How does one implement a Hadoop Mapper in Scala 2.9.0?

Question

When I migrated to Scala 2.9.0 from 2.8.1, all of the code was functional except for the Hadoop mappers. Because I had some wrapper objects in the way, I distilled down to the following example:

import org.apache.hadoop.mapreduce.{Mapper, Job}


object MyJob {
  def main(args:Array[String]) {
    val job = new Job(new Configuration())
    job.setMapperClass(classOf[MyMapper])

  }
}

class MyMapper extends Mapper[LongWritable,Text,Text,Text] {
  override def map(key: LongWritable, value: Text, context: Mapper[LongWritable,Text,Text,Text]#Context) {

  }
}

When I run this in 2.8.1, it runs quite well (and I have plenty of production code in 2.8.1. In 2.9.0 I get the following compilation error:

error: type mismatch;
found   : java.lang.Class[MyMapper](classOf[MyMapper])
required: java.lang.Class[_ <: org.apache.hadoop.mapreduce.Mapper]
job.setMapperClass(classOf[MyMapper])

The failing call is when I call setMapperClass on the Job object. Here's the definition of that method:

public void setMapperClass(java.lang.Class<? extends org.apache.hadoop.mapreduce.Mapper> cls) throws java.lang.IllegalStateException { /* compiled code */ }

The definition of the Mapper class itself is this:

public class Mapper<KEYIN, VALUEIN, KEYOUT, VALUEOUT>

Does anyone have a sense of what I'm doing wrong? It looks to me like the type is fundamentally correct: MyMapper does extend Mapper, and the method wants something that extends Mapper. And it works great in 2.8.1...

Answer 1

Silly as it seems, you can work around the problem by defining the Mapper before the Job. The following compiles:

import org.apache.hadoop._
import org.apache.hadoop.io._
import org.apache.hadoop.conf._
import org.apache.hadoop.mapreduce._

class MyMapper extends Mapper[LongWritable,Text,Text,Text] {
  override def map(key: LongWritable, value: Text, context: Mapper[LongWritable,Text,Text,Text]#Context) {
  }
}

object MyJob {
  def main(args:Array[String]) {
    val job = new Job(new Configuration())
    job.setMapperClass(classOf[MyMapper])
  }
}