我已经找到了解决这个错误的办法。错误是因为未在中设置序列化类型 Configuration 为了 MapDriver mapDriver . 我必须使用以下命令显式设置序列化：

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

希望这能帮助任何有类似问题的人！

首先，值得测试序列化/反序列化是否真的如预期的那样工作。
在不知道如何编写测试的情况下，下面这个简单的测试在mrunit 0.9.0-cubuating和junit 4.10中运行良好：

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());
    }
}

它使用自定义可写键（customrecord）测试身份Map器和reducer。
注意，该键实现了writeablecomparable，并重写hashcode和equals。