Hadoop--MapReduce实现WordCount全步骤

1. 新建maven项目
   1. +Create New Project… ->Maven -> Next
      
      
   2. 填写好GroupId和ArtifactId 点击Next -> Finish
2. 编写wordcount项目
   1. 建立项目结构目录：右键java -> New -> package 输入package路径（本例是com.hadoop.wdcount）建立package。类似的方式在创建好的package下建立三个类WordcountMain、WordcountMapper、WordcountReducer
      
      2、 编写pom.xml配置（引入要用到的hadoop的jar包）

    4.0.0

    sa.hadoop
    wordcount
    1.0-SNAPSHOT

    
        
            org.apache.hadoop
            hadoop-common
            2.7.7

        

        
            org.apache.hadoop
            hadoop-hdfs
            2.7.7
        
		
            org.apache.hadoop
            hadoop-mapreduce-client-common
            2.7.7
        

        
            org.apache.hadoop
            hadoop-mapreduce-client-core
            2.7.7
        
    
    

3、 编写项目代码
完成刚刚建立的三个类中的逻辑实现。
(1) WordcountMapper.java

package com.hadoop.wdcount;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;

import java.io.IOException;

public class WordcountMapper extends org.apache.hadoop.mapreduce.Mapper<LongWritable,Text,Text,IntWritable> {


    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
        String line=value.toString();
        String[] words=line.split(" ");
        for (String word:words){
            context.write(new Text(word),new IntWritable(1));
        }
    }
}

(2)WordcountReducer.java

package com.hadoop.wdcount;

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

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

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

    @Override
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
        Integer counts=0;
        for (IntWritable value:values){
            counts+=value.get();
        }
        context.write(key,new IntWritable(counts));
    }
}

(3)WordcountMain.java

package com.hadoop.wdcount;

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;

public class WordcountMain {
    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        Job job = Job.getInstance(conf, "wordcount");
        job.setJarByClass(WordcountMain.class);
        job.setMapperClass(WordcountMapper.class);
        job.setReducerClass(WordcountReducer.class);
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);
        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        boolean flag = job.waitForCompletion(true);
        if (!flag) {
            System.out.println("wordcount failed!");
        }
    }
}

3. 将项目打包成jar

   1. 右键项目名称 -> Open Module Settings
      
   2. Artifacts -> + -> JAR -> From modules with dependencies…
      
   3. 填写Main Class（点击…选择WordcountMain），然后选择extract to the target JAR，点击OK。
      
   4. 勾选include in project build ，其中Output directory为最后的输出目录，下面output layout是输出的各jar包，点击ok
      
   5. 点击菜单Build——>Build Aritifacts…
   6. 选择Build，结果可到前面4的output目录查看或者项目结构中的out目录
      

4. 执行验证（这里采用win环境下的hadoop2.7.6作为例子，wsl暂时未验证）

   1. 先在创建jar包路径下（C:\Users\USTC\Documents\maxyi\Java\wordcount\out\artifacts\wordcount_jar）建立一个input1.txt文档，并添加内容“I believe that I will succeed!”并保存。等会儿要将该txt文件上传到hadoop。

   2. 运行hadoop打开所有节点
      cd hadoop-2.7.6/sbin
start-all.cmd

   3. 运行成功后，来到之前创建的jar包路径，将编写好的txt文件上传到hadoop
      cd /
cd C:\Users\USTC\Documents\maxyi\Java\wordcount\out\artifacts \wordcount_jar
hadoop fs -put ./input1.txt /input1
      可以用以下代码查看是否上传成功。
      hadoop fs -ls /
      4、 删除wordcount.jar/META-INF/LICENSE，否则不能创建hadoop运行时不能创建license，会报错。
      5、 运行wordcount
      hadoop jar wordcount.jar com.hadoop.wdcount.WordcountMain /input1 /output2
      jar 命令后有四个参数，
      第一个wordcount.jar是打好的jar包
      第二个com.hadoop.wdcount.WordcountMain是java工程总编写的主类，这里要包含package路径
      第三个/input1是刚刚上传的输入
      第四个/output2是wordcount的输出（一定要新建，不能重用之前建立的）
      6、 下载输出文件查看结果是否正确
      hadoop fs -get /output2