Java tutorial
/** * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.hadoop.examples; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.StringTokenizer; import org.apache.hadoop.conf.Configuration; import org.apache.hadoop.conf.Configured; import org.apache.hadoop.fs.FileSystem; 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.TaskCounter; 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; import com.google.common.base.Charsets; public class WordMedian extends Configured implements Tool { private double median = 0; private final static IntWritable ONE = new IntWritable(1); /** * Maps words from line of text into a key-value pair; the length of the word * as the key, and 1 as the value. */ public static class WordMedianMapper extends Mapper<Object, Text, IntWritable, IntWritable> { private IntWritable length = new IntWritable(); /** * Emits a key-value pair for counting the word. Outputs are (IntWritable, * IntWritable). * * @param value * This will be a line of text coming in from our input file. */ public void map(Object key, Text value, Context context) throws IOException, InterruptedException { StringTokenizer itr = new StringTokenizer(value.toString()); while (itr.hasMoreTokens()) { String string = itr.nextToken(); length.set(string.length()); context.write(length, ONE); } } } /** * Performs integer summation of all the values for each key. */ public static class WordMedianReducer extends Reducer<IntWritable, IntWritable, IntWritable, IntWritable> { private IntWritable val = new IntWritable(); /** * Sums all the individual values within the iterator and writes them to the * same key. * * @param key * This will be a length of a word that was read. * @param values * This will be an iterator of all the values associated with that * key. */ public void reduce(IntWritable key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException { int sum = 0; for (IntWritable value : values) { sum += value.get(); } val.set(sum); context.write(key, val); } } /** * This is a standard program to read and find a median value based on a file * of word counts such as: 1 456, 2 132, 3 56... Where the first values are * the word lengths and the following values are the number of times that * words of that length appear. * * @param path * The path to read the HDFS file from (part-r-00000...00001...etc). * @param medianIndex1 * The first length value to look for. * @param medianIndex2 * The second length value to look for (will be the same as the first * if there are an even number of words total). * @throws IOException * If file cannot be found, we throw an exception. * */ private double readAndFindMedian(String path, int medianIndex1, int medianIndex2, Configuration conf) throws IOException { FileSystem fs = FileSystem.get(conf); Path file = new Path(path, "part-r-00000"); if (!fs.exists(file)) throw new IOException("Output not found!"); BufferedReader br = null; try { br = new BufferedReader(new InputStreamReader(fs.open(file), Charsets.UTF_8)); int num = 0; String line; while ((line = br.readLine()) != null) { StringTokenizer st = new StringTokenizer(line); // grab length String currLen = st.nextToken(); // grab count String lengthFreq = st.nextToken(); int prevNum = num; num += Integer.parseInt(lengthFreq); if (medianIndex2 >= prevNum && medianIndex1 <= num) { System.out.println("The median is: " + currLen); br.close(); return Double.parseDouble(currLen); } else if (medianIndex2 >= prevNum && medianIndex1 < num) { String nextCurrLen = st.nextToken(); double theMedian = (Integer.parseInt(currLen) + Integer.parseInt(nextCurrLen)) / 2.0; System.out.println("The median is: " + theMedian); br.close(); return theMedian; } } } finally { if (br != null) { br.close(); } } // error, no median found return -1; } public static void main(String[] args) throws Exception { ToolRunner.run(new Configuration(), new WordMedian(), args); } @Override public int run(String[] args) throws Exception { if (args.length != 2) { System.err.println("Usage: wordmedian <in> <out>"); return 0; } setConf(new Configuration()); Configuration conf = getConf(); Job job = Job.getInstance(conf, "word median"); job.setJarByClass(WordMedian.class); job.setMapperClass(WordMedianMapper.class); job.setCombinerClass(WordMedianReducer.class); job.setReducerClass(WordMedianReducer.class); job.setOutputKeyClass(IntWritable.class); job.setOutputValueClass(IntWritable.class); FileInputFormat.addInputPath(job, new Path(args[0])); FileOutputFormat.setOutputPath(job, new Path(args[1])); boolean result = job.waitForCompletion(true); // Wait for JOB 1 -- get middle value to check for Median long totalWords = job.getCounters().getGroup(TaskCounter.class.getCanonicalName()) .findCounter("MAP_OUTPUT_RECORDS", "Map output records").getValue(); int medianIndex1 = (int) Math.ceil((totalWords / 2.0)); int medianIndex2 = (int) Math.floor((totalWords / 2.0)); median = readAndFindMedian(args[1], medianIndex1, medianIndex2, conf); return (result ? 0 : 1); } public double getMedian() { return median; } }