写一端mapperreduce代码

以下是一个示例 MapReduce 代码：

Mapper：

public class MapperClass 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);
    }
  }
}

Reducer：

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

Driver：

public class DriverClass {
  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    Job job = Job.getInstance(conf, "word count");
    job.setJarByClass(DriverClass.class);
    job.setMapperClass(MapperClass.class);
    job.setCombinerClass(ReducerClass.class);
    job.setReducerClass(ReducerClass.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);
  }
}

这段代码实现了一个简单的单词计数 MapReduce 任务。Mapper 读取输入文件中的每一行文本，将每个单词作为 key，值为 1，并输出到 Reducer 进行处理。Reducer 对相同单词的值进行累加，输出每个单词及其出现次数。Driver 则设置了输入输出路径等参数，并启动 MapReduce 任务。