@Override
        public ListenableFuture<String[]> reload(final String k, String[] oldValue)
          throws Exception {

          return executor.submit(new Callable<String[]>() {
            @Override
            public String[] call() throws Exception {
              return getGroupStrings(k);
            }
          });
        }
Provide an instance of a user. Allows custom {@link User} creation.
hbase-common/src/main/java/org/apache/hadoop/hbase/security/UserProvider.java