在Eclipse中运行WordCount示例时出现ClassNotFoundException

Question

I'm trying to run the exemplary code for WordCount map/reduce job. I'm running it on Hadoop 1.2.1. and I'm running it from my Eclipse. Here is the code I try to run:

package mypackage;

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.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.Reducer.Context;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;


public class WordCount {

    public static class Map extends
            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, Context context)
                throws IOException, InterruptedException {
            String line = value.toString();
            StringTokenizer tokenizer = new StringTokenizer(line);
            while (tokenizer.hasMoreTokens()) {
                word.set(tokenizer.nextToken());
                context.write(word, one);
            }
        }
    }

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

        public void reduce(Text key, Iterable<IntWritable> values,
                Context context) throws IOException, InterruptedException {
            int sum = 0;
            for (IntWritable val : values) {
                sum += val.get();
            }
            context.write(key, new IntWritable(sum));
        }
    }

    public static void main(String[] args) throws Exception {
        Configuration conf = new Configuration();
        conf.set("mapred.job.tracker", "maprfs://,y_address");
        conf.set("fs.default.name", "hdfs://my_address");

        Job job = new Job(conf, "wordcount");
        job.setJarByClass(WordCount.class);

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

        job.setMapperClass(Map.class);
    job.setReducerClass(Reduce.class);

        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);

        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        job.waitForCompletion(true);    
    }   
}

Unfortunatelly, running this code ends up with the following error:

13/11/04 13:27:53 INFO mapred.JobClient: Task Id : attempt_201310311611_0005_m_000000_0, Status : FAILED java.lang.RuntimeException: java.lang.ClassNotFoundException: com.rf.hadoopspikes.WordCount$Map at org.apache.hadoop.conf.Configuration.getClass(Configuration.java:857) at org.apache.hadoop.mapreduce.JobContext.getMapperClass(JobContext.java:199) at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:718) at org.apache.hadoop.mapred.MapTask.run(MapTask.java:364) 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:415) at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190) at org.apache.hadoop.mapred.Child.main(Child.java:249)

I understand that the WordClass cannot be found but I have no idea how to make this work. Any ideas?

Answer 1

When running this directly from Eclipse, you need to make sure the classes have been bundled into a Jar file (for which hadoop then copies up to HDFS). Your error most probably relates to the fact that your Jar hasn't been built, or at runtime the classes are being run from the output directory and not the bundled jar.

Try and export the classes into a jar file, and then run your WordCount class from that Jar file. You could also look into using the Eclipse Hadoop plugin that i think handles all this form you. Final option would be to bundle the jar and then launch from the command line (as outlined in the various Hadoop tutorials)