MapReduce程序如何输出txt文件

MapReduce程序输出txt文件的过程和普通的文件输出过程类似，只需要在Reduce函数中通过Java API将结果写入指定路径的文件即可。以下是一个简单的MapReduce程序输出txt文件的示例：

public class WordCount {
  public static class TokenizerMapper extends Mapper<Object, Text, Text, IntWritable>{
    private final static IntWritable one = new IntWritable(1);
    private 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);
      }
    }
  }
  
  public static class IntSumReducer extends Reducer<Text,IntWritable,Text,IntWritable> {
    private 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);
    }
  }
  
  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    Job job = Job.getInstance(conf, "word count");
    job.setJarByClass(WordCount.class);
    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(IntSumReducer.class);
    job.setReducerClass(IntSumReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    
    // 设置输出格式为TextOutputFormat，这样输出的结果会以文本的形式存储在指定路径下
    job.setOutputFormatClass(TextOutputFormat.class);
    
    // 设置输出路径为指定的txt文件所在的目录，注意这里不是txt文件本身的路径
    FileOutputFormat.setOutputPath(job, new Path("hdfs://localhost:9000/output"));
    
    // 执行MapReduce任务
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

在上面的示例中，我们通过`job.setOutputFormatClass(TextOutputFormat.class)`设置了输出格式为`TextOutputFormat`，并且通过`FileOutputFormat.setOutputPath(job, new Path("hdfs://localhost:9000/output"))`设置了输出路径为指定的txt文件所在的目录，注意这里不是txt文件本身的路径。最终执行该程序后，就会将结果以文本的形式存储在指定路径下。