MapReduce實作WordCount程式

1.Mapper程式

import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
//											 k1			  v1	k2		v2
public class WordCountMapper extends Mapper<LongWritable, Text, Text, IntWritable> {

	@Override
	protected void map(LongWritable k1, Text v1,Context context)
			throws IOException, InterruptedException {
		/*
		 *context 表示Mapper的上下文
		 *上文：HDFS
		 *下文：Reducer
		 */
		//資料
		String data = v1.toString();
		
		//分詞操作
		String[] words = data.split(" ");
		for (String w : words) {
			context.write(new Text(w), new IntWritable(1));
		}
	}



}
           

2.Reducer程式

import java.io.IOException;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
//												k3		v3		  k4	v4
public class WordCountReducer extends Reducer<Text, IntWritable, Text, IntWritable> {

	@Override
	protected void reduce(Text k3, Iterable<IntWritable> v3, Context context)
			throws IOException, InterruptedException {
		/*
		 * context:Reducer的上下文
		 * 上文是：Mapper
		 * 下文是：HDFS
		 */
		//對v3求和
		int total = 0;
		for (IntWritable v : v3) {
			total += v.get();
		}
		//輸出k4單詞 v4頻率
		context.write(k3, new IntWritable(total));
	}

	
}
           

3.送出任務的主程式

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 {

	/**
	 * @param args
	 * @throws Exception 
	 */
	public static void main(String[] args) throws Exception {
		// 建立一個Job
		Job job = Job.getInstance(new Configuration());
		job.setJarByClass(WordCountMain.class);//main方法所在的class
		
		//指定job的mapper <k2,v2>
		job.setMapperClass(WordCountMapper.class);
		job.setMapOutputKeyClass(Text.class);
		job.setMapOutputValueClass(IntWritable.class);
		
		//指定job的reducer <k4,v4>
		job.setReducerClass(WordCountReducer.class);
		job.setOutputKeyClass(Text.class);
		job.setOutputValueClass(IntWritable.class);
		
		//指定job的輸入和輸出
		FileInputFormat.setInputPaths(job, new Path(args[0]));
		FileOutputFormat.setOutputPath(job, new Path(args[1]));
		
		//執行job
		job.waitForCompletion(true);
	}

}