4、第一 个mapReduce程序

第一步，map，TokenizerMapper：

package com.Kevin.hadoop;

import java.io.IOException;
import java.util.StringTokenizer;

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

public class TokenizerMapper
extends Mapper<Object,Text,Text,IntWritable>{

IntWritable one = new IntWritable(1);
Text word = new Text();

public void map(Object key,Text value,Context context)
throws IOException,InterruptedException{

StringTokenizer itr = new StringTokenizer(value.toString());
while(itr.hasMoreTokens())
{
word.set(itr.nextToken());
context.write(word, one);
}
}
}

继承Mapper<Object,Text,Text,IntWritable>，Object输入数据的key，如该行数据的偏移量,Text输入数据的值,Text输出数据的key,IntWritable输出数据的值；

IntWritable one = new IntWritable(1);设置每个key的value都为1

Text word = new Text();创建key对象

StringTokenizer itr = new StringTokenizer(value.toString());java中用来分割字符串的类

context.write(word, one);context保存key-value，留待reduce处理

第二步，reduce，IntSumReducer

package com.Kevin.hadoop;

import java.io.IOException;

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

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

IntWritable result = new IntWritable();

public void reduce(Text key,Iterable<IntWritable> values,Context context)
throws IOException,InterruptedException{

int sum = 0;
for(IntWritable val:values)
{
sum += val.get();
}
result.set(sum);
context.write(key, result);

}

}

for(IntWritable val:values)
{
sum += val.get();
}循环读取key对应的所有value，并将其累加，最后将结果写入到context中
第三步，WordCount

package com.Kevin.hadoop;

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 org.apache.hadoop.util.GenericOptionsParser;

public class WordCount {

public static void main(String[] args) throws Exception
{
Configuration conf = new Configuration();
String[] otherArgs = new GenericOptionsParser(conf,args).getRemainingArgs();
if(2 != otherArgs.length)
{
System.err.println("Usage: wordcount<in><out>");
System.exit(2);
}

Job job = new Job(conf,"wordcount");
job.setJarByClass(WordCount.class);
job.setMapperClass(TokenizerMapper.class);
job.setReducerClass(IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
System.exit(job.waitForCompletion(true)?0:1);
}

}

configuration类：读写和保存各种配置资源

job.setMapperClass(TokenizerMapper.class);设置map

job.setReducerClass(IntSumReducer.class);设置reduce

job.setOutputKeyClass(Text.class);设置输出key

job.setOutputValueClass(IntWritable.class);设置输出value

FileInputFormat.addInputPath(job, new Path(otherArgs[0]));设置输入文件路径

FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));设置输出文件路径