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一、源代码

1. WordCountMapper类

package org.example.wordcounttemplate;

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> {

    //新建输出文本对象(输出的key类型)
    private Text text = new Text();
    //新建输出IntWritable对象(输出的value类型)
    private IntWritable intWritable = new IntWritable( 1);


    /**
     * 重写map方法
     * @param key 文本的索引
     * @param value 文本值
     * @param context 上下文对象
     * @throws IOException
     * @throws InterruptedException
     */
    @Override
    protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {

        //获取拆分后的一行文本

        //mysql mysql value value value
        String line = value.toString();

        //根据分隔符进行单词拆分
        String[] words = line.split( " ");

        //循环创建键值对
        for (String word : words){

            //输出key值设置
            text.set (word) ;

            //进行map输出
            //igeek igeek -> <igeek ,1> <igeek,1>
            context.write(text,intWritable);
        }

    }
}

2. WordCountReducer类

package org.example.wordcounttemplate;

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> {

    //输出value对象
    private IntWritable valueOut = new IntWritable();


    /**
     * 重写reduce方法
     * @param key 单词值
     * @param values 单词出现的次数集合
     * @param context   上下文对象
     * @throws IOException
     * @throws InterruptedException
     */
    @Override
    protected void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {

        //每个单词出现的次数
        int sum= 0;

        //<igeek,(1,1)>
        for (IntWritable value : values){

        //累计单词出现的数量
            sum += value.get();
        }

        //进行封装
        valueOut.set(sum);

        // reduce输出
        context.write(key, valueOut);


    }
}

3. WordCountDriver类

package org.example.wordcounttemplate;
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;

/**
 * 充当mapreduce任务的客户端，用于提交任务
 */

public class WordCountDriver {
    public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
//        1.获取配置信息，获取job对象实例
        Configuration conf=new Configuration();
        Job job=Job.getInstance(conf);

//        2.关联本Driver得jar路径
        job.setJarByClass(WordCountDriver.class);

//        3.关联map和reduce
        job.setMapperClass(WordCountMapper.class);
        job.setReducerClass(WordCountReducer.class);

//        4.设置map得输出kv类型
        job.setMapOutputKeyClass(Text.class);
        job.setMapOutputValueClass(IntWritable.class);

//        5.设置最终输出得kv类型
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(IntWritable.class);

//        6.设置输入和输出路径
        FileInputFormat.setInputPaths(job,new Path(args[0]));
        FileOutputFormat.setOutputPath(job,new Path(args[1]));

//        7.提交job
        boolean result=job.waitForCompletion(true);
        System.out.println(result?"任务提交成功":"任务提交失败");

    }


}

4. pom.xml

重点是更改添加打包插件依赖

<plugins>
    <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.6.1</version>
        <configuration>
            <source>1.8</source>
            <target>1.8</target>
        </configuration>
    </plugin>
    <plugin>
        <artifactId>maven-assembly-plugin</artifactId>
        <configuration>
            <descriptorRefs>
                <descriptorRef>jar-with-dependencies</descriptorRef>
            </descriptorRefs>
        </configuration>
        <executions>
            <execution>
                <id>make-assembly</id>
                <phase>package</phase>
                <goals>
                    <goal>single</goal>
                </goals>
            </execution>
        </executions>
    </plugin>
</plugins>

pom.xml文件内容如下：

<?xml version="1.0" encoding="UTF-8"?>

<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>org.example</groupId>
  <artifactId>mapreduce_demo</artifactId>
  <version>1.0-SNAPSHOT</version>

  <name>mapreduce_demo</name>
  <!-- FIXME change it to the project's website -->
  <url>http://www.example.com</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties>

  <dependencies>
    <!-- https://mvnrepository.com/artifact/org.apache.hadoop/hadoop-client -->
    <dependency>
      <groupId>org.apache.hadoop</groupId>
      <artifactId>hadoop-client</artifactId>
      <version>3.1.3</version>
    </dependency>

  </dependencies>
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.6.1</version>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
        </configuration>
      </plugin>
      <plugin>
        <artifactId>maven-assembly-plugin</artifactId>
        <configuration>
          <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
          </descriptorRefs>
        </configuration>
        <executions>
          <execution>
            <id>make-assembly</id>
            <phase>package</phase>
            <goals>
              <goal>single</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>

</project>

二、相关操作和配置

1. 项目打包

2. 带参测试

在本地执行成功：

3. 上传打包后的jar包和测试文档

上传打包后的带依赖jar包（第二个）和测试文档Hello.txt 到linux系统及hdfs上

cd /opt/jar/
ll

jar包改名：

mv mapreduce_demo-1.0-SNAPSHOT-jar-with-dependencies.jar wordcount.jar
ll

 cd /opt/file/
 ll

4. 增大虚拟内存

进行MapReduce操作时，可能会报溢出虚拟内存的错误

beyond the 'VIRTUAL’memory limit.
Current usage: 32.7 MB of 1 GB physical memory used;
2.3 GB of 2.1 GB virtual memory used. Killing container.

解决:

在mapred-site.xml中添加如下内容

	<!-- 是否对容器强制执行虚拟内存限制 -->
	<property>
        <name>yarn.nodemanager.vmem-check-enabled</name>
        <value>false</value>
        <description>Whether virtual memory limits will be enforced for containers</description>
    </property>
	
	
	<!-- 为容器设置内存限制时虚拟内存与物理内存之间的比率 -->
    <property>
        <name>yarn.nodemanager.vmem-pmem-ratio</name>
        <value>5</value>
        <description>Ratio between virtual memory to physical memory when setting memory limits for containers</description>
    </property>

cd /opt/softs/hadoop3.1.3/etc/hadoop/
vim mapred-site.xml

分发到另外两台服务器虚拟机

scp mapred-site.xml root@bigdata04:/opt/softs/hadoop3.1.3/etc/hadoop/

scp mapred-site.xml root@bigdata05:/opt/softs/hadoop3.1.3/etc/hadoop/

5.启动集群

[root@bigdata03 hadoop]# start-dfs.sh

[root@bigdata05 ~]# start-yarn.sh

6.在hdfs上创建输入文件夹和上传测试文档Hello.txt

hadoop fs -ls /
hadoop fs -mkdir /input

hadoop fs -put Hello.txt  /input
hadoop fs -ls  /input

7. 利用jar包在hdfs实现文本计数

 cd /opt/jar/
 ll
 
 hadoop jar wordcount.jar org.example.wordcounttemplate.WordCountDriver /input/Hello.txt /output  

注意：输出目录需不存在，让他执行命令时自行创建

8. 查看计算统计结果

hadoop fs -ls  /output
hadoop fs -cat  /output/part-r-00000

对照文章：
大数据作业4（含在本地实现wordcount案例）
https://blog.csdn.net/m0_48170265/article/details/130029532?spm=1001.2014.3001.5501