Wednesday, May 25, 2011

Word Count Example with Hadoop – 0.20


For detailed understanding on the working and control flow of this example refer

Mapper Class  - WordCountMapper.java

import java.io.IOException;
import java.util.StringTokenizer;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapreduce.Mapper;

public class WordCountMapper extends Mapper<LongWritable, Text, Text, IntWritable>
{
            //hadoop supported data types
      private final static IntWritable one = new IntWritable(1);
      private Text word = new Text();
         
           public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException
           {
             //taking one line at a time and tokenizing the same
               String line = value.toString();
               StringTokenizer tokenizer = new StringTokenizer(line);
           
             //iterating through all the words available in that line and forming the key value pair
               while (tokenizer.hasMoreTokens())
               {
                  word.set(tokenizer.nextToken());
                  //sending to output collector which inturn passes the same to reducer
                  context.write(word, one);
               }
           }
         
 }

Reducer Class - WordCountReducer.java

import java.io.IOException;
import java.util.Iterator;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapreduce.Reducer;

public class WordCountReducer extends Reducer<Text, IntWritable, Text, IntWritable>
{
      //Reduce method for just outputting the key from mapper as the value from mapper is just an empty string   
      public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException
      {
            int sum = 0;
            /*iterates through all the values available with a key and add them together and give the
            final result as the key and sum of its values*/
            for (IntWritable value : values)
            {
                  sum += value.get();

            }
            context.write(key, new IntWritable(sum));
       }
}

Driver Class - WordCount.java

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;



public class WordCount extends Configured implements Tool
{
      public int run(String[] args) throws Exception
      {
            //getting configuration object and setting job name
            Configuration conf = getConf();
        Job job = new Job(conf, "Word Count hadoop-0.20");
      
        //setting the class names
        job.setJarByClass(WordCount.class);
        job.setMapperClass(WordCountMapper.class);
        job.setReducerClass(WordCountReducer.class);

        //setting the output data type classes
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);

        //to accept the hdfs input and outpur dir at run time
        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        return job.waitForCompletion(true) ? 0 : 1;
    }

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

34 comments:

  1. Thank for the example, i would like to know what's the role for each class.
    thanks and regards

    ReplyDelete
  2. Hi Mehdi

    As the name suggests
    - Mapper
    - Reducer
    - Driver

    Driver is a trigger point. To know more on mapper and reduce please have a look at http://kickstarthadoop.blogspot.com/2011/04/word-count-hadoop-map-reduce-example.html

    For a detailed understanding I'd recommend "Hadoop - The definitive guide by Tom White"

    ReplyDelete
    Replies
    1. how do i configure hadoop using java so that i can perform these wordcount n other tasks ??

      Delete
  3. As the name suggests
    WordCountMapper.java - Mapper
    WordCountReducer.java - Reducer
    WordCount.java - Driver

    ReplyDelete
  4. How do you extend this functionality to an xml input file instead of a text file?

    ReplyDelete
  5. Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/commons/logging/LogFactory
    at org.apache.hadoop.conf.Configuration.(Configuration.java:139)
    at WordCount.main(WordCount.java:39)
    Caused by: java.lang.ClassNotFoundException: org.apache.commons.logging.LogFactory
    at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:323)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:268)
    ... 2 more

    ReplyDelete
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