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