演示非常简单。什么时候用 hadoop jar hia-1.0-SNAPSHOT.jar cha1.CharCount ，但有例外：

Error: java.lang.RuntimeException: java.lang.ClassNotFoundException: Class cha1.CharCount$CMapper not found
    at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:1587)
    at org.apache.hadoop.mapreduce.task.JobContextImpl.getMapperClass(JobContextImpl.java:186)
    at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:715)
    at org.apache.hadoop.mapred.MapTask.run(MapTask.java:338)
    at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:157)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1408)
    at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:152)
Caused by: java.lang.ClassNotFoundException: Class cha1.CharCount$CMapper not found
    at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:1493)
    at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:1585)
    ... 8 more

这是演示代码

package cha1;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.ByteWritable;
import org.apache.hadoop.io.LongWritable;
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;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

/**
 * User: mzang
 * Date: 10/31/13
 * Time: 4:21 PM
 */
public class CharCount extends Configured implements Tool {

    class CMapper extends Mapper<LongWritable, Text, ByteWritable, LongWritable> {

        @Override
        protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
            byte[] content = value.getBytes();
            Map<ByteWritable, Integer> map = new HashMap<ByteWritable, Integer>();
            for (byte b : content) {
                ByteWritable bw = new ByteWritable(b);
                Integer c = map.get(bw);
                if (c == null) {
                    map.put(bw, 1);
                } else {
                    c++;
                    map.put(bw, c);
                }
            }
            for (Map.Entry<ByteWritable, Integer> entry : map.entrySet()) {
                context.write(entry.getKey(), new LongWritable(entry.getValue()));
            }
        }
    }

    class CCombiner extends Reducer<ByteWritable, LongWritable, ByteWritable, LongWritable> {

        @Override
        protected void reduce(ByteWritable key, Iterable<LongWritable> values, Context context) throws IOException, InterruptedException {

            long sum = 0;
            for (LongWritable longWritable : values) {
                sum += longWritable.get();
            }

            context.write(key, new LongWritable(sum));

        }
    }

    class CReducer extends Reducer<ByteWritable, LongWritable, Text, Text> {

        @Override
        protected void reduce(ByteWritable key, Iterable<LongWritable> values, Context context) throws IOException, InterruptedException {

            long sum = 0;
            for (LongWritable longWritable : values) {
                sum += longWritable.get();
            }

            context.write(new Text(String.valueOf(key.get())), new Text(String.valueOf(sum)));
        }
    }

    public int run(String[] args) throws IOException, ClassNotFoundException, InterruptedException {

        Job job = Job.getInstance(this.getConf());
        FileOutputFormat.setOutputPath(job, new Path("/tmp/test/output"));

        FileInputFormat.setInputPaths(job, new Path("/tmp/test/input"));

        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);
        job.setOutputFormatClass(TextOutputFormat.class);
        job.setMapperClass(CMapper.class);
        job.setCombinerClass(CCombiner.class);
        job.setReducerClass(CReducer.class);

        job.submit();

        return job.waitForCompletion(true) ? 0 : 1;
    }

    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        conf.addResource("hdfs-site.xml");
        int res = ToolRunner.run(conf, new CharCount(), args);

        System.exit(res);
    }

}

它应该可以在apachehadoop上运行。
我查了hadoop日志。它什么也没说，但很多尝试都失败了。
然后我试着检查一下 hadoop fs -ls /tmp/hadoop-yarn/staging/history/ . 但是两个目录done和done是空的。
jar应该分发给hdfs，并由每个任务跟踪器下载，对吗？类charcount$cmapper应该在jar中。