MyJob.java

import java.io.IOException;
import java.util.Iterator;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.FileInputFormat;
import org.apache.hadoop.mapred.FileOutputFormat;
import org.apache.hadoop.mapred.JobClient;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.KeyValueTextInputFormat;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.mapred.TextOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;


public class MyJob extends Configured implements Tool{
  
	public static class MapClass extends MapReduceBase implements Mapper<Text, Text, Text, Text>{
		@Override
		public void map(Text key, Text value, OutputCollector<Text, Text> output, Reporter reporter) throws IOException {
			output.collect(value, key);
		}	
	}
	
	
	public static class Reduce extends MapReduceBase implements Reducer<Text, Text, Text, Text>{
		@Override
		public void reduce(Text key, Iterator<Text> values, OutputCollector<Text, Text> output, Reporter reporter) throws IOException {
			String csv = "";
			while (values.hasNext()) {
				if(csv.length() > 0)
					csv +=",";
				csv += values.next().toString();
			}
			output.collect(key, new Text(csv));
		}
	}
	

	@Override
	public int run(String[] args) throws Exception {
		Configuration conf = getConf();
		
		JobConf job = new JobConf(conf, MyJob.class);
		
		Path in = new Path(args[0]);
		Path out = new Path(args[1]);
		FileInputFormat.setInputPaths(job, in);
		FileOutputFormat.setOutputPath(job, out);
		
		job.setJobName("myJob");
		job.setMapperClass(MapClass.class);
		job.setReducerClass(Reduce.class);
		
		job.setInputFormat(KeyValueTextInputFormat.class);
		job.setOutputFormat(TextOutputFormat.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(Text.class);
		job.set("key.value.separator.in.input.line", ",");
		
		JobClient.runJob(job);
		
		return 0;
	}
	

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