我对hadoop还比较陌生，在尝试运行作业时遇到classnotfoundexception，我很难理解。我使用的是这里的标准教程，下面是我的wordcount类（运行在ubuntu 16.04 hadoop 2.7.3分布式集群模式下）：

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.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
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 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);
    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

为了保持组织有序，我在~/.bashrc文件中添加了几个路径：

hduser@mynode:~$ cd $HADOOP_CODE
hduser@mynode:/usr/local/hadoop/code$

这是$hadoop\u主目录下的一个目录。为了编译wordcount.java文件，我运行了：

hduser@mynode:/usr/local/hadoop$ hadoop com.sun.tools.javac.Main $HADOOP_CODE/WordCount.java
hduser@mynode:/usr/local/hadoop$ jar cf wc.jar $HADOOP_CODE/WordCount*.class

然后我试着：

hduser@mynode:/usr/local/hadoop$ hadoop jar $HADOOP_CODE/wc.jar $HADOOP_CODE/WordCount /home/hduser/input /home/hduser/output/wordcount

爆炸时出现以下错误：

Exception in thread "main" java.lang.ClassNotFoundException: /usr/local/hadoop/code/WordCount

编辑这个给了我同样的错误：

hduser@mynode:/usr/local/hadoop/code$ hadoop jar $HADOOP_CODE/wc.jar WordCount /home/hduser/input /home/hduser/output/wordcount

为了让它正常运行，我将wordcount.java文件移到了默认的hadoop（$hadoop\u home）文件夹中。我也从这里和这里知道，解决方案是添加一个包到文件。
我想理解的是为什么这是解决办法。没有包名的情况下，hadoop在哪里查找指定的包，为什么我不能传递完整的路径让它正确运行？这可能是一个基本的java问题（很抱歉，我来自python世界），但是包名在编译过程中做了什么，使得我可以在没有路径名的情况下运行，但是去掉包名意味着它必须在默认目录中？我不想为我运行的每个作业都添加包名。如能解释，将不胜感激！