深入探究如何使用Java编写MapReduce程序

目录

• MapReduce的原理
• Map阶段
• Reduce阶段
• Shuffle阶段
• MapReduce程序实现
• 总结

接下来，我们编写Reduce函数。Reduce函数将具有相同键的值相加，并将结果作为键值对输出。以下是Reduce函数的代码：

javaCopy codepublic static class Reduce extends Reducer<Text, IntWritable, Text, IntWritable> {
  public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
    int sum = 0;
    for (IntWritable value : values) {
      sum += value.get();
    }
    context.write(key, new IntWritable(sum));

最后，我们将Map函数和Reduce函数组合起来，并将它们作为MapReduce程序的一部分提交给Hadoop集群。以下是完整的MapReduce程序：

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.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 class Map extends Mapper<LongWritable, Text, Text, IntWritable> {
    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();

    public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
      String line = value.toString();
      StringTokenizer tokenizer = new StringTokenizer(line);
      while (tokenizer.hasMoreTokens()) {
        word.set(tokenizer.nextToken());
        context.write(word, one);
      }
    }
  }

  public static class Reduce extends Reducer<Text, IntWritable, Text, IntWritable> {
    public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
      int sum = 0;
      for (IntWritable value : values) {
        sum += value.get();
      }
      context.write(key, new IntWritable(sum));
    }
  }

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    Job job = Job.getInstance(conf, "wordcount");
    job.setJarByClass(WordCount.class);
    job.setMapperClass(Map.class);
    job.setCombinerClass(Reduce.class);
    job.setReducerClass(Reduce.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }

}

在上面的代码中，我们首先定义了Map类和Reduce类，然后在main函数中将它们组合起来，使用Job类将程序提交给Hadoop集群进行处理。我们使用FileInputFormat和FileOutputFormat指定输入和输出路径。

总结

本文介绍了MapReduce的原理和使用Java编写MapReduce程序的方法。MapReduce是一个强大的并行编程模型，可用于处理大规模数据集。如果你正在处理大数据集，那么MapReduce可能是你的首选方案。

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