Hadoop只输出Key不输出Value的小技巧‏

用NullWritable即可。

 

 

       public static class XxxMapper

                extends Mapper<LongWritable, XxxRecord, LongWritable, NullWritable>

                {

                        private LongWritable uid = new LongWritable();

 

                        public void map(LongWritable key, XxxRecord value, Context context)

                                throws IOException, InterruptedException

                                {

                                        uid.set(value.uid);

                                        context.write(uid, NullWritable.get());

                                }

                }

 

 

说只输出Value不输出Key也类似。但没试过。不过我曾纠结于为什么K/V之间必须是Tab分隔呢？用这个可以轻松搞定了，呵呵。