我为mapreduce文本排序编写了这样的代码：

public static class SortMapper extends Mapper<Object, Text, Text, Text> {
    private Text citizenship = new Text();

    @Override
    public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
        citizenship.set(value.toString().split(",")[11]);
        context.write(citizenship, value);
    }
}

public static class PrintReducer extends Reducer<Text, Text, NullWritable, Text> {

    @Override
    protected void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
        Iterator<Text> valIt = values.iterator();

        while (valIt.hasNext()) {
            Text value = valIt.next();
            context.write(NullWritable.get(), value);
        }
    }
}

public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    Job job = Job.getInstance(conf, "Football Sort");
    job.setJarByClass(FootballSort.class);
    job.setMapperClass(SortMapper.class);
    job.setCombinerClass(PrintReducer.class);
    job.setReducerClass(PrintReducer.class);
    job.setMapOutputKeyClass(Text.class);
    job.setMapOutputValueClass(Text.class);
    job.setOutputKeyClass(NullWritable.class);
    job.setOutputValueClass(Text.class);
    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
}

但它总是很吸引人
第26、34行出现ioexception原因：org.apache.hadoop.io.nullwriteable类不是org.apache.hadoop.io.text类