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下面通过WordCount，WordMean等几个例子讲解MapReduce的实际应用，编程环境都是以Hadoop MapReduce为基础

一、WordCount

WordCount用于计算文件中每个单词出现的次数，非常适合采用MapReduce进行处理，处理单词计数问题的思路很简单，在 Map阶段处理每个文本split中的数据,产生<word,1> 这样的键-值对，在Reduce阶段对相同的关键字求和,最后生成所有的单词计数 。

运行示意图如下

运行结果如下 

 二、WordMean

对上面例子的代码稍作修改，改成计算所有文件中单词的平均长度，单词长度的定义是单词的字符个数，现在HDFS集群中有大量的文件，需要统计所有文件中所出现单词的平均长度。

三、Grep

还是进行大规模文本中单词的相关操作，现在希望提供类似Linux系统中的Grep命令的功能，找出匹配目标串的所有文件，并统计出每个文件中出现目标字符串的个数。

在 Map阶段根据提供的文件split信息、给定的每个字符串输出 <filename,1> 这样 的键-值对信息

在 Reduce阶段根据filename对 Map阶段产生的结果进行合并

运行效果如下

 四、代码

部分代码如下 全部代码请点赞关注收藏后评论区留言私信~

package alibook.odps;

import java.io.IOException;
import java.util.Iterator;

import com.aliyun.odps.data.Record;
import com.aliyun.odps.data.TableInfo;
import com.aliyun.odps.mapred.JobClient;
import com.aliyun.odps.mapred.MapperBase;
import com.aliyun.odps.mapred.ReducerBase;
import com.aliyun.odps.mapred.conf.JobConf;
import com.aliyun.odps.mapred.utils.InputUtils;
import com.aliyun.odps.mapred.utils.OutputUtils;
import com.aliyun.odps.mapred.utils.SchemaUtils;

public class wordcount {

  public static class TokenizerMapper extends MapperBase {
    private Record word;
    private Record one;

    @Override
    public void setup(TaskContext context) throws IOException {
      word = context.createMapOutputKeyRecord();
      one = context.createMapOutputValueRecord();
      one.set(new Object[] { 1L });
      System.out.println("TaskID:" + context.getTaskID().toString());
    }

    @Override
    public void map(long recordNum, Record record, TaskContext context)
        throws IOException {
      for (int i = 0; i < record.getColumnCount(); i++) {
        word.set(new Object[] { record.get(i).toString() });
        context.write(word, one);
      }
    }
  }

  /**
   * A combiner class that combines map output by sum them.
   **/
  public static class SumCombiner extends ReducerBase {
    private Record count;

    @Override
    public void setup(TaskContext context) throws IOException {
      count = context.createMapOutputValueRecord();
    }

    @Override
    public void reduce(Record key, Iterator<Record> values, TaskContext context)
        throws IOException {
      long c = 0;
      while (values.hasNext()) {
        Record val = values.next();
        c += (Long) val.get(0);
      }
      count.set(0, c);
      context.write(key, count);
    }
  }

  /**
   * A reducer class that just emits the sum of the input values.
   **/
  public static class SumReducer extends ReducerBase {
    private Record result = null;

    @Override
    public void setup(TaskContext context) throws IOException {
      result = context.createOutputRecord();
    }

    @Override
    public void reduce(Record key, Iterator<Record> values, TaskContext context)
        throws IOException {
      long count = 0;
      while (values.hasNext()) {
        Record val = values.next();
        count += (Long) val.get(0);
      }
      result.set(0, key.get(0));
      result.set(1, count);
      context.write(result);
    }
  }

  public static void main(String[] args) throws Exception {
    if (args.length != 2) {
      System.err.println("Usage: WordCount <in_table> <out_table>");
      System.exit(2);
    }

    JobConf job = new JobConf();

    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(SumCombiner.class);
    job.setReducerClass(SumReducer.class);

    job.setMapOutputKeySchema(SchemaUtils.fromString("word:string"));
    job.setMapOutputValueSchema(SchemaUtils.fromString("count:bigint"));

    InputUtils.addTable(TableInfo.builder().tableName(args[0]).build(), job);
    OutputUtils.addTable(TableInfo.builder().tableName(args[1]).build(), job);

    JobClient.runJob(job);
  }

}

pom.xml文件代码如下

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>alibook</groupId>
  <artifactId>odps</artifactId>
  <version>0.0.1</version>
  <packaging>jar</packaging>

  <name>odps</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
    
    <dependency>
      <groupId>com.aliyun.odps</groupId>
      <artifactId>odps-sdk-core</artifactId>
      <version>0.23.3-public</version>
    </dependency>
    
    <dependency>
      <groupId>com.aliyun.odps</groupId>
      <artifactId>odps-sdk-commons</artifactId>
      <version>0.23.3-public</version>
    </dependency>
  
    <dependency>
      <groupId>com.aliyun.odps</groupId>
      <artifactId>odps-sdk-mapred</artifactId>
      <version>0.23.3-public</version>
    </dependency>
  
  </dependencies>
</project>

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