MapReduce 排序程序中的 NullPointerException

Question

我知道 SortComparator 用于按键对映射输出进行排序。我写了一个自定义 SortComparator 来理解 MapReduce 框架 better.This 是我的 WordCount class 和自定义 SortComparator class.

package bananas;

import java.io.FileWriter;
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.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Partitioner;
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
       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 class MyPartitoner extends Partitioner<Text, IntWritable>{

    @Override
    public int getPartition(Text key, IntWritable value, int numPartitions) {


        return Math.abs(key.hashCode()) % numPartitions;
    }  
  }

  public static class MySortComparator2 extends WritableComparator{

      protected MySortComparator2() {
          super();
          }

      @SuppressWarnings({ "rawtypes" })
    @Override
      public int compare(WritableComparable w1,WritableComparable w2){

          return 0;
      }
  }

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    Job job = Job.getInstance(conf, "word count");
    job.setJarByClass(WordCount.class);
    job.setSortComparatorClass(MySortComparator2.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]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

但是当我执行这个时我得到了这个错误

Error: java.lang.NullPointerException
    at org.apache.hadoop.io.WritableComparator.compare(WritableComparator.java:157)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.compare(MapTask.java:1265)
    at org.apache.hadoop.util.QuickSort.fix(QuickSort.java:35)
    at org.apache.hadoop.util.QuickSort.sortInternal(QuickSort.java:87)
    at org.apache.hadoop.util.QuickSort.sort(QuickSort.java:63)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.sortAndSpill(MapTask.java:1593)
    at org.apache.hadoop.mapred.MapTask$MapOutputBuffer.flush(MapTask.java:1482)
    at org.apache.hadoop.mapred.MapTask$NewOutputCollector.close(MapTask.java:720)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:790)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
    at org.apache.hadoop.mapred.YarnChild.run(YarnChild.java:163)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:415)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1628)
    at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)

我的自定义 SortComparator class 看起来不错。映射完成后，MySortComparator2 的比较方法应该接收 "Text" 键作为输入，因为我返回 0，所以不会进行排序。这是我所期望的see/observe。我遵循了这些教程

http://codingjunkie.net/secondary-sort/

http://blog.zaloni.com/secondary-sorting-in-hadoop

http://www.bigdataspeak.com/2013/02/hadoop-how-to-do-secondary-sort-on_25.html

在此先感谢我希望能提供一些帮助。

Answer 1

你也需要implement/override这个方法：

public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
    // per your desired no-sort logic
    return 0;
}

我认为您的比较器的构造方式使得超级实现中提到的变量为空（这是为支持排序而调用的方法，而不是您在上面编写的方法）。这就是你得到空指针异常的原因。通过使用不使用变量的实现覆盖该方法，您可以避免异常。

Answer 2

正如 Chris Gerken 所说，您需要在扩展 WritableComparator 时覆盖此方法或实现 RawComparator 而不是 WritableComparator。

public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
    return 0;
}

正如您所说，您不想看到要进行的排序，但是如果您 return 0 这意味着每次 MapReduce 尝试 sort/compare 它都会将每个键视为同一件事，所以，您将只接收一个键值对，这将是地图任务中最先完成的第一个键和输入文件中带有单词数的值。希望你明白我在说什么。如果你的输入是这样的

why are rockets cylindrical

你的减少输出将是

why  4

因为它假定所有内容都是同一个密钥。希望对您有所帮助。

Answer 3

实际上，MySortComparator2 构造函数有问题。代码应该看起来像

protected MySortComparator2() {
      super(Text.class, true);
}

其中第一个参数是您的密钥 class，第二个参数的值确保 WritableComparator 以 WritableComparator.compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) 可以调用 MySortComparator2.compare(WritableComparable a, WritableComparable b)[=14= 的方式实例化]

MapReduce 排序程序中的 NullPointerException

NullPointerException in MapReduce Sorting Program

java

sorting

hadoop

mapreduce