Hadoop MapReduce运行报错：WordCount$Map类未找到求助

我编译并尝试运行以下WordCount代码：

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(Reduce.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);
    }
}

执行命令：

yarn jar wc.jar WordCount /user/ubuntu/books/holmes.txt /user/ubuntu/output3

出现如下错误：

Exception in thread "main" java.lang.NoClassDefFoundError: WordCount$Map
    at WordCount.main(WordCount.java:43)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.hadoop.util.RunJar.run(RunJar.java:323)
    at org.apache.hadoop.util.RunJar.main(RunJar.java:236) 
Caused by: java.lang.ClassNotFoundException: WordCount$Map
    at java.net.URLClassLoader.findClass(URLClassLoader.java:387)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:418)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:351)
    ... 7 more

我不清楚WordCount$Map的来源，请问我遗漏了什么？

什么是WordCount$Map？ #

WordCount$Map是Java内部类的标准命名格式，对应你代码里定义在WordCount类内部的Map静态内部类。Java编译内部类时，会单独生成一个名为外部类名$内部类名.class的字节码文件，所以编译你的代码后，应该得到三个文件：WordCount.class、WordCount$Map.class、WordCount$Reduce.class。

错误原因 #

这个报错的核心是Hadoop运行时找不到WordCount$Map的字节码文件，大概率是打包jar的时候没把内部类的.class文件包含进去，或者编译时没生成这些文件。

解决步骤 #

1. 检查编译产物：先看编译后目录里有没有WordCount$Map.class和WordCount$Reduce.class，如果没有，重新编译，编译命令要带上Hadoop的依赖路径：

   javac -cp $(hadoop classpath) WordCount.java
   

2. 正确打包jar：打包时要把所有相关的.class文件都加进去，别只打包主类的.class。可以用下面的命令：

   jar cvf wc.jar *.class
   

   或者指定文件打包：

   jar cvf wc.jar WordCount.class WordCount$Map.class WordCount$Reduce.class
   

3. 验证jar内容：打包完用jar tf wc.jar查看jar里的文件列表，确认WordCount$Map.class和WordCount$Reduce.class在里面。
4. 重新运行：确认jar没问题后，再执行原来的yarn命令即可。

内容的提问来源于stack exchange，提问作者onlyjamiejay