Simple subclass of MRUnit's MapReduceDriver to fix NPE when using MultipleOutputs

MultiOutputMapReduceDriver.java

package org.apache.hadoop.mrunit.mapreduce;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mrunit.mapreduce.MapDriver;
import org.apache.hadoop.mrunit.mapreduce.MapReduceDriver;
import org.apache.hadoop.mrunit.types.Pair;


@SuppressWarnings("rawtypes")
public class MultiOutputMapReduceDriver<K1, V1, K2 extends Comparable, V2, K3, V3> extends
        MapReduceDriver<K1, V1, K2, V2, K3, V3> {

    public MultiOutputMapReduceDriver() {
        super();
    }

    public MultiOutputMapReduceDriver(Mapper<K1, V1, K2, V2> m, Reducer<K2, V2, K3, V3> r) {
        super(m, r);
    }

    @Override
    public List<Pair<K3, V3>> run() throws IOException {

        List<Pair<K2, V2>> mapOutputs = new ArrayList<Pair<K2, V2>>();

        // run map component
        for (Pair<K1, V1> input : inputList) {
          LOG.debug("Mapping input " + input.toString() + ")");

          mapOutputs.addAll(new MapDriver<K1, V1, K2, V2>(getMapper()).withInput(
                  input).withCounters(getCounters()).withConfiguration(configuration).run());
        }

        List<Pair<K2, List<V2>>> reduceInputs = shuffle(mapOutputs);
        List<Pair<K3, V3>> reduceOutputs = new ArrayList<Pair<K3, V3>>();

        for (Pair<K2, List<V2>> input : reduceInputs) {
          K2 inputKey = input.getFirst();
          List<V2> inputValues = input.getSecond();
          StringBuilder sb = new StringBuilder();
          formatValueList(inputValues, sb);
          LOG.debug("Reducing input (" + inputKey.toString() + ", "
              + sb.toString() + ")");

          reduceOutputs.addAll(new MultiOutputReduceDriver<K2, V2, K3, V3>(getReducer())
                  .withCounters(getCounters()).withConfiguration(configuration)
                  .withInputKey(inputKey).withInputValues(inputValues).run());
        }

        return reduceOutputs;

    }

}

MultiOutputReduceDriver.java

package org.apache.hadoop.mrunit.mapreduce;

import static org.mockito.Mockito.*;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.TaskAttemptID;
import org.apache.hadoop.mrunit.mapreduce.ReduceDriver;
import org.apache.hadoop.mrunit.mapreduce.mock.MockReduceContextWrapper;
import org.apache.hadoop.mrunit.types.Pair;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;

public class MultiOutputReduceDriver<K1, V1, K2, V2> extends ReduceDriver<K1, V1, K2, V2> {

    public MultiOutputReduceDriver() {
        super();
    }

    public MultiOutputReduceDriver(Reducer<K1, V1, K2, V2> r) {
        super(r);
    }

    @Override
    public List<Pair<K2, V2>> run() throws IOException {
      List<Pair<K1, List<V1>>> inputs = new ArrayList<Pair<K1, List<V1>>>();
      inputs.add(new Pair<K1, List<V1>>(inputKey, getInputValues()));

      try {
        MockReduceContextWrapper<K1, V1, K2, V2> wrapper =
          new MockReduceContextWrapper<K1, V1, K2, V2>(inputs, getCounters(), getConfiguration());
        Reducer<K1, V1, K2, V2>.Context context = wrapper.getMockContext();

        when(context.getTaskAttemptID()).thenAnswer(new Answer<TaskAttemptID>() {
           @Override
            public TaskAttemptID answer(InvocationOnMock invocation) throws Throwable {
               return new TaskAttemptID();
            }
        });

        getReducer().run(context);
        return wrapper.getOutputs();
      } catch (InterruptedException ie) {
        throw new IOException(ie);
      }
    }

}

Can you tell me how to use these two files ?

Don't use them, upgrade to mrunit 1.1.0