I have the code below, this code is a hadoop code in map reduce, I am not writing this, it is copied from:

https://hadoop.apache.org/docs/r2.7.5/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html#Example:_WordCount_v2.0 )

Could you help me explain the purpose of each method in this code and explain some of the classes used in the code? For example, Contex class, Text class, or anyline of the code that you think you can give me an insight about it. Also, in other normal code, when the code scanning .txt file, the programmer must giving the path to the file within the code. However, in this code, I don't see anywhere that mentions the path, so I don't know how it understand where can it get the .txt file. If possible, please help me explain in as much detail as possible. I really appreciate it. I am new to Hadoop so this is very important to me.

import java.io.BufferedReader; import java.io.FileReader; import java.io.IOException; import java.net.URI; import java.util.ArrayList; import java.util.HashSet; import java.util.List; import java.util.Set; import java.util.StringTokenizer;

import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.fs.Path; import org.apache.hadoop.io.IntWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Job; import org.apache.hadoop.mapreduce.Mapper; import org.apache.hadoop.mapreduce.Reducer; import org.apache.hadoop.mapreduce.lib.input.FileInputFormat; import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat; import org.apache.hadoop.mapreduce.Counter; import org.apache.hadoop.util.GenericOptionsParser; import org.apache.hadoop.util.StringUtils;

public class WordCount2 {

public static class TokenizerMapper extends Mapper{

static enum CountersEnum { INPUT_WORDS }

private final static IntWritable one = new IntWritable(1); private Text word = new Text();

private boolean caseSensitive; private Set patternsToSkip = new HashSet();

private Configuration conf; private BufferedReader fis;

@Override public void setup(Context context) throws IOException, InterruptedException { conf = context.getConfiguration(); caseSensitive = conf.getBoolean("wordcount.case.sensitive", true); if (conf.getBoolean("wordcount.skip.patterns", true)) { URI[] patternsURIs = Job.getInstance(conf).getCacheFiles(); for (URI patternsURI : patternsURIs) { Path patternsPath = new Path(patternsURI.getPath()); String patternsFileName = patternsPath.getName().toString(); parseSkipFile(patternsFileName); } } }

private void parseSkipFile(String fileName) { try { fis = new BufferedReader(new FileReader(fileName)); String pattern = null; while ((pattern = fis.readLine()) != null) { patternsToSkip.add(pattern); } } catch (IOException ioe) { System.err.println("Caught exception while parsing the cached file '" + StringUtils.stringifyException(ioe)); } }

@Override public void map(Object key, Text value, Context context ) throws IOException, InterruptedException { String line = (caseSensitive) ? value.toString() : value.toString().toLowerCase(); for (String pattern : patternsToSkip) { line = line.replaceAll(pattern, ""); } StringTokenizer itr = new StringTokenizer(line); while (itr.hasMoreTokens()) { word.set(itr.nextToken()); context.write(word, one); Counter counter = context.getCounter(CountersEnum.class.getName(), CountersEnum.INPUT_WORDS.toString()); counter.increment(1); } } }

public static class IntSumReducer extends Reducer { private IntWritable result = new IntWritable();

public void reduce(Text key, Iterable values, Context context ) throws IOException, InterruptedException { int sum = 0; for (IntWritable val : values) { sum += val.get(); } result.set(sum); context.write(key, result); } }

public static void main(String[] args) throws Exception { Configuration conf = new Configuration(); GenericOptionsParser optionParser = new GenericOptionsParser(conf, args); String[] remainingArgs = optionParser.getRemainingArgs(); if (!(remainingArgs.length != 2 || remainingArgs.length != 4)) { System.err.println("Usage: wordcount [-skip skipPatternFile]"); System.exit(2); } Job job = Job.getInstance(conf, "word count"); job.setJarByClass(WordCount2.class); job.setMapperClass(TokenizerMapper.class); job.setCombinerClass(IntSumReducer.class); job.setReducerClass(IntSumReducer.class); job.setOutputKeyClass(Text.class); job.setOutputValueClass(IntWritable.class);

List otherArgs = new ArrayList(); for (int i=0; i < remainingArgs.length; ++i) { if ("-skip".equals(remainingArgs[i])) { job.addCacheFile(new Path(remainingArgs[++i]).toUri()); job.getConfiguration().setBoolean("wordcount.skip.patterns", true); } else { otherArgs.add(remainingArgs[i]); } } FileInputFormat.addInputPath(job, new Path(otherArgs.get(0))); FileOutputFormat.setOutputPath(job, new Path(otherArgs.get(1)));

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