MapReducer中不同key输入一个Reducer进行计算时reducer方法运行了几次？

Question

写了一个普通的WordCount在运行时发现有多个不同key输入后，在reducer中加入一个控制台显示语句，该语句只会输出一次，但是如果只执行了一次reduce方法key是如何实现遍历的？

map和reduce方法

     private static class MyMapper extends Mapper<LongWritable, Text, MedcineWritable, NullWritable>{
         static MedcineWritable outkey = new MedcineWritable();;
         static NullWritable outval = NullWritable.get();
         static String[] tmp ;
         @Override
         protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
             tmp = value.toString().split(" ");
            context.getCounter("Mapper_line","Total_line").increment(1);
            try {
                outkey.setDate(tmp[0]);
                outkey.setCid((tmp[1]));
                outkey.setSid(Integer.parseInt(tmp[2]));
                outkey.setName(tmp[3]);
                outkey.setNumber(Double.parseDouble(tmp[4]));
                outkey.setExceptprice(Double.parseDouble(tmp[5]));
                outkey.setRealprice(Double.parseDouble(tmp[6]));
                context.getCounter("Mapper_line","good_line").increment(1);
                System.out.println(outkey);
                context.write(outkey, outval);
            }
            catch (ArrayIndexOutOfBoundsException e){
                context.getCounter("Mapper_line","bad_line").increment(1);
            }catch (ClassFormatError e){
                context.getCounter("Mapper_line","bad_line").increment(1);
            }
         }
     }
     private static class MyReducer extends Reducer<MedcineWritable,NullWritable,MedcineWritable,NullWritable>{
         @Override
         protected void reduce(MedcineWritable key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
             System.out.println("上面是遍历map的输出，下面是reduce的输出");
             System.out.println(key);
         }
     }

Answer 1

如果是在reduce方法里面加的输出，理论上应该是有几个key就输出几次的。如果是加在reduce的类的初始化方法里面，应该是有几个reduce就输出几次