How to format the output being written by Mapreduce in Hadoop

Question

I am trying to reverse the contents of the file by each word. I have the program running fine, but the output i am getting is something like this

1   dwp
2   seviG
3   eht
4   tnerruc
5   gnikdrow
6   yrotcerid
7   ridkm
8   desU
9   ot
10  etaerc

I want the output to be something like this

dwp seviG eht tnerruc gnikdrow yrotcerid ridkm desU
ot etaerc

The code i am working with

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

    import org.apache.hadoop.fs.Path;
    import org.apache.hadoop.conf.*;
    import org.apache.hadoop.io.*;
    import org.apache.hadoop.mapred.*;
    import org.apache.hadoop.util.*;

    public class Reproduce {

    public static int temp =0;
    public static class ReproduceMap extends MapReduceBase implements Mapper<LongWritable, Text, IntWritable, Text>{
        private Text word = new Text();
        @Override
        public void map(LongWritable arg0, Text value,
                OutputCollector<IntWritable, Text> output, Reporter reporter)
                throws IOException {
            String line = value.toString().concat("\n");
            StringTokenizer tokenizer = new StringTokenizer(line);
            while (tokenizer.hasMoreTokens()) {
                word.set(new StringBuffer(tokenizer.nextToken()).reverse().toString());
                temp++;
                output.collect(new IntWritable(temp),word);
              }

        }

    }

    public static class ReproduceReduce extends MapReduceBase implements Reducer<IntWritable, Text, IntWritable, Text>{

        @Override
        public void reduce(IntWritable arg0, Iterator<Text> arg1,
                OutputCollector<IntWritable, Text> arg2, Reporter arg3)
                throws IOException {
            String word = arg1.next().toString();
            Text word1 = new Text();
            word1.set(word);
            arg2.collect(arg0, word1);

        }

    }

    public static void main(String[] args) throws Exception {
    JobConf conf = new JobConf(WordCount.class);
    conf.setJobName("wordcount");

    conf.setOutputKeyClass(IntWritable.class);
    conf.setOutputValueClass(Text.class);

    conf.setMapperClass(ReproduceMap.class);
    conf.setReducerClass(ReproduceReduce.class);

    conf.setInputFormat(TextInputFormat.class);
    conf.setOutputFormat(TextOutputFormat.class);

    FileInputFormat.setInputPaths(conf, new Path(args[0]));
    FileOutputFormat.setOutputPath(conf, new Path(args[1]));

    JobClient.runJob(conf);

  }
}

How do i modify my output instead of writing another java program to do that

Thanks in advance

Answer 1

Here is a simple code demonstrate the use of custom FileoutputFormat

public class MyTextOutputFormat extends FileOutputFormat<Text, List<IntWritable>> {
      @Override
      public org.apache.hadoop.mapreduce.RecordWriter<Text, List<Intwritable>> getRecordWriter(TaskAttemptContext arg0) throws IOException, InterruptedException {
         //get the current path
         Path path = FileOutputFormat.getOutputPath(arg0);
         //create the full path with the output directory plus our filename
         Path fullPath = new Path(path, "result.txt");
     //create the file in the file system
     FileSystem fs = path.getFileSystem(arg0.getConfiguration());
     FSDataOutputStream fileOut = fs.create(fullPath, arg0);

     //create our record writer with the new file
     return new MyCustomRecordWriter(fileOut);
  }
}

public class MyCustomRecordWriter extends RecordWriter<Text, List<IntWritable>> {
    private DataOutputStream out;

    public MyCustomRecordWriter(DataOutputStream stream) {
        out = stream;
        try {
            out.writeBytes("results:\r\n");
        }
        catch (Exception ex) {
        }  
    }

    @Override
    public void close(TaskAttemptContext arg0) throws IOException, InterruptedException {
        //close our file
        out.close();
    }

    @Override
    public void write(Text arg0, List arg1) throws IOException, InterruptedException {
        //write out our key
        out.writeBytes(arg0.toString() + ": ");
        //loop through all values associated with our key and write them with commas between
        for (int i=0; i<arg1.size(); i++) {
            if (i>0)
                out.writeBytes(",");
            out.writeBytes(String.valueOf(arg1.get(i)));
        }
        out.writeBytes("\r\n");  
    }
}

Finally we need to tell our job about our ouput format and the path before running it.

job.setOutputKeyClass(Text.class);
job.setOutputValueClass(ArrayList.class);
job.setOutputFormatClass(MyTextOutputFormat.class);
FileOutputFormat.setOutputPath(job, new Path("/home/hadoop/out"));

Answer 2

我们可以通过编写自定义文件outputformat类来自定义输出

Answer 3

you can use NullWritable as a output value. NullWritable is just a placeholder Since you don't want number to be displayed as a part of your output. I have given modified reducer class. Note :- need to add import statement for NullWritable

public static class ReproduceReduce extends MapReduceBase implements Reducer<IntWritable, Text,  Text, NullWritable>{

            @Override
            public void reduce(IntWritable arg0, Iterator<Text> arg1,
                    OutputCollector<Text, NullWritable> arg2, Reporter arg3)
                    throws IOException {
                String word = arg1.next().toString();
                Text word1 = new Text();
                word1.set(word);
                arg2.collect(word1, new NullWritable());

            }

        }

and change the driver class or main method

conf.setOutputKeyClass(Text.class);
conf.setOutputValueClass(NullWritable.class);

Answer 4

In Mapper key temp is incremented for each word value, So each word is processed as a separate key-value pair.

Below steps should solve the problem 1) In Mapper just remove the temp++, so that all the reversed words will have the key as 0 (temp =0).

2) Reducer receives the key 0 and list of reversed strings. In reducer set the key to NullWritable and write the output.

Answer 5

What you can try is take one constant key (or simply nullwritable) and pass this as a key and your complete line as a value(you can reverse it in mapper class or you can also reverse it in the reducer class as well). so your reducer will receive a constant key (or place holder if you have used nullwritable as a key) and complete line. Now you can simply reverse the line and write it to output file. By not using tmp as a key you avoid writing unwanted numbers in your output file.