MapReduce程序——wordCount

参考原文:

http://www.cnblogs.com/little-YTMM/p/4396008.html

import java.io.IOException;
import java.util.StringTokenizer;
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.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 class TokenizerMapper
    /*
     * Mapper的前两个参数为输入K1,V1。后两个参数为输出K2,V2.
     * 本例中K1为文章ID且不重要,V1为文章内容,因此是Text类型。K2为输出的单词因此也为Text类型,V2为计数,因此为IntWritable。
     * 由于key和value在程序运行时需要被Hadoop框架序列化,因此必须实现Writable接口。
     * 另外key必须实现WritableComparable,这样框架才能对其进行排序。
     */
       extends Mapper<Object, Text, Text, IntWritable>{
    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();
    /*
     * LongWritable 为输入的key的类型
     * Text 为输入value的类型
     * Text-IntWritable 为输出key-value键值对的类型
     */
    public void map(Object key, Text value, Context context //这里前两个参数对应K1,V1。即输入键值对。
                    ) throws IOException, InterruptedException {
      StringTokenizer itr = new StringTokenizer(value.toString());  // 将TextInputFormat生成的键值对转换成字符串类型
      while (itr.hasMoreTokens()) {
        word.set(itr.nextToken());
        context.write(word, one); //这里word和one对应输出的K2,V2
      }
    }
  }
  public static class IntSumReducer
    /*
     * 和Mapper一样,Reducer的前两个参数为输入K1,V1。后两个参数为输出K2,V2.
     */
       extends Reducer<Text,IntWritable,Text,IntWritable> {
    private IntWritable result = new IntWritable();
    /*
     * Text-IntWritable 来自map的输入key-value键值对的类型
     * Text-IntWritable 输出key-value 单词-词频键值对
     */
    public void reduce(Text key, Iterable<IntWritable> values, //这里前两个参数对应K1,V1。即输入键值对。
                       Context context
                       ) throws IOException, InterruptedException {
      int sum = 0;
      for (IntWritable val : values) {
        sum += val.get();
      }
      result.set(sum);
      context.write(key, result);
    }
  }
  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();  // job的配置
    Job job = Job.getInstance(conf, "word count");  // 初始化Job
    job.setJarByClass(WordCount.class);
    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(IntSumReducer.class);
    job.setReducerClass(IntSumReducer.class);  
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    FileInputFormat.addInputPath(job, new Path(args[0]));  // 设置输入路径
    FileOutputFormat.setOutputPath(job, new Path(args[1]));  // 设置输出路径
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}
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