简介

Combiner是Mapper和Reducer之外的组件。
Combiner是在Reducer运行之前，对Mapper数据进行处理的。

Wordcount实例

WordCountMapper

package com.neve.Combiner;

import org.apache.hadoop.io.IntWritable;

import org.apache.hadoop.io.LongWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.Mapper;

import java.io.IOException;

public class WordCountMapper  extends Mapper<LongWritable, Text,Text, IntWritable>{

    private Text outk = new Text();

    //每次读到一个单词都为1

    private IntWritable outv = new IntWritable(1);

    @Override

    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {

        //1.将text换为string

        String line = value.toString();

        //2.分割

        String[] words = line.split(" ");

        //3.输出

        for (String word : words) {

            //将String转换为Text

            outk.set(word);

            //写出

            context.write(outk, outv);

        }

    }

}

WordCountReducer

package com.neve.Combiner;

import org.apache.hadoop.io.IntWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;

public class WordCountReducer extends Reducer<Text, IntWritable,Text,IntWritable> {

    private IntWritable outv = new IntWritable();

    @Override

    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {

        int sum = 0;

        for (IntWritable value : values) {

            sum += value.get();

        }

        outv.set(sum);

        context.write(key,outv);

    }

}

WordCountCombiner

package com.neve.Combiner;

import org.apache.hadoop.io.IntWritable;

import org.apache.hadoop.io.Text;

import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;

public class WordCountCombiner extends Reducer<Text, IntWritable,Text,IntWritable> {

    private IntWritable outv = new IntWritable();

    @Override

    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {

        int sum = 0;

        for (IntWritable value : values) {

            sum += value.get();

        }

        outv.set(sum);

        context.write(key,outv);

    }

}

WordCountDriver

package com.neve.Combiner;

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.lib.input.FileInputFormat;

import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;

public class WordCountDriver {

    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {

        //1.创建配置

        Configuration configuration = new Configuration();

        //2.创建job

        Job job = Job.getInstance(configuration);

        //3.关联驱动类

        job.setJarByClass(WordCountDriver.class);

        //4.关联mapper和reducer类

        job.setMapperClass(WordCountMapper.class);

        job.setReducerClass(WordCountReducer.class);

        //5.设置mapper的输出值和value

        job.setMapOutputKeyClass(Text.class);

        job.setMapOutputValueClass(IntWritable.class);

        //6.设置最终的输出值和value

        job.setOutputKeyClass(Text.class);

        job.setOutputValueClass(IntWritable.class);

        //7.设置输入输出路径

        FileInputFormat.setInputPaths(job,new Path("F:\\Workplace\\IDEA_Workplace\\hadoopstudy2\\input"));

        FileOutputFormat.setOutputPath(job,new Path("F:\\Workplace\\IDEA_Workplace\\hadoopstudy2\\output"));

        //设置combiner

        job.setCombinerClass(WordCountCombiner.class);

        //8.提交job

        job.waitForCompletion(true);

    }

}

可以看到combiner与reducer类相同，便可直接将reducer类当做combiner使用（该案例）。

巴特西

Combiner-Reduce之前处理过程

简介

Wordcount实例

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