我在理解如何将简单hadoop的输出写回elasticsearch时遇到了一些困难。
作业配置为：

job.setOutputFormatClass(EsOutputFormat.class);
job.setOutputKeyClass(NullWritable.class);
job.setOutputValueClass(MapWritable.class);

减速机有：

final DoubleWritable average = new DoubleWritable(sum / size);

final MapWritable output = new MapWritable();
output.put(key, average);
context.write(NullWritable.get(), output);

但我有一个莫名其妙的例外：

14/08/15 16:59:54 INFO mapreduce.Job: Task Id : attempt_1408106733881_0013_r_000000_2, Status : FAILED Error: 
  org.elasticsearch.hadoop.EsHadoopIllegalArgumentException: 
  [org.elasticsearch.hadoop.serialization.field.MapWritableFieldExtractor@5796fabe] cannot extract value from object [org.apache.hadoop.io.MapWritable@dcdb8e97]    
    at org.elasticsearch.hadoop.serialization.bulk.TemplatedBulk$FieldWriter.write(TemplatedBulk.java:49)   
    at org.elasticsearch.hadoop.serialization.bulk.TemplatedBulk.writeTemplate(TemplatedBulk.java:101)  
    at org.elasticsearch.hadoop.serialization.bulk.TemplatedBulk.write(TemplatedBulk.java:77)   
    at org.elasticsearch.hadoop.rest.RestRepository.writeToIndex(RestRepository.java:130)   
    at org.elasticsearch.hadoop.mr.EsOutputFormat$EsRecordWriter.write(EsOutputFormat.java:161)     
    at org.apache.hadoop.mapred.ReduceTask$NewTrackingRecordWriter.write(ReduceTask.java:558)

在context.write（）调用中。
我有点困惑，知道吗？