摘要：MapReduce程序进行单词计数。
关键词：MapReduce程序 单词计数
数据源：人工构造英文文档file1.txt,file2.txt。
file1.txt 内容
Hello Hadoop
I am studying the Hadoop technology
file2.txt内容
Hello world
The world is very beautiful
I love the Hadoop and world
问题描写叙述：
统计人工构造的英文文档中单词的频数，要求输出的结果依照单词字母的顺序进行排序。
解决方式：
1 开发工具：VM10+ Ubuntu12.04+ Hadoop1.1.2
2 设计思路：把英文文档内容且分成单词，然后把全部同样的单词聚集在一起，最后计算各个单词的频数。
程序清单：
package com.wangluqing;
import java.io.IOException;
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.util.GenericOptionsParser;
public class WordCount {
public static class TokenizerMapper extends Mapper<Object,Text,Text,IntWritable> {
private final static IntWritable one = new IntWritable(1);
private Text word = new Text();
public void map(Object key, Text value, Context context) throws IOException,InterruptedException {
StringTokenizer its = new StringTokenizer(value.toString());
while (its.hasMoreTokens()) {
word.set(its.nextToken());
context.write(word,one);
}
}
}
public static class IntSumReducer extends Reducer<Text,IntWritable,Text,IntWritable> {
private IntWritable result = new IntWritable();
public void reduce(Text key, Iterable<IntWritable> 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();
String[] otherArgs = new GenericOptionsParser(conf,args).getRemainingArgs();
if(otherArgs.length !=2 ) {
System.err.println("Usage:wordcount<in><out>");
System.exit(2);
}
Job job = new Job(conf,"word count");
job.setJarByClass(WordCount.class);
job.setMapperClass(TokenizerMapper.class);
job.setCombinerClass(IntSumReducer.class);
job.setReducerClass(IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job,new Path(otherArgs[0]));
FileOutputFormat.setOutputPath(job,new Path(otherArgs[1]));
System.exit(job.waitForCompletion(true)?0:1);
}
}
3 运行程序
1）创建输入文件夹
hadoop fs -mkdir wordcount_input
2）上传本地英文文档
hadoop fs -put /usr/local/datasource/article/* wordcount_input
3）编译WordCount.java程序，把结果存放在当前文件夹的WordCount文件夹下。
root@hadoop:/usr/local/program/hadoop# javac -classpath hadoop-core-1.1.2.jar:lib/commons-cli-1.2.jar -d WordCount WordCount.java
4） 将编译结果打成Jar包
jar -cvf wordcount.jar WordCount/ .
5）执行WordCount程序，输入文件夹为wordcount_input，输出文件夹为wordcount_output。
hadoop jar wordcount.jar com.wangluqing.WordCount wordcount_input wordcount_output
6） 查看各个单词频数结果
root@hadoop:/usr/local/program/hadoop# hadoop fs -cat wordcount_output/part-r-00000
Hadoop 3
Hello 2
I 2
The 1
am 1
and 1
beautiful 1
is 1
love 1
studying 1
technology 1
the 2
very 1
world 3
总结：
WordCount程序是最简单也是最具代表性的MapReduce程序，一定程度上MapReduce设计的初衷，即对日志文件的分析。
Resource：
1 http://www.wangluqing.com/2014/03/hadoop-mapreduce-programapp2/
2 《Hadoop实战 第二版》陆嘉恒著 第5章 MapReduce应用案例