private String getUniqKey(FanOutOneBlockAsyncDFSOutput output)
        throws IllegalAccessException, InvocationTargetException {
        return (String) getUniqKeyMethod.invoke(output.getDummyStream());
      }
Helper class for implementing {@link FanOutOneBlockAsyncDFSOutput}.
hbase-asyncfs/src/main/java/org/apache/hadoop/hbase/io/asyncfs/FanOutOneBlockAsyncDFSOutputHelper.java