@Override
  public String toString() {
    return providers.stream().map((p) -> p.getClass().getName())
      .collect(Collectors.joining(", ", "providers=[", "], selector=")) + selector.getClass();
  }
Chooses the best authentication provider and corresponding token given the HBase cluster
identifier and the user.
hbase-client/src/main/java/org/apache/hadoop/hbase/security/provider/SaslClientAuthenticationProviders.java