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