我也有过类似的情况。我所做的是这样的：

int numberOfReduceCalls = 5
IntWritable outKey = new IntWritable();
Random random = new Random();
public void map(LongWritable key, Text value, Context context)
                      throws IOException, InterruptedException {
    // use a random integer within a limit
    outKey.set( random.nextInt(numberOfReduceCalls) );  
    context.write(outKey, value);
}