環境要求
說明：本文檔為wordcount的mapreduce job編寫及執行文檔。

操作系統：Ubuntu14 x64位
Hadoop：Hadoop 2.7.0
Hadoop官網：http://hadoop.apache.org/releases.html
MapReduce參照官網步驟：
http://hadoop.apache.org/docs/current/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html#Source_Code

本章基於前一篇文章《hadoop2.7.0實踐-環境搭建》。

1.安裝Eclipse
1)下載eclipse
官網：http://www.eclipse.org/

2)解壓eclipse包

$tar -xvf eclipse-jee-mars-R-linux-gtk-x86_64.tar.gz

3)啟動eclipse
4)寫測試程序

public class TestMore {

    public static void main(String[] args) {
        System.out.println("hello world!");
        System.out.println("I‘m so glad to see that"
 
);
    }
}

2.編寫wordcount
1)jar包引入
eclipse的lib中引入的jar包
hadoop包下的share/hadoop下的各個文件夾都有jar包
hadoop-2.7.0/share/hadoop/common/hadoop-common-2.7.0.jar
hadoop-2.7.0/share/hadoop/mapreduce/hadoop-mapreduce-client-core-2.7.0.jar

2)編寫worcount程序
相應源代碼

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;

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 itr = new StringTokenizer(value.toString());
      while (itr.hasMoreTokens()) {
        word.set(itr.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();
    Job job = Job.getInstance(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(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

3)導出jar包
取名wc.jar，直接導出到hadoop文件夾下。

3.執行wordcount
1)啟動dfs服務
參照文件《hadoop2.7.0實踐-環境搭建》。
進入hadoop文件夾，用cd命令。

$sbin/start-dfs.sh

相應查看網頁：http://localhost:50070/
2)準備文件
hadoop-2.7.0/wctest/input文件夾中放入待統計文件file01
輸入內容：hello world bye world

//創建hdfs文件夾。操作命令相似本地操作

$ bin/hdfs fs -mkdir /user
$ bin/hdfs fs -mkdir /user/a

//復制本地文件到hdfs中

$ bin/hdfs fs -put wctest/input /user/a/input

//備註：相應文件夾刪除命令例如以下

delete dir：bin/hadoop fs -rm -f -r /user/a/input

相應文件http://localhost:50070/
3)啟動yarn服務

$ sbin/start-yarn.sh

4)執行wordcount程序

$ bin/hadoop jar wc.jar WordCount /user/a/input /user/a/output

5)查看結果

$ bin/hadoop fs -cat /user/a/output/part-r-00000
bye 1
hello   1
world   2

常見錯誤及說明
1)未啟動yarn時執行MapReduce程序

原因：已經配置了yarn,但沒有啟動引起的
調整：啟動一下yarn

$ sbin/start-yarn.sh

hadoop2.7.0實踐- WordCount