MAPREDUCE 简单入门

• 什么是MAPREDUCE ：

1. MapReduce 八个字的核心的思想分而治之，

• Mapreduce简单的工作原理：

1. mapredue 有maptask、reducetask组成
2. 一个切片一个mapreduce,
3. reduceTask 的默认是一个，可以设置多个
   1. 设置过程job.setNumReduceTask(3);
   2. reduce 分区规则：
      1. 根据可以的(value.hashcode()%reduce_num) 得到分区号。
      2. 好处：同一个key发给同一个reduce ,最后所有的reduce合并就会得到最终结果。
4. 在同一个map_reduce阶段，一个过程仅可出现一次。

• 

• 创建一个简单的MapReduce程序。

1. 重写map方法

public class MyMap extends Mapper<LongWritable, Text,Text,IntWritable>{
    private Text outputKey = null;
    private IntWritable outputValue = null;
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
       String line = value.toString();
       String[] words = line.split(" ");
        outputKey = new Text();
        outputValue = new IntWritable(1);
        for(String word:words){
            outputKey.set(word);
            context.write(outputKey,outputValue);
        }
    }
}

重写reducer方法

package com.dousil.hadoop.demo.MyReduce;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

import java.io.IOException;

public class MyReduce extends Reducer<Text, IntWritable,Text,IntWritable> {
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        int count =0;
        for(IntWritable value:values){
            count +=value.get();
        }
        context.write(key,new IntWritable(count));
    }
}

1. 组装代码package com.dousil.hadoop.demo.MyWordCounter

import com.dousil.hadoop.demo.MyMap.MyMap;
import com.dousil.hadoop.demo.MyReduce.MyReduce;
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.FileOutputStream;
import java.io.IOException;

public class MyWordCounter {
    public static void main(String args[]) throws IOException {
        Configuration conf = new Configuration() ;
        Job job = Job.getInstance(conf);
        FileInputFormat.setInputPaths(job,new Path(args[0]));
        FileOutputFormat.setOutputPath(job,new Path(args[1]));
        //设置自己Mapper类
        job.setMapperClass(MyMap.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);
        //设置reduce类
        job.setReducerClass(MyReduce.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);

        //job提交到yarn去运行
        boolean result =false;
        try{
            result = job.waitForCompletion(true);//true，和false控制台是否打印调试信息
        }catch(Exception e){
            e.printStackTrace();
        }
        System.out.println(result);
    }
}