Mapreduce实例-Top Key

 1 public class TopK extends Configured implements Tool {

    public static class TopKMapper extends Mapper<Object, Text, NullWritable, LongWritable> {

        public static final int K = 100;
        private TreeMap<Long, Long> tm = new TreeMap<Long, Long>();

        @Override
        protected void map(Object key, Text value, Context context) throws IOException, InterruptedException {
           try {
                long k = Integer.parseInt(value.toString().substring(0, 9));
                tm.put(k, k);
                if (tm.size() > K) {
                    tm.remove(tm.firstKey());
                }
            } catch (Exception e) {
                context.getCounter("TopK", "errorlog").increment(1);
            }
        }

        @Override
        protected void cleanup(Context context) throws IOException, InterruptedException {
            for (Long text : tm.values()) {
                context.write(NullWritable.get(), new LongWritable(text));
            }
        }
    }

    public static class TopKReducer extends Reducer<NullWritable, LongWritable, NullWritable, LongWritable> {

        public static final int K = 100;
        private TreeMap<Long, Long> mt = new TreeMap<Long, Long>();

        @Override
        protected void reduce(NullWritable key, Iterable<LongWritable> values, Context context)
                throws IOException, InterruptedException {
            for (LongWritable value : values) {
                mt.put(value.get(), value.get());
                if (mt.size() > K) {
                    mt.remove(mt.firstKey());
                }
            }
            for (Long val : mt.descendingKeySet()) {
                context.write(NullWritable.get(), new LongWritable(val));
            }
        }

    }

    @Override
    public int run(String[] args) throws Exception {
        Configuration conf = getConf();
        Job job = new Job(conf, "TopKNum");
        job.setOutputKeyClass(NullWritable.class);
        job.setOutputValueClass(LongWritable.class);
        job.setMapperClass(TopKMapper.class);
        job.setReducerClass(TopKReducer.class);
        job.setJarByClass(TopK.class);
        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));
        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);

        return job.waitForCompletion(true) ? 0 : 1;
    }

    
    public static void main(String[] args) throws IOException, InterruptedException {
        try {
            if (args.length < 2) {
                System.err.println("ERROR: Parameter format length ");
                System.exit(0);
            }
            int ret = ToolRunner.run(new TopK(), args);
            System.exit(ret);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

上面是求最大100个，如果求最小的100 个数，改map和reduce中的mt.remove(mt.firstKey());为mt.remove(mt.lastKey()）

来自：http://blog.csdn.net/liuzhoulong/article/details/11175381