Hadoop学习笔记 2 - MapReduce 简单实例

1.2 MapReduce开发实例

MapReduce 执行过程，如下图，（先由Mapper进行map计算，将数据进行分组，然后在由Reduce进行结果汇总计算）

直接上代码

package com.itbuilder.hadoop.mr;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class WordCount {

	public static void main(String[] args) throws Exception {

		//构建一个JOB对象
		Job job = Job.getInstance(new Configuration());
		
		//注意：main方法所在的类
		job.setJarByClass(WordCount.class);
		
		//设置Mapper相关属性
		job.setMapperClass(WCMapper.class);
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(LongWritable.class);
		FileInputFormat.setInputPaths(job, new Path(args[0]));
		
		//设置Reducer相关属性
		job.setReducerClass(WCReducer.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(LongWritable.class);
		FileOutputFormat.setOutputPath(job, new Path(args[1]));
		
		//提交任务
		job.waitForCompletion(true);
	}

	
	
	public static class WCMapper extends Mapper<LongWritable, Text, Text, LongWritable> {

		
		public WCMapper() {
			super();
		}

		@Override
		protected void map(LongWritable key, Text value,
				Mapper<LongWritable, Text, Text, LongWritable>.Context context)
				throws IOException, InterruptedException {
			String line = value.toString();
			
			String words[] = line.split(" ");
			
			for (String word : words) {
				context.write(new Text(word), new LongWritable(1));
			}
			
		}
		
		
	}
	
	public static class WCReducer extends Reducer<Text, LongWritable, Text, LongWritable> {

		
		public WCReducer() {
			super();
		}

		@Override
		protected void reduce(Text k2, Iterable<LongWritable> v2,
				Reducer<Text, LongWritable, Text, LongWritable>.Context arg2)
				throws IOException, InterruptedException {
			long counter = 0;
			for (LongWritable count : v2) {
				counter += count.get();
			}
			arg2.write(k2, new LongWritable(counter));
		}

	}
	
	
}

需要注意：

WCMapper、WCReducer 作为内部类，必须是静态的内部类

pom.xml 中的jar包依赖

<dependencies>
  	<dependency>
  		<groupId>junit</groupId>
  		<artifactId>junit</artifactId>
  		<version>4.11</version>
  		<scope>test</scope>
  	</dependency>
  	<dependency>
  		<groupId>org.apache.hadoop</groupId>
  		<artifactId>hadoop-mapreduce-client-core</artifactId>
  		<version>2.7.1</version>
  	</dependency>
  	<dependency>
  		<groupId>org.apache.hadoop</groupId>
  		<artifactId>hadoop-common</artifactId>
  		<version>2.7.1</version>
  	</dependency>
  	<dependency>
  		<groupId>org.apache.hadoop</groupId>
  		<artifactId>hadoop-hdfs</artifactId>
  		<version>2.7.1</version>
  	</dependency>
  	
  	<dependency>
  		<groupId>org.apache.hadoop</groupId>
  		<artifactId>hadoop-yarn-common</artifactId>
  		<version>2.7.1</version>
  	</dependency>
  	<dependency>
  		<groupId>org.apache.hadoop</groupId>
  		<artifactId>hadoop-yarn-client</artifactId>
  		<version>2.7.1</version>
  	</dependency>
  </dependencies>