java.lang.NullPointerException using MRUnit. Custom key serialization error

Question 1

I have found a solution to this error. The error was because the type of serialization had not been set in the Configuration for MapDriver mapDriver. I had to explicitly set the serialization using the following:

Configuration conf = new Configuration();
conf.set("io.serializations","org.apache.hadoop.io.serializer.JavaSerialization," 
            + "org.apache.hadoop.io.serializer.WritableSerialization");
mapDriver.setConfiguration(conf);

Hope this helps anyone with a similar problem!

Question 2

First it's worth to test whether the serialization / deserialization works really as expected.
Without knowing how you wrote the test, the following simple one works well with MRUnit 0.9.0-incubating and JUnit 4.10 :

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

import junit.framework.Assert;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.Text;
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.mapreduce.ReduceDriver;
import org.apache.hadoop.mrunit.types.Pair;
import org.junit.Before;
import org.junit.Test;

public class TestCustom {

    private MapDriver<CustomRecord, Text, CustomRecord, Text> mapDriver;

    private Mapper<CustomRecord, Text, CustomRecord, Text> map = 
        new Mapper<CustomRecord, Text, CustomRecord, Text>();

    private Reducer<CustomRecord, Text, CustomRecord, Text> reduce = 
        new Reducer<CustomRecord, Text, CustomRecord, Text>();

    private ReduceDriver<CustomRecord, Text, CustomRecord, Text> reduceDriver
        = ReduceDriver.newReduceDriver(reduce);

    private MapReduceDriver<CustomRecord, Text, CustomRecord, 
      Text, CustomRecord, Text> mapReduceDriver;

    private Configuration conf = new Configuration();

    //test data
    private Pair<CustomRecord, Text> data;

    //shuffled and sorted data
    private static List<Pair<CustomRecord, List<Text>>> shuffledData; 

    @Before
    public void init() {

        mapDriver = MapDriver.newMapDriver(map);
        mapReduceDriver = MapReduceDriver.newMapReduceDriver(map, reduce);
        mapDriver.withConfiguration(conf);
        initData();
    }

    private void initData() {

        CustomRecord key = new CustomRecord("first", 1);
        Text value = new Text("key1");
        data = new Pair<CustomRecord, Text>(key, value);
    }

    @Test
    public void testMapper() throws IOException {

        mapDriver.withInput(data);
        //expected output result
        mapDriver.withOutput(data); 
        mapDriver.runTest(true);

        //shuffle and sort
        List<Pair<CustomRecord, Text>> pairs = 
          new ArrayList<Pair<CustomRecord, Text>>();
        pairs.add(data);

        shuffledData = mapReduceDriver.shuffle(pairs);

    }

    @Test
    public void testReducer() throws IOException {

        // feed input to one single reduce call
        Pair<CustomRecord, List<Text>> pair = shuffledData.get(0);
        reduceDriver.withInput(pair.getFirst(), pair.getSecond());

        //reducer's output
        List<Pair<CustomRecord, Text>> result = reduceDriver.run();

        Assert.assertEquals("Key mismatch!", 
          data.getFirst(), result.get(0).getFirst());
        Assert.assertEquals("Value mismatch!", 
          data.getSecond(), result.get(0).getSecond());
    }
}

It tests the identity mapper and reducer with custom Writable as key (CustomRecord).
Note, that the key implements WritableComparable, and overrides hashCode and equals.