CODEWITHSUNDEEP
javahadoopmapreduceclasscastexception
Abhi
Abhi

Reputation: 1255

ClassCastException:java.lang.Exception: java.lang.ClassCastException in mapred

I am writing a mapreduce application which takes input in (key, value) format and just displays the same data as output from reducer.

This is the sample input:

1500s   1
1960s   1
Aldus   1

In the below code, I am specifying the input format using <<>> and specified the delimiter as tab in the main(). When I run the code, I am running into the error message: 

java.lang.Exception: java.lang.ClassCastException: org.apache.hadoop.io.Text cannot be cast to org.apache.hadoop.io.LongWritable
at org.apache.hadoop.mapred.LocalJobRunner$Job.runTasks(LocalJobRunner.java:462)
at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:522)
Caused by: java.lang.ClassCastException: org.apache.hadoop.io.Text cannot be cast to org.apache.hadoop.io.LongWritable
at cscie63.examples.WordDesc$KVMapper.map(WordDesc.java:1)
at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:145)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:787)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:243)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

Tried different things to debug but nothing helped. 

public class WordDesc {

  public static class KVMapper 
        extends Mapper<Text, LongWritable, Text, LongWritable>{
    private final static IntWritable one = new IntWritable(1);
    private Text word = new Text();
    public void map(Text key, LongWritable value , Context context
                    ) throws IOException, InterruptedException {
      context.write(key,value);
      }
    }


  public static class KVReducer 
       extends Reducer<Text,LongWritable,Text,LongWritable> {

    private IntWritable result = new IntWritable();
    public void reduce(Text key, LongWritable value, 
                       Context context
                       ) throws IOException, InterruptedException {
      context.write(key, value);
    }
  }

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    conf.set("mapreduce.input.keyvaluelinerecordreader.key.value.separator", "\t");
            String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
    if (otherArgs.length < 2) {
      System.err.println("Usage: wordcount <in> [<in>...] <out>");
      System.exit(2);
    }
    Job job = new Job(conf, "word desc");
    job.setInputFormatClass(KeyValueTextInputFormat.class);
    job.setJarByClass(WordDesc.class);
    job.setMapperClass(KVMapper.class);
    job.setCombinerClass(KVReducer.class);
    job.setReducerClass(KVReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(LongWritable.class);
    for (int i = 0; i < otherArgs.length - 1; ++i) {
      FileInputFormat.addInputPath(job, new Path(otherArgs[i]));
    }
    FileOutputFormat.setOutputPath(job,
      new Path(otherArgs[otherArgs.length - 1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

Upvotes: 2

Views: 775

Answers (1)

vefthym
vefthym

Reputation: 7462

I guess that this line job.setInputFormatClass(KeyValueTextInputFormat.class); tells your program to treat your input as key value pairs of Text. Therefore, when you require your input value to be a LongWritable you get this Exception.

A quick fix would be to read your input as Text and then, if you want to use a LongWritable, parse it using:

public static class KVMapper 
    extends Mapper<Text, Text, Text, LongWritable>{
    private final static LongWritable val = new LongWritable();
    public void map(Text key, Text value, Context context) {
      val.set(Long.parseLong(value.toString()));
      context.write(key,val);
    }
}

What it does is the following: value is Text, then value.toString() gives the String representation of this Text and then Long.parseLong() parses this String as long. Finally, val.set(), transforms it to a a LongWritable.

By the way, I don't think that you need a Reducer for that... You could make it faster by setting the number of reduce tasks to 0.

Upvotes: 1

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