapred.lib.IdentityReducer;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

/**
hadoop的map/reduce例子，演示了用CompositeInputFormat类来对两个数据集进行join操作，可以是inner join；outerjoin；也可以是override join。
 * This is the trivial map/reduce program that does absolutely nothing
 * other than use the framework to fragment and sort the input values.
 *
 * To run: bin/hadoop jar build/hadoop-examples.jar join
 *            [-m <i>maps</i>] [-r <i>reduces</i>]
 *            [-inFormat <i>input format class</i>] 
 *            [-outFormat <i>output format class</i>] 
 *            [-outKey <i>output key class</i>] 
 *            [-outValue <i>output value class</i>] 
 *            [-joinOp &lt;inner|outer|override&gt;]
 *            [<i>in-dir</i>]* <i>in-dir</i> <i>out-dir</i> 
 */
public class Join extends Configured implements Tool {

  static int printUsage() {
    System.out.println("join [-m <maps>] [-r <reduces>] " +
                       "[-inFormat <input format class>] " +
                       "[-outFormat <output format class>] " + 
                       "[-outKey <output key class>] " +
                       "[-outValue <output value class>] " +
                       "[-joinOp <inner|outer|override>] " +
                       "[input]* <input> <output>");
    ToolRunner.printGenericCommandUsage(System.out);
    return -1;
  }

  /**driver代码
   * The main driver for sort program.
   * Invoke this method to submit the map/reduce job.
   * @throws IOException When there is communication problems with the 
   *                     job tracker.
   */
  public int run(String[] args) throws Exception {
    JobConf jobConf = new JobConf(getConf(), Sort.class);
    jobConf.setJobName("join");

    jobConf.setMapperClass(IdentityMapper.class);  //设置map类是IdentityMapper      
    jobConf.setReducerClass(IdentityReducer.class);//设置reduce类是IdentityReducer

    JobClient client = new JobClient(jobConf);
    ClusterStatus cluster = client.getClusterStatus();
    int num_maps = cluster.getTaskTrackers() * 
                   jobConf.getInt("test.sort.maps_per_host", 10);
    int num_reduces = (int) (cluster.getMaxReduceTasks() * 0.9);
    String sort_reduces = jobConf.get("test.sort.reduces_per_host");
    if (sort_reduces != null) {
       num_reduces = cluster.getTaskTrackers() * 
                       Integer.parseInt(sort_reduces);
    }
    Class<? extends InputFormat> inputFormatClass = 
      SequenceFileInputFormat.class;
    Class<? extends OutputFormat> outputFormatClass = 
      SequenceFileOutputFormat.class;
    Class<? extends WritableComparable> outputKeyClass = BytesWritable.class;
    Class<? extends Writable> outputValueClass = TupleWritable.class;
    String op = "inner"; //join是inner join 
    List<String> otherArgs = new ArrayList<String>();
    for(int i=0; i < args.length; ++i) {
      try {
        if ("-m".equals(args[i])) {
          num_maps = Integer.parseInt(args[++i]);
        } else if ("-r".equals(args[i])) {
          num_reduces = Integer.parseInt(args[++i]);
        } else if ("-inFormat".equals(args[i])) {
          inputFormatClass = 
            Class.forName(args[++i]).asSubclass(InputFormat.class);
        } else if ("-outFormat".equals(args[i])) {
          outputFormatClass = 
            Class.forName(args[++i]).asSubclass(OutputFormat.class);
        } else if ("-outKey".equals(args[i])) {
          outputKeyClass = 
            Class.forName(args[++i]).asSubclass(WritableComparable.class);
        } else if ("-outValue".equals(args[i])) {
          outputValueClass = 
            Class.forName(args[++i]).asSubclass(Writable.class);
        } else if ("-joinOp".equals(args[i])) {
          op = args[++i];
        } else {
          otherArgs.add(args[i]);
        }
      } catch (NumberFormatException except) {
        System.out.println("ERROR: Integer expected instead of " + args[i]);
        return printUsage();
      } catch (ArrayIndexOutOfBoundsException except) {
        System.out.println("ERROR: Required parameter missing from " +
            args[i-1]);
        return printUsage(); // exits
      }
    }

    // Set user-supplied (possibly default) job configs
    jobConf.setNumMapTasks(num_maps);
    jobConf.setNumReduceTasks(num_reduces);

    if (otherArgs.size() < 2) {
      System.out.println("ERROR: Wrong number of parameters: ");
      return printUsage();
    }

    FileOutputFormat.setOutputPath(jobConf, 
      new Path(otherArgs.remove(otherArgs.size() - 1)));
    List<Path> plist = new ArrayList<Path>(otherArgs.size());
    for (String s : otherArgs) {
      plist.add(new Path(s));
    }

    jobConf.setInputFormat(CompositeInputFormat.class);//设置输入格式类是CompositeInputFormat
    jobConf.set("mapred.join.expr", CompositeInputFormat.compose(
          op, inputFormatClass, plist.toArray(new Path[0])));
//设置CompositeInputFormat的参数，包括join类型，输入格式类型。
    jobConf.setOutputFormat(outputFormatClass);

    jobConf.setOutputKeyClass(outputKeyClass);
    jobConf.setOutputValueClass(outputValueClass);

    Date startTime = new Date();
    System.out.println("Job started: " + startTime);
    JobClient.runJob(jobConf);
    Date end_time = new Date();
    System.out.println("Job ended: " + end_time);
    System.out.println("The job took " + 
        (end_time.getTime() - startTime.getTime()) /1000 + " seconds.");
    return 0;
  }

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

}