public GroupedVectorStore() throws IOException {
   super(new NullOutputFormat());
   Env.inBackground(
       new Env.BackgroundProcedure() {
         @Override
         public void execute(Configuration conf) throws IOException {
           tvgen =
               VectorUtils.createVectorGenerator(
                   Env.getProperty(GroupedVectorStore.class, "vector.type"));
           writers = new HashMap<Object, SequenceFile.Writer>();
           keyWritable = new IntWritable();
           valueWritable = new VectorWritable();
         }
       });
 }