Question

解决方案：使用更好的教程 - http://hadoop.apache.org/mapreduce/docs/r0.22.0/mapred_tutorial.html

我刚刚开始使用MapReduce，我遇到了一个我无法通过Google回答的奇怪错误。我正在制作一个基本的WordCount程序，但是当我运行它时，我在Reduce期间遇到以下错误：

java.lang.RuntimeException: java.lang.NoSuchMethodException: org.apache.hadoop.mapred.Reducer.<init>()
at org.apache.hadoop.util.ReflectionUtils.newInstance(ReflectionUtils.java:115)
at org.apache.hadoop.mapred.ReduceTask.runOldReducer(ReduceTask.java:485)
at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:420)
at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:396)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1121)
at org.apache.hadoop.mapred.Child.main(Child.java:249)

WordCount程序是Apache MapReduce教程中的程序。我在Mountain Lion上以伪分布式模式运行Hadoop 1.0.3，我认为所有这些都正常工作，因为这些示例都正常执行。有什么想法吗？

编辑：这是我的参考代码：

package mrt;

import java.io.IOException;
import java.util.*;

import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.*;

public class WordCount {
public static class Map extends MapReduceBase implements 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, OutputCollector<Text,IntWritable> output, Reporter reporter)
      throws IOException{

        String line = value.toString();
        StringTokenizer tokenizer = new StringTokenizer(line);
        while(tokenizer.hasMoreTokens()){
            word.set(tokenizer.nextToken());
            output.collect(word,one);
        }
    }
}

public static class Reduce extends MapReduceBase implements Reducer<Text, IntWritable, Text, IntWritable> {

    public void reduce(Text key, Iterator<IntWritable> values, OutputCollector<Text, IntWritable> output, Reporter reporter)
      throws IOException{

        int sum = 0;

        while(values.hasNext()){
            sum += values.next().get();
        }

        output.collect(key, new IntWritable(sum));

    }
}

public static void main(String[] args) throws Exception{

    JobConf conf = new JobConf(WordCount.class);
    conf.setJobName("Wordcount");

    conf.setOutputKeyClass(Text.class);
    conf.setOutputValueClass(IntWritable.class);

    conf.setMapperClass(Map.class);
    conf.setCombinerClass(Reduce.class);
    conf.setReducerClass(Reducer.class);

    conf.setInputFormat(TextInputFormat.class);
    conf.setOutputFormat(TextOutputFormat.class);

    FileInputFormat.setInputPaths(conf, new Path(args[0]));
    FileOutputFormat.setOutputPath(conf, new Path(args[1]));

    JobClient.runJob(conf);

}
}

Answer 1

问题不是您选择的API。完全支持stable（mapred。*）和不断发展的（mapreduce。*）API，框架本身也会对两者进行测试，以确保不会在各版本之间出现回归/破坏。

问题在于这一行：

conf.setReducerClass(Reducer.class);

当你应该设置Reducer接口的实现时，你将Reducer接口设置为Reducer。将其更改为：

conf.setReducerClass(Reduce.class);

将修复它。

Answer 2

检查以确保您使用的是hadoop.mapreduce包而不是hadoop.mapred包。 mapred包较旧，并且在类上具有与当前版本mapreduce类不同的方法。

Hadoop MapReduce的新手，在mapred.Reducer上获取NoSuchMethodException。<init>（）</init>

2 个答案: