编译hadoop版的hello,world

cd ~/src

mkdir classes

javac -classpath ~/hadoop-0.20.2/hadoop-0.20.2-core.jar WordCount.java -d classes

jar -cvf WordCount.jar -C classes/ .

hadoop jar WordCount.jar com.codestyle.hadoop.WordCount input output

hadoop fs -ls output

hadoop fs -cat output/part-00000

要点：

编译WordCount.java时必须通过classpath指定hadoop的库文件。指定源码输出到classes目录

打包class文件成为jar文件

通过hadoop调用jar文件执行MapReduce, 内容输出到output目录
（如果该目录存在，则要先删掉这个目录）在命令参数中必须指定包名+类名

WordCount.java

package com.codestyle.hadoop;

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 WordCount {
public static class Map extends MapReduceBase implements Mapper<LongWritable, Text, Text, IntWritable> {

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

     private Text word = new Text();
public void map(LongWritable key, Text value, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException {

       String line = value.toString();

       StringTokenizer tokenizer = new StringTokenizer(line);

       while (tokenizer.hasMoreTokens()) {

         word.set(tokenizer.nextToken());

         output.collect(word, one);

       }

     }

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

     public void reduce(Text key, Iterator<IntWritable> values, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException {

       int sum = 0;

       while (values.hasNext()) {

         sum += values.next().get();

       }

       output.collect(key, new IntWritable(sum));

     }

   }
public static void main(String[] args) throws Exception {

     JobConf conf = new JobConf(WordCount.class);

     conf.setJobName("wordcount");
conf.setOutputKeyClass(Text.class);

     conf.setOutputValueClass(IntWritable.class);
conf.setMapperClass(Map.class);

     conf.setReducerClass(Reduce.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);

   }

}

查看执行结果

[email protected]:~/src$ hadoop fs -cat output/part-00000

Hadoop    1

Hello    2

World    1

参考资料：

http://www.cnblogs.com/xia520pi/archive/2012/05/16/2504205.html

http://blog.csdn.net/xw13106209/article/details/6862480

http://blog.csdn.net/turkeyzhou/article/details/8121601

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